EARLY ORTHOCERATOID CEPHALOPODS FROM THE ARGENTINE PRECORDILLERA (LOWER–MIDDLE ORDOVICIAN)

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J. Paleont., 81(6), 2007, pp. 1266–1283Copyright � 2007, The Paleontological Society0022-3360/07/0081-1266$03.00

EARLY ORTHOCERATOID CEPHALOPODS FROM THE ARGENTINEPRECORDILLERA (LOWER–MIDDLE ORDOVICIAN)

BJORN KROGER,1 MATILDE S. BERESI,2 AND ED LANDING3

1Museum fur Naturkunde, Institut fur Palaontologie, Invalidenstraße 43, D-10115 Berlin, Germany, �[email protected]�;2Cricyt-Ianigla, Departamento de Paleontologıa, Avda. Ruiz leal s/n, 5500 Mendoza, Argentina, �[email protected]�; and

3New York State Museum, State Education Department, Albany, New York 12230, �[email protected]�

ABSTRACT—The Early and Middle Ordovician Orthocerida and Lituitida of Precordilleran Argentina are described, and their systematics andpaleogeographic significance are revised. These cephalopods show a strong affinity to coeval faunas of North China, suggesting a location ofthe Precordillera at middle latitudes in the Southern Hemisphere east of the North China block and relatively close to the Gondwanan marginduring the early Middle Ordovician. The descriptive terminology of characters of the septal necks, the position and shape of the siphuncule,and the shape of the connecting ring is improved. The distribution of these characters support an emendation of the Baltoceratidae, Sactorth-oceratidae, and Proteoceratidae. Braulioceras n. gen. (Sactorthoceratidae) and Palorthoceras n. gen. (Orthoceratidae) are erected. The newspecies Braulioceras sanjuanense, Eosomichelinoceras baldisii, Gangshanoceras villicumense, and Rhynchorthoceras minor are proposed. Pa-lorthoceras n. gen. from the Lower Ordovician Oepikodus evae Zone represents the earliest known orthocerid.

INTRODUCTION

ORTHOCERIDAN NAUTILOIDS are the stem group of coleoids,the dominant group of modern cephalopods. Of the 139

modern cephalopod genera, only the Recent Nautilus does notbelong to the coleoids (Sweeney and Roper, 1998). Coleoids arecharacterized by a single pair of gills and a small number of armsand radular teeth, while all cochleate coleoids have a sphericalprotoconch. These characters unite the coleoids with the ammo-noids, bactritoids, and orthoceridans as taxa of the Neocephalo-poda (Engeser, 1996). The earliest known neocephalopods areknown from the early Tulean of the Arenigian Series of Wales(Evans, 2005). However, these early representatives remain veryrare until the late Arenigian when the first orthoconic shells witha central siphuncle appear which are referred to the Orthocerati-dae McCoy, 1844. By the earliest Middle Ordovician, a complexand diverse fauna of Orthocerida and closely related taxa of theLituitida was established. However, the details of the evolutionaryhistory of this group during the critical Early-Middle Ordovicianboundary interval in the Great Ordovician Radiation are poorlyknown.

Over the last few decades, MSB collected a suite of nautiloidsfrom the Lower–Middle Ordovician of the Argentine Precordil-lera. This report concentrates on the revision and comparison ofthe Orthocerida and Lituitida of this collection in order to providenew data on the initial radiation of the Neocephalopoda. Thisrevision also provided new information on the paleogeographicalrelationships of the Precordillera in the Early–Middle Ordovician.These data are of particular interest because the paleogeographiclocation of the Precordillera during the Early Paleozoic is highlydisputed.

The Precordilleran rocks comprise an enigmatic, allochthonoussuccession that represents an Early Paleozoic microcontinentcalled the Precordillera or Cuyania terrane. Two conflicting hy-potheses explain the origin, history, and final collision of the mi-crocontinent with the Gondwana margin. The first is the widelyaccepted Laurentian microcontinent model of Astini et al. (1995)and Thomas and Astini (1996). By this model, the Precordillerarepresent a terrane of Laurentian origin that collided with thewestern Argentina margin of Gondwana during the Ordovician(Thomas and Astini, 1996, 2003). However, the Cambrian–Or-dovician history and movement of the terrane from its separationfrom Laurentia until its collision with Gondwana is not known(e.g., Astini et al., 1995; Fanning et al., 2004). The second hy-pothesis considers the Precordillera to be autochthonous and afragment of Gondwana (Baldis et al., 1989; Acenolaza and To-selli, 1999; Acenolaza et al., 2002, Finney et al., 2003a). Recent

U-Pb geochronology of detrital zircons has indicated a Gondwan-an provenance for Lower Cambrian and Upper Ordovician sand-stones of Precordilleran Argentina (Finney et al., 2003b), and sup-ports the autochthonous Gondwanan model for the Precordilleraterrane.

The Lower-Middle Ordovician sedimentary rocks of the Ar-gentine Precordillera represent a subsiding carbonate platformwith a diverse and relatively complete fossil record (Albanesi etal., 1999). Numerous reports of different aspects of the complexfaunal history of this succession have been published (e.g., Beresiand Rigby, 1993; Benedetto, 1998; Ottone et al., 1999; Carrera,2001; Albanesi and Ortega, 2002). The rich nautiloid fauna of thePrecordilleran carbonates was early noted by A. W. Stelzner(1885). However, the Ordovician cephalopods of PrecordilleranArgentina remain poorly known, and were only remarked on ina general review of South American nautiloid faunas (Acenolazaet al., 1977; Acenolaza and Beresi, 2002). Interestingly, their re-view emphasized that the Precordilleran cephalopod fauna isunique to this region and differs considerably from better known,coeval, Early-Middle Ordovician faunas from Baltoscandia andLaurentia. Indeed, the Precordilleran cephalopod faunas contrastwith associated biotic groups which are dominated by cosmopol-itan taxa. In this report, we provide the first detailed overview ofthe Precordilleran nautiloids.

GEOLOGICAL SETTING AND AGE OF THE FAUNA

The eastern and central belts of Precordilleran Argentina arecharacterized by a thick succession of Lower-Middle Ordovicianplatform carbonates. The stratigraphic synthesis of PrecordilleranLower Paleozoic sediments is extraordinarily good, and has in-volved biostratigraphic and sedimentary data and incorporatedmagmatic events. The uppermost unit of these carbonates is thefossil-rich San Juan Formation which conformably overlies theLa Silla Formation. The conodont biostratigraphy of the San JuanFormation was established by Serpagli (1974), Lehnert (1995) tospan the upper Tremadocian (Paltodus deltifer Zone) to the lowerDarriwilian (Eoplacognathus suecicus Zone). The top of the SanJuan Formation is diachronous, and extends into the Oepikodusevae/Oepikodus ‘‘communis’’ zones in the northern sections (e.g.,Guandacol River) to the higher Eoplacognatus variabilis Zone insouthern sections (e.g., Villicum Range) (Sarmiento, 1985; Hun-icken and Ortega, 1987; Albanesi et al., 1999; Albanesi and Or-tega, 2002). The base of the formation is characterized by a bio-turbated bioclastic wackestone with a mollusk-dominatedmacrofauna (gastropods, nautiloids), that is interbedded with thin-to medium-bedded bio-intraclastic grainstones to packstones andintraclast rudstones. The middle part of the San Juan Formationis a thick transgressive unit that includes a bioturbated bioclastic

1267KROGER ET AL.—ORDOVICIAN ORTHOCERATOID CEPHALOPODS

FIGURE 1—Regional overview of outcrop area and localities of the SanJuan Formation in the Precordillera, western Argentina.

and fossiliferous wackestone with a rich fauna (brachiopods, tri-lobites, pelmatozoans, gastropods, ostracodes, nautiloids, lithistidsponges, receptaculitids, bryozoans, conodonts, green algae, andGirvanella-type fossils). The upper boundary of the San Juan For-mation is represented by a gray interval of an often nodular wack-estone and mudstone. (For a detailed documentation of the sedi-mentology see Beresi, 1992; Keller, 1994; Canas, 1995.) Two reefhorizons have been identified. The lower reef horizon (upper Tre-madocian) occurs near the base of the formation in the northernPrecordillera (Canas and Carrera, 1993). The upper reef horizonoccurs in the lower Middle Ordovician (approximately Baltoniod-us triangularis–Paroistodus originalis Zone) (Keller and Flugel,1996).

The San Juan Formation was deposited on an open carbonateshelf, and was bounded to the west by continental slope and oce-anic basin deposits (Beresi, 1986). The diverse marine fauna andthe lack of features indicative of restricted marine conditions sug-gest low-energy, subtidal conditions on an open platform duringthe entire interval of San Juan Formation deposition. The calcar-eous algae and cyanobacteria indicate deposition within the photiczone. The diachronous facies change that defines the top of theformation implies local epeirogenic subsidence as a mechanismresponsible for producing accommodation space, rather than sim-ple eustatic rise, during San Juan deposition. The nautiloid asso-ciations occur mainly in the middle (O. evae Zone) and upperparts (E. suecicus Zone) of the San Juan Formation. Nautiloidconchs represent approximately 5% of the total macrofauna in thelower part of the sequence and up to 20% in the upper levels.The nautiloids are commonly associated with gastropods, bra-chiopods, trilobites, bivalves, and diverse algae. The nautiloidsexamined in this report were collected at four outcrops (Fig. 1,2).

LOCALITIES

Quebrada Talacasto section, central Precordillera, San JuanProvince.⎯The Talacasto section (Fig. 2) is a roadcut on NationalRoad 40, approximately 70 km northwest of San Juan near thevillage of Iglesias. The succession spans the Lower-Middle Or-dovician carbonate sequences of the La Silla and San Juan For-mations and the uppermost Ordovician–Silurian siliciclastics ofthe La Chilca Formation. The Talacasto section is a classical out-crop in the Argentine Precordillera (Kayser, 1876; Stelzner, 1885).Stelzner (1885) noted the rich cephalopod fauna at Talacasto, andthe entire succession at Talacasto, and particularly the San JuanFormation, has been subjected to numerous paleontological andsedimentological studies (e.g., Beresi, 1981; Herrera and Bene-detto, 1991; Sanchez et al., 1996). K-bentonites from the sectionhave been described by Huff et al. (1995). At Talacasto Creek,the San Juan Formation is 350 m thick. It consists of fossiliferouswackestone–intraclastic packstone in the lower and middle part,and nodular limestones in the upper member. In the middle partof the sequence, the conodont Oepikodus evae (Lindstrom, 1955)occurs, whereas the top of the succession is significantly younger(Eoplacognathus suecicus Zone). The medium- to thick-beddedmud-wackestones of the Oepikodus evae Zone yield a rich benthicmacrofauna with occasional nautiloids. However, the main fossil-iferous interval constitutes the nodular limestones of the uppermember, which has common sponge biostromes in the Talacastoarea (Beresi, 1986). A thin-bedded, platy, black mudstone-wack-stone that alternates with marlstone represents the top of the for-mation. A major unconformity separates the San Juan Formationfrom younger deposits (uppermost Ashgillian of the La ChilcaFormation).

Quebrada Gustavo section, eastern Precordillera, San JuanProvince.⎯The Gustavo Creek section occurs in the VillicumRange of the eastern Precordillera (Peralta and Beresi, 1999). The

upper member of the San Juan Formation and the boundary in-terval with the overlaying upper Darriwilian siliciclastics are ex-posed at Gustavo Creek. The upper member is composed of ar-gillaceous lime-mudstones, fossiliferous wackestones,K-bentonites, and calcarenites. At the top of the upper member,Eoplacognathus suecicus Zone (middle Darriwilian) conodontsoccur (Sarmiento, 1985). The 9 m thick upper member is char-acterized by a trilobite- and brachiopod-dominated association(assemblage A.I of Peralta and Beresi, 1999) at the base, followedby a lithistid sponge-dominated association (assemblage A.II ofPeralta and Beresi, 1999). Five meters above the base of the uppermember, the fauna is predominantly composed of longiconic or-thocones (endoceridans, ellesmeroceridans, and orthoceridans)(assemblage A.III of Peralta and Beresi, 1999). Above this fos-siliferous interval is 6 m of nearly macrofossil-free argillaceousmudstone with interbedded, thin K-bentonites. The upper memberis capped by a distinctive, ca. 0.5 m thick grainstone bed thatcontains abundant crinoid ossicles and orthocone nautiloids (pre-dominantly endoceridans with conches up to 0.6 m long).

Don Braulio section, eastern Precordillera, San Juan Prov-ince.⎯Don Braulio Creek is a classic locality for Ordovician stra-ta on the eastern slope of the Villicum Range. The San Juan

1268 JOURNAL OF PALEONTOLOGY, V. 81, NO. 6, 2007

FIGURE 2—Schematic overview, correlation, and nautiloid occurrences of the localities in the San Juan Formation, Precordilleran Argentina.

Formation is 380 m thick at this locality. The formation is com-posed of a lower nodular grey wackestone lithofacies, a middle,thick-bedded packstone-wackestone lithofacies, and upper fossil-iferous wackestones and mudstones which abruptly change attheir top into bioclastic (crinoidal) grainstones and packstones inwhich nautiloids are concentrated. Ash beds intercalated with theuppermost beds (7 m) have been correlated with K-bentonites atother localities in the Precordillera (Huff et al., 1995). The top ofthe San Juan Formation is an omission surface overlain by 12 mof a rhythmically bedded, limestone-marlstone sequence of theGualcamayo Formation (Eoplacognathus suecicus Zone) (Baldisand Beresi, 1981). In the middle San Juan Formation, the Oepi-kodus evae Zone has been rocognized, and the top of the for-mation lies in the Eoplacognathus suecicus Zone (Sarmiento,1985). The Oepikodus evae Zone contains a nautiloid fauna char-acterized by larger endocerids. The nautiloid diversity is highest

at the top of the San Juan Formation, while the lowest Gualca-mayo Formation yields mainly small orthocerids and few brevi-cones.

Cerro Viejo, Huaco area, eastern Precordillera, San JuanProvince.⎯The Cerro Viejo de Huaco is located in the CentralPrecordillera between 30�11�40� and 30�15�30�S, and 68�34�30�and 68�35�20�W. The San Juan Formation at Cerro Viejo is 260m thick, and is overlain conformably by the Los Azules Shale(middle Darriwilian–Upper Ordovician). Yellowish K-bentonitelayers occur at the top of the carbonate sequence and in the lowerpart of the graptolitic, dark Los Azules Shale. Hunicken and Or-tega (1987) referred the uppermost San Juan Formation to theMiddle Ordovician Eoplacognathus suecicus Zone. However, newcollections indicate correlation with the lowest Middle OrdovicianLenodus variabilis Zone (Ottone et al., 1999). The San Juan For-mation in the Cerro Viejo is composed of lower nodular wackestones


with chert horizons, middle intraclast packstones-grainstones, andupper marly wackestones and mudstones. The macrofauna is com-posed of articulate brachiopods, trilobites, gastropods, bryozoans,sponges, calcareous algae such as Nuia, and nautiloids. Ortho-ceras sp. and Cyrtoceras sp. were cited from this locality by Bor-rello and Gareca (1951).

PALEOBIOGEOGRAPHY OF PRECORDILLERAN CEPHALOPODS

Endoceridans followed by orthoceridans are the most commonnautiloids of the San Juan Formation. However, orthoceridan di-versity is clearly higher than that of the endoceridans, and theyare the preferred cephalopods for paleogeographic comparisons.

The paleogeographical pattern of orthoceridan nautiloids duringthe Paleozoic is highly provincial (e.g., Foerste, 1929; Gnoli,2002). The utility of orthoceridans in paleogeographical investi-gations has not been qualitatively or quantitatively tested. How-ever, it is evident that orthoceridans, despite their supposed plank-tic lifestyle in parts of their ontogeny (Westermann, 1999), weresensitive to biogeographical barriers such as cold currents likeother cephalopods (Crick, 1990). The embryonic shell of ortho-ceridans is small and often shows a constriction at a certainstage—features that strongly suggest the development of paralar-vae (Ristedt, 1968; Kroger, 2006). The development of paralarvaein orthoceridans strongly enhanced the probability of widespreadgeographical distribution, a probability that was constrained byoceanic currents. Similarly, the geographical distribution of Re-cent coleoids with planktic paralarvae is strongly controlled bywater currents (e.g., Rocha et al., 1999; Gonzalez et al., 2005).By analogy with coleoids, orthoceridans should have potential forpaleogeographical investigations.

Few paleogeographical syntheses provide data on the possiblepositions and movement of the Precordilleran terrane. The resultsof these investigations are useful in the Early Ordovician but arehighly equivocal for the Middle Ordovician. Pliomerid and pro-sopiscid trilobites from the Precordillera demonstrate a peri-Gondwana affinity in the Middle Ordovician, but a biogeograph-ically very ambiguous picture is generally provided by thesetrilobites (Edgecombe et al., 1999). Fortey and Cocks (2003)mention the strongly Laurentian aspect of Precordilleran EarlyOrdovician brachiopods. Tychsen and Harper (2004) demonstrat-ed that Precordilleran orthid brachiopods show a strong Lauren-tian aspect during the late Early Ordovician, but commonly in-clude widespread to cosmopolitan taxa during the MiddleOrdovician, and thus provide little paleogeographical information.Similarly, Precordilleran trilobites show clear Laurentian affinitiesin the Early Ordovician, but include a unique mixture of faunalsignatures in the Middle Ordovician with warm-water Baltic,Avalonian, and West Gondwanan taxa appearing (Fortey andCocks, 2003).

Orthid brachiopods (Tychsen and Harper, 2004) and conodonts(Zhen and Percival, 2003) of the Precordillera are dominated bywidespread and cosmopolitan taxa, and this makes detailed pa-leogeographical reconstructions difficult. It is notable that Zhenand Percival (2003) grouped the Precordilleran conodonts togeth-er with those of South China within a ‘‘Temperate Domain’’grouping. However, the time span of this ‘‘domain’’ comprisesthe entire Ordovician, and thus is hardly useful in tracing theEarly–Middle Ordovician paleogeographic movements of the Pre-cordillera. Terminal Lower–lower Middle Ordovician conodontsfrom the San Juan Formation (Serpagli, 1974) have been shownto be dominated by an association of taxa known from the Balticplatform and from Laurentian continental slope successions inwestern Nevada, the Marathon Basin of Texas, and the Taconicallochthons in eastern New York, Quebec, and western New-foundland (see Landing and Ludvigsen, 1984; Landing, 1976).These conodont data are comparable with ‘‘temperate’’ and un-restricted marine environments. Li and Servais (2002) placed the

Early-Middle Ordovician acritarchs of Argentina in their peri-Gondwana province, but gave no information from which regiontheir Argentina data were obtained.

The only known Precordilleran Early Ordovician orthocerid,Palorthoceras kayseri n. gen. and sp., is also known from theGreat Basin of the western United States. This is consistent withbrachiopod data that indicate strong Laurentian affinities of thePrecordilleran faunas during this time interval (Benedetto, 1998).However, the Middle Ordovician orthoceridans known from thePrecordillera are strongly non-Laurentian in paleogeographic af-finity. Of the six Darriwilian orthoceridan genera and species,only Cochlioceras avus Eichwald, 1860, is known from Laurentia.However, C. avus is cosmopolitan, and is known also from Si-beria, Baltica, and North China. One Precordilleran genus, Rhyn-chorthoceras Remele, 1882, is known from Baltica and NorthChina. Of the remaining taxa, four orthoceridan species and onegenus are endemic. The remaining two Middle Ordovician Pre-cordilleran orthoceridan genera known from the Precordillera(Eosomichelinoceras J.-Y. Chen, 1974; Gangshanoceras Zou,1988) were previously known only from China. It is significantthat Braulioceras n. gen., an endemic Precordilleran genus, showsstrong affinities to Sactorthoceras coreanicum Kobayashi, 1927,from the North China block. The known orthoceridans of thePuna Basin (Cecioni, 1965) of northwest Argentina differ com-pletely at the species level. The Puna Basin has yielded only thecosmopolitan genus Cochlioceras Eichwald, 1860 and the endem-ic genus Belloceras Cecioni, 1965. Thus, the Middle Ordovicianorthoceridans of the Precordillera show a clear affinity to coevalfaunas of North China. This apparent affinity with North Chinamay only represent an artifact of current knowledge because Mid-dle Ordovician Gondwanan cephalopod faunas are poorly known.A Precordilleran-North China connection may in reality representa peri-Gondwana faunal province, a speculation that requiresmore data.

Nevertheless, the strong affinities of Precordilleran and NorthChinese orthoceridan faunas put a spotlight on the unresolvedquestion of the longitudinal paleo-positions of the Precordilleraand North China. A review of the available literature reveals anambiguous picture. In the widely accepted models of Niocaill etal. (1997), Fortey and Cocks (2003), and Cocks and Torsvik(2004), the Precordillera drifted away from low-latitude Lauren-tia, which is distant from high-latitude West Gondwana and Ava-lonia, and toward the Argentine margin of Gondwana. In thesemodels the closest neighbors to the Precordilleran terrane are Lau-rentia to the northeast, Baltica to the east, and West Gondwanato the south. The position of North China is not given, but anentirely Northern Hemisphere position is supposed. This modelfollows the paleomagnetic reconstruction of Niocaill et al. (1997),which provides no specific data for the North China block. Ty-chsen and Harper (2004) locate the North China block in theSouthern Hemisphere somewhere between Baltica and the easternmargin of Gonwana, but no specific data are given for the Pre-cordillera. Li and Servais (2002) place the North China block onthe northeastern edge of Gondwana close to Australia and in thenorthern hemisphere. In the paleomagnetic reconstructions ofHuang et al. (2000) and Yang et al. (2002), the North China blockseparated from the eastern margin of Gondwana in the earliestOrdovician and moved eastward in the middle latitudes of thesouthern hemisphere toward the Argentinian margin of Gond-wana. In the latter models, North China drifted relatively close tothe Appalachian margin of Laurentia in the Middle Ordovicianand lay just west of the supposed trajectory of the Precordillera(Astini et al., 1995; Thomas and Astini, 1996).

Thus, the models of Huang et al. (2002), Yang et al. (2002),and Li and Servais (2002) are in good agreement with the distri-bution of Middle Ordovician orthoceridans. Moreover, the recon-struction of ocean currents by Barnes (2004), which supposed an


FIGURE 3—Main characters of the septal neck, connecting ring, and shapeof the siphuncle of some Early-Middle Ordovician orthoconic nautiloids. Or-thoconic nautiloids with thin concave siphuncular segments are classifiedwithin the Protocycloceratidae of the Ellesmeroceratida. See text for discus-sion.

eastward-directed peri-Gondwana current, supports the faunal pat-tern of Middle Ordovician orthoceridans.

CONCH TERMINOLOGY

The terminology used herein to describe nautiloid conchs isderived from a series of recent reports (Kroger and Mapes, 2005,2007; Kroger, 2006; Kroger and Isakar, 2006) that describe a par-ticular fauna or association of orthoceridan cephalopods. Thesepapers are part of a general revision of orthoceridan cephalopods.This revision is part of a planned cladistic analysis of Paleozoicnautiloids. The crucial requirement of this approach is a robustdetermination and designation of characters. In order to establishan unequivocal morphological terminology, it is necessary to dis-cuss some character designations below.

Position of the siphuncle.⎯The conventional terminology fordesignation of the siphuncle position relies on the position of thesoft body axis (Sweet, 1964). Therefore, the position of the si-phuncle within the phragmocone is designated as ventral, central,or dorsal. This practice is possible only when the soft body of anectocochleate cephalopod is oriented with the anterior-posterioraxis parallel to the growth axis. The venter can be determinedonly by reference to the position of the hyponomic sinus at theaperture. However, the hyponomic sinus is not known or is un-developed in many conchs. Thus, many cyrtoconic forms have anunknown aperture, and the ventral side is simply equated with theside of the shell where the siphuncle is located. To avoid this typeof unsubstantiated conclusion, it is proposed herein that the po-sition of the siphuncle be designated with reference to the conchcurvature if the position of the hyponomic sinus is not known.This is possible even in orthoconic shells, because no completelystraight shell exists. Thus, the siphuncle can be on the convexside of the growth axis of the shell, on the concave side, and inthe center. The terms ‘‘prosiphonal’’ and ‘‘antisiphonal’’ designatethe position of an element of the conch with reference to thesiphuncle. An element is antisiphonal, when it is positioned at theopposite side of the conch with reference to the siphuncle.

Septal neck shape.⎯An achoantic septal neck designates a con-dition in which septal necks are absent—that is, septal perfora-tions have the same diameter as the siphuncular tube (Fig. 3).Loxochoanitic septal necks point obliquely inward and backwardfrom a relatively large septal perforation toward the narrower si-phuncle (curvature of the septal neck �90�). Suborthochoaniticseptal necks are short and recurved; the curvature of the septalneck is �90� and �180�. Orthochoanitic septal necks are orientedparallel to the growth axis (curvature of the septal neck 90�), andare shorter than half the length of the siphuncular segment. Cyr-tochoanitic septal necks are recurved; they slope into an expandedsiphuncular tube (curvature of the septal neck �180�). Hemi-choanitic septal necks point toward the growth axis as ortho-choanitic septal necks, but are longer and comprise more thanhalf the length of a siphuncular segment. Holochoanitic septal

necks are longer than the distance between two successive cham-bers, and are always parallel to the growth axis.

Siphuncular tube shape.⎯The siphuncular tube consists of seg-ments that span two adjacent septa. The tube wall is the con-necting ring. The thickness of the connecting ring can vary sig-nificantly along its length in different taxa. Its thickness is usuallygreatest midway between the septa. A connecting ring that isthickest at midlength may have a concave inner wall and/or aconvex outer wall. The shape of the siphuncular tube reflects theshape of the siphuncle that occupied and produced the siphunculartube. Indeed, the shape of the siphuncle reflects the pressure re-gimes of the siphuncle and chamber at the time of production ofthe siphuncular tube. The contact between the siphuncle and con-necting ring reflects this condition. Therefore the inner surface ofthe connecting ring is considered to be the main character thatdetermined the shape of the siphuncular tube. The shape of thetube is described as ‘‘concave’’ when the minimum cross sectiondiameter of the siphuncle is between two adjacent septa. It is‘‘tubular’’ when the cross section diameter of the siphuncular seg-ment is nearly constant. It is ‘‘convex’’ when the siphuncular tubeis expanded within the chambers.

Connecting ring shape.⎯The connecting ring is described as‘‘straight’’ when the outer and inner surfaces of the ring are par-allel along the entire length of the segment; the ring is ‘‘annular’’when the thickness of the connecting ring is greatest near mid-length of the segment. The connecting ring can be ‘‘wedgeshaped’’ with its greatest thickness at its adapical or adoral end.

MATERIAL

The material comprises 41 specimens collected by MSB duringnumerous field trips in the San Juan Precordillera in the last fewdecades. The material is reposited in the paleontological collec-tion of the CRICYT-IANIGLA, Mendoza, Argentina (repositorynumbers, PI-IANIGLA: No), and the INGEO, Facultad de Cien-cias Exactas, Fısicas y Naturales de la Universidad Nacional deSan Juan (INGEO-PI: No).

SYSTEMATIC PALEONTOLOGY

Order ORTHOCERIDA Kuhn, 1940Family BALTOCERATIDAE Kobayashi, 1935, emend.

Emended diagnosis.⎯Orthocones with nearly straight suturesand septal spacing with orthoceridan aspect. Siphuncle marginalor located between center and margin in some advanced forms.Siphuncular tube generally wide, in some forms narrow; tubularor slightly expanded between septa; no diaphragms present. Con-necting ring thin compared with ellesmeroceridans, and Sactorth-oceratidae. Septal necks suborthochoanitic or orthochoanitic.Apex spheroidal with constriction. Endosiphuncular deposits con-sist of tubular calcareous rods typically depressed circular in sec-tion. Cameral deposits typically mural, episeptal, restricted toadapical half of phragmocone.

Comparison.⎯The Orthoceratidae also have a thin connectingring, but differ from the Baltoceratidae in having a central ornearly central siphuncle, in having strongly surpressed endosi-phuncular deposits, and in a generally narrower siphuncular di-ameter. The Protocycloceratidae differ from the Baltoceratidae inhaving endosiphuncular diaphragms and concave siphuncular seg-ments. The Sactorthoceratidae differ from the Baltoceratidae inhaving thicker connecting rings, and a narrower septal spacing.

Genera included.⎯Bactroceras Holm, 1899; CochliocerasEichwald, 1860; Cyrtobaltoceras Flower, 1964; Eosomichelino-ceras, J.-Y. Chen, 1974; Hedstreomoceras Foerste, 1930; Meta-baltoceras Flower, 1964; Microbaltoceras Flower, 1964; ?Ogy-goceras Ulrich et al., 1944; Rangeroceras Hook and Flower,1977; Rhabdiferoceras Flower, 1964; Sinocochlioceras Qi, 1980;Veneficoceras Hook and Flower, 1977.

Occurrence.⎯Upper Lower–Middle Ordovician; North China, Korea, Si-beria, Norway, Laurentian USA, Argentina.


Discussion.⎯The diagnosis of the family largely follows Frey(1995). Herein, we include an apex type that is known in Hed-stroemoceras (Balashov, 1957) and Bactroceras (Dzik, 1982) anda connecting ring structure that is known in Cochlioceras (Mutvei,2002) and Hedstroemoceras (Kroger and Mutvei, 2005). Thesespecific characters are similar to those in known orthoceridans.The Baltoceratidae share with the Orthoceratidae a spherical apexthat lacks a cicatrix, and a thin connecting ring that consists ofan inner calcified-perforate and outer spherulitic-prismatic layer.Consequently, the Baltoceratidae must be assigned to the Ortho-cerida. In addition, we restrict the Baltoceratidae to forms with asiphuncle that is tubular or slightly expanded within the chambers.This is a consequence of the emendation of the Ellesmeroceridaby Kroger and Mutvei (2005). Kroger and Mutvei (2005) assignednautiloids that display a siphuncle with a concave outline of si-phuncular segments to the Ellesmerocerida. Therefore, the straightconchs of Amsleroceras Hook and Flower, 1977; CyptendocerasUlrich and Foerste, 1936; Pachendoceras Ulrich and Foerste,1936; Rioceras Flower, 1964; and Robsonoceras Ulrich and Foer-ste, 1936 fall within the Ellesmerocerida as originally proposedby Flower (1964). Dzik’s (1984) synonymization of Rioceras withCochlioceras Eichwald, 1860, and the classification of Pachen-doceras and Robsonoceras within the Baltoceratidea overlookedthe important differences of the siphuncular shape of these genera.

The structure and thickness of the connecting ring of Carter-soceras Flower, 1964 and Murrayoceras Foerste, 1926, whichwere assigned to the Baltoceratidae by Flower (1964) and sub-sequent authors, differ considerably from that of other Baltocer-atidae, where they are much thicker and layered (compare Flower,1964, pl. 26, figs. 8–11). Similarly thick, layered connecting ringsare known in the Sactorthoceratidae. Cartersoceras, Murrayocer-as, and Wolungoceras Kobayashi, 1931 share a narrow septalspacing and expanded siphuncular segments with other Sactortho-ceratidae (compare Fig. 3). Therefore, we assign Cartersoceras,Murrayoceras, and Wolungoceras to the Sactorthoceratidae.

When Flower (1964) described the Baltoceratidae, he empha-sized the occurrence of an endosiphuncular rod that he considereda unique feature of the family. However, the rods occur in straightnautiloids with large siphuncles in the upper Lower-Middle Or-dovician that have very different septal necks, connecting rings,and siphuncular tube shapes. Beneath Cochlioceras, the endosi-phuncular rod occurs in Cyptendoceras which has a connectingring typical of Ellesmeroceratida and is present in Murrayoceras,which displays the connecting ring of typical Sactorthoceratidae.Therefore, we consider the endosiphuncular rods as a synapo-morphy of the Baltoceratidae and the Sactorthoceratidae, and afeature that was already developed in the Ellesmeroceratidae.

The discrimination between the Orthoceratidae and Baltocera-tidae is problematical at first glance. Genera like Eosomicheli-noceras, Nilssonoceras Kroger, 2004; and Kinnekulloceras Kro-ger, 2004 are very similar. The latter two, which are assigned tothe Orthoceratidae, differ from the baltoceratid Eosomichelino-ceras only in having annulosiphuncular deposits and a siphunclethat is closer to the center than to the conch margin. It seemssubjective to separate these genera into two families. However,annulosiphuncular deposits are characteristic of many younger or-thoceratid genera (e.g., Geisonoceras Hyatt, 1884), while balto-cerid endosiphuncular deposits seem to be restricted to endosi-phuncular rods. Moreover, the strongly excentric position of thesiphuncle in Eosomichelinoceras is a feature characteristic of theBaltoceratidae, while the siphuncle position of orthocerids is gen-erally close to the center.

Genus COCHLIOCERAS Eichwald, 1860, emend.Type species.⎯Orthoceras avus Eichwald, 1860, from the Or-

thoceratite Limestone of Ropsha near St. Petersburg, Russia.Emended diagnosis.⎯Straight, smooth baltoceratids with large,

marginal, tubular or slightly expanded siphuncle (more than

0.25% of shell diameter). Connecting ring thin compared withProtocycloceras Hyatt (in Zittel and Eastman, 1900) or Sactorth-oceras Kobayashi, 1934. Septal spacing of orthoceridan aspect(i.e., two to three septa in a length comparable to conch diameter).Septal necks orthochoanitic. Endosiphuncular deposits in apicalparts of nearly mature specimens include an irregular lining androd on side of siphuncle facing conch margin. Cameral depositsmural, episeptal.

Comparison.⎯Hedstroemoceras differs from Cochlioceras inhaving clearly expanded siphuncular segments and a narrowersiphuncle. Bactroceras differs from Cochlioceras in having amuch narrower marginal siphuncle. Cartersoceras and Murrayo-ceras differ from Cochlioceras in having thick connecting rings.

Occurrence.⎯Upper Lower–Middle Ordovician; Baltoscandia, Siberia,North China, western Argentina, and Laurentian North America.

Discussion.⎯Cochlioceras, by its original description and asemended by Balashov (1955), only included orthocones with alarge marginal siphuncle and short septal necks. However, severalspecies exist in which the siphuncle is slightly or considerablydisplaced from the conch margin. Although several species havebeen assigned to Cochlioceras in which the siphuncle is not incontact with the shell margin (such as Cochlioceras roemeri Dzik,1984), an explicit emendation of the genus that allows inclusionof forms with a siphuncle slightly displaced from the conch mar-gin has not existed until this report. Thus, we have changed thediagnosis in order to make up this inconsistency.

COCHLIOCERAS AVUS Eichwald, 1860Figure 4.4, 4.5

Cochlioceras avus EICHWALD, 1860, p. 1251, pl. 48, fig. 4a, 4b; BALASHOV,1953, p. 207; 1955, pl. 6, fig. 7; CHANG, 1957, p. 38, 54; BALASHOV AND

ZHURAVLEVA, 1962, pl. 6, fig. 7; SALADZIUS, 1966, p. 36, table 1, pl. 3,fig. 8a, b; DZIK, 1984, p. 17, pl. 1, fig. 1, text-figs. 1, e, h, 2.10; KING,1999, p. 140, text-fig. 2.

Endoceras avus (EICHWALD, 1860). FOORD, 1888, p. 146.Cochlioceras sinense CHANG, 1957, p. 55, pl. 2, fig. 2a, b.Cochlioceras sp. ACENOLAZA AND BERESI, 2002, p. 115, pl. 2, fig. L.

Diagnosis (after Balashov, 1955).⎯Cochlioceras species withconchs with apical angle of approximately 3�, conch slightly com-pressed, ornamented with fine transverse striae (6–8/mm). Suturesstraight. Three to four septa in a length comparable to conchdiameter. Siphuncle tubular or slightly expanded, marginal, withdiameter approximately one-third of conch diameter. Septal necksorthochoanitic.

Comparison.⎯Cochlioceras avus differs from all other speciesof the genus in having a siphuncle that is in contact with theconch.

Description.⎯Single specimen from San Juan Formation is 46 mm long,with maximum cross section diameter of 10 mm; diameter at the adapical endis 7.2 mm. Conch is circular in cross section. Outer shell not preserved.Specimen has 19 chambers, resulting in approximately four chambers percross section diameter. Suture lines straight. Curvature of septa shallow. Septalnecks ortochoanitic. Siphuncle marginal, measures approximately 0.4 of crosssection diameter. Sipuncular segments tubular or very slightly expanded with-in the chambers.

Material examined.⎯One specimen IANIGLA-PI No 920 V6, lower Dar-riwilian, Don Braulio Creek, Villicum Range, Precordillera, Argentina.

Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus zones,Darriwilian, Middle Ordovician; Baltoscandia, China, western Argentina.

Discussion.⎯The specimen has a siphuncle that is very widecompared with that of other Cochlioceras avus conchs. However,the intraspecific variation of the siphuncular diameter in C. avusis not known. The only known species of the genus with a verywide diameter is C. yuhangense Zou, 1987, from Zhejiang Prov-ince, North China, but it differs considerably from C. avus by itsexpanded siphuncular segments. Therefore, we consider the Pre-cordilleran specimen to be a true representitative of C. avus. Wesynonymize C. sinense, with C. avus because the diagnostic char-acters given by Chang (1957) for C. sinense are based on minimaldifferences of siphuncular diameter and a narrower septal spacing


FIGURE 4—Baltoceratidae of the San Juan Formation, Precordillera, Argentina. 1–3, Cochlioceras sp., INGEO-PI 686 Vi11, lower Darriwilian, GustavoCreek, Villicum Range; note the compressed cross section and the unique lobate suture lines; 1, polished section of the siphuncle on prosiphuncular side ofthe conch, 3; 2, antisiphuncular view, 2; 3, lateral view, 2; 4, 5, Cochlioceras avus Eichwald, 1860, PIANIGLA-PI No 920 V6, lower Darriwilian, DonBraulio Creek, Villicum Range; 4, lateral view, 2; 5, polished section of the siphuncle on a plane slightly oblique to the growth axis and cutting the siphuncleobliquely within the prosiphuncular half of the conch, distance of the siphuncle from conch margin reflects plane of section, 2.2; 6–9, Eosomichelinocerasbaldisii n. sp., from Eoplacognathus suecicus Zone, upper member, San Juan Formation; Talacasto, and Cerro Viejo; 6, holotype, PI-IANIGLA No 923 Ta 10,median section, note the asymmetrical shape of the siphuncle, 3; 7, detail of same specimen showing the thin connecting ring and a septal neck that slightlytips toward the siphuncle’s center, 10; 8, specimen PI-IANIGLA No 922 CV4, lateral view, 2; 9, same specimen, antisphuncular view, 2.


in a single specimen. The septal spacing in C. sinense is approx-imately six chambers over a distance comparable to the conchdiameter. Although the intraspecific variation of C. avus is un-known, it seems reasonable to synonymize it with C. sinense.

COCHLIOCERAS sp.Figure 4.1–4.3

Cochlioceras sp. ACENOLAZA AND BERESI, 2002, p. 115, pl. 2, fig. O.

Description.⎯Single specimen is 25 mm long, with maximum cross sec-tion diameter of 16 mm, and 12 mm diameter at adapical end. Conch clearlycompressed in cross section with short axis/long axis ratio of 0.8. Outer shellnot preserved. Fragment has three chambers, resulting in approximately twochambers over a distance comparable to cross section diameter. Sutures withshallow lateral lobes. Curvature of septa strong. Septal necks orthochoanitic.Siphuncle marginal, diameter approximately 0.4 times cross section diameter.Siphuncular segments tubular or very slightly expanded within chambers.

Material examined.⎯One specimen INGEO-PI 686 Vi11, lower Darriwil-ian, Gustavo Creek, Villicum Range, Precordillera, Argentina.

Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus Zone,Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯The specimen differs from conchs of the 10known species of Cochlioceras because of its strongly com-pressed cross section and its slightly lobate sutures. However,based on the single fragmentary specimen, the proposal of a newspecies is not justified.

Genus EOSOMICHELINOCERAS Chen, 1974

Type species.⎯Eosomichelinoceras huananense J.-Y. Chen,1974, from the Middle Ordovician of Southwest China.

Proposed diagnosis.⎯Smooth or transversally lirate slender,orthoconic baltoceratids with narrow, tubular siphuncle. Siphuncleis strongly excentric, with distance of more than 0.2 of conchdiameter from shell wall, and with thickness 0.2% or less of shelldiameter. Connecting ring is thin compared with Protocyclocerasand Sactorthoceras. Septal necks are orthochoanitic. Endosiphun-cular or cameral deposits are not known.

Comparison.⎯Eosomichelinoceras differs from all other bal-toceratids in having a narrow, tubular, excentric siphuncle. Hed-stroemoceras differs from Eosomichelinoceras in having widersiphuncular segments that are more expanded within the cham-bers. Wolungoceras differs from Eosomichelinoceras in having awider, more tubular siphuncle with a thicker connecting ring anda clearly narrower septal spacing. The Baltoscandian orthoceri-dans Nilssonoceras Kroger, 2004, and Kinnekulloceras Kroger,2004, differ in having annular endosiphuncular deposits and amore central siphuncle. Bactroceras differs in having a siphunclethat is close to the conch margin or marginal.

Occurrence.⎯Upper Lower–Middle Ordovician; southwestern China, Ti-bet, North China, and western Argentina.

Discussion.⎯Referring any species to Eosomichelinoceras isproblematical because the genus is defined only very briefly byJ.-Y. Chen (1974, p. 142). Moreover, J.-Y. Chen (1974) referredto ‘‘Chen (1964)’’ for original description of the genus and forproposal of the genotype ‘‘Eosomichelinoceras huananense Chen,1964.’’ However, we have been unable to find a ‘‘Chen (1964)’’report. Thus, the figures given in J.-Y. Chen (1974, pl. 61, figs.1–3) are the earliest illustrations of the type. Consequently, sub-sequent authors refer to the genus as Eosomichelinoceras J.-Y.Chen, 1974 (e.g., T.-E. Chen, 1987). Evans (2005, text-fig. 8)pinpointed in a very detailed description of Bactroceras that allknown specimens referred to Bactroceras are characterized by asiphuncular position very close to the conch margin (distance ofthe siphuncle from conch wall between 5% and 15% of the conchdiameter). Only in very early growth stages the position is morevariable in Bactroceras. The distance of the siphuncle of Eosom-ichelinoceras from the conch margin is clearly above 15% ofconch diameter, which can serve as a distinction between the twogenera. However, there is some possibility that a closer investi-gation of the type species will reveal annulosiphuncular deposits.

If so, Nilssonoceras or Kinnekulloceras must be synonymizedwith Eosomichelinoceras. However, despite these difficulties, agenus such as Eosomichelinoceras is needed in order to classifynautiloids similar to Bactroceras, but with a siphuncle that is con-siderably removed from the conch margin.

EOSOMICHELINOCERAS BALDISII n. sp.Figure 4.6–4.9

Diagnosis.⎯Eosomichelinoceras species with compressedconch with low apical angle (3�–5�). Sutures with shallow laterallobe. Three to four septa occur over a length comparable to crosssection diameter. Siphuncle is eccentric, located approximatelymidway between conch wall and center, with diameter ca. 0.2times conch cross section. Siphuncular segments are slightly ex-panded toward conch wall. Septal necks are orthochoanitic, tiltedslightly toward center of siphuncle. Endosiphuncular and cameraldeposits are not known.

Description.⎯Conch apical angle varies from 5� in fragment PI-IANIGLANo 922 CV4 (maximum cross section diameter 10 mm) to 1.9� in holotype.Holotype conch has largest cross section diameter (15.5 mm). Conch crosssection compressed with short axis/long axis ratio of 0.83 (in holotype). Su-tures with shallow lateral lobes. Septal spacing ranges from three (PI-IANIG-LA No 921 CV2, 922 CV4) to four septa (holotype) over a distance compa-rable to cross section diameter. Septal necks orthochoanitic, slightly tiltedtoward siphuncule center. Siphuncular segments tubular in lateral view, slight-ly expanded in transverse view toward conch. Siphuncule diameter ca. 0.15times conch cross section diameter (in holotype). Siphuncle displaced 7 mmfrom conch wall where holotype conch has 17 mm cross section diameter.Connecting ring thin.

Etymology.⎯In honor of the late Bruno Alberto Juan Baldis, a regionalgeologist at the Universidad Nacional de San Juan, Argentina, and the teacherof MSB.

Type.⎯Specimen PI-IANIGLA No 923 Ta 10 from Eoplacognathus sue-cicus Zone, upper member of the San Juan Formation; Talacasto Creek, Cen-tral Precordillera, Argentina.

Other material examined.⎯Two specimens from Cerro Viejo, Huaco area,Eoplacognathus variabilis Zone, San Juan Formation at PI-IANIGLA 921,922.

Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus zones,Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion and comparison.⎯Eosomichelinoceras baldisii n.sp. is unique in its genus because the shapes of the septal necks,which are slightly tilted toward the center of the siphuncle andthe slightly expanded siphuncular segments.

Eosomichelinoceras baldisi n. sp. occupies an intermediatemorphological position between Eosomichelinoceras and Hed-stroemoceras in displaying partly expanded siphuncular segments.However, the narrow siphuncule and the slender orthoconic shelljustify an assignment to Eosomichelinoceras. Eosomichelinocerasbaldisi n. sp. from Precordilleran Argentina is the first record ofthe genus outside of China.

Family SACTORTHOCERATIDAE Flower, 1946, emend.Emended diagnosis.⎯Orthocerids with smooth or annulated,

orthoconic or slightly cyrtoconic conchs with straight sutures.Relatively close septal spacing of ellesmeroceridan aspect (five ormore chambers over a distance comparable to conch cross sectiondiameter). Siphuncle is central or located between conch wall andcenter, but never marginal. Siphuncle is narrow, tubular, or slight-ly expanded between the septa. Connecting rings are relativelythicker than in other Orthocerida. Septal necks are suborthochoan-itic or orthochoanitic. Endosiphuncular deposits occur in apicalparts of nearly mature specimens, and either form an irregularlining or are composed of endosiphuncular rods. Cameral depositsare mural and episeptal in apical portions the conch.

Comparison.⎯The connecting rings in the Sactorthoceratidaeare thinner than those in the Ellesmeroceratidae, but thicker thanthose of the Orthoceratidae. The Clinoceratidae differ in havinga siphuncle that is never central and in displaying a peculiar lon-giconic to cyrtoconic fusiform shell shape. The Dawsonoceratidaediffer by the regular presence of endosiphuncular deposits, thinner


connecting rings, and a wider septal spacing. In addition, theDawsonoceratidae display much less variation in the shape of thesiphuncular tube. Dawsonoceratid siphuncular tubes are alwaysslightly expanded, but show a definite constriction at the transitioninto the septal neck. The Baltoceratidae differ from the Sactorth-oceratidae in having thinner connecting rings and generally amore tubular siphuncle. The Apocrinoceratidae differ in havingstrongly curved conchs and cyrtochoanitic septal necks. The Po-lymeridae and Rangeroceratidae differ from the Sactorthocerati-dae in having characteristic endosiphuncular and cameral deposits(see Evans 2005).

Genera included.⎯Braulioceras n. gen.; Cartersoceras Flower,1964; Centroonoceras Kobayashi, 1934; Glenisteroceras Flower(in Flower and Teichert, 1957); Leptoplatophrenoceras Zou andChen (in J.-Y. Chen and Zou, 1984); Murrayoceras Foerste, 1926;Sactorthoceras Kobayashi, 1934; Scipioceras Zhuravleva, 1964;Sigmocycloceras Kobayashi, 1934; Wennanoceras J.-Y. Chen,1976; ?Wolungoceras Kobayashi, 1931.

Occurrence.⎯Upper Lower–Middle Ordovician; North China, Korea, Si-beria, Norway, Laurentian North America (New York), western Argentina.

Discussion.⎯Flower’s (1946) original definition clearly em-phasized suborthochoanitic septal necks as a definitive characterof the family. However, Flower (1962, p. 30) later emended theSactorthoceratidae so that it only included forms that are verysimilar to the genotype of Sactorthoceras, S. gonioseptum Ko-bayashi, 1927.

Unfortunately, the genotype of Sactorthoceras is very peculiarbecause its septal necks are geniculate and form a distinct wedge.With this emendation, the family was essentially restricted to adegraded monotypic taxon with limited utility. The original def-inition of Sactorthoceras (see Kobayashi, 1934) allowed muchwider morphologic variation than that of Flower’s (1962) emend-ed family. Kobayashi (1934) included three morphological groupsin the genus: 1) a group characterized by S. shimamurai Kobay-ashi, 1934, with a tubular siphuncle; 2) a group with S. tenuicur-vatum Kobayashi, 1934, with a slightly expanded siphuncule; and3) a group characterized by S. gonioseptum, with a constrictedsiphuncular tube. Flower’s (1962) emendation restricted the entirefamily to Sactorthoceras forms comparable to S. gonioseptum. Werecommend that Sactorthoceras should be restricted to suborth-ochoanitic forms of the S. tenuicurvatum and S. gonioseptumgroups, and that all of Kobayashi’s (1934) Sactorthoceras speciesrepresent a family-level taxon that includes all three groups. Thus,our emendation of the Sactorthoceratidae invokes Flower’s (1946)original diagnosis of the group.

Since the erection of the family, several similar Middle Ordo-vician orthocones have been proposed, mainly from sections inNorth China. Some of these forms are similar to Protocycloceras,but show a tubular or slightly expanded siphuncle (e.g., Wennan-oceras, P. gangshanense Zou, 1988). Other forms are similar toMichelinoceras Foerste, 1932, but show a much narrower septalspacing and thicker connecting rings (e.g., Sactorthoceras shi-mamurai). The problematical Ellesmeroceras tchungense Balash-ov, 1962, from the upper Lower Ordovician of the Siberian Plat-form, has a siphuncle that is located very close to the shell walland displays orthochoanitic septal necks and a tubular siphunclesimilar to that in some Baltoceratidae. However, the very narrowseptal spacing clearly distinguishes E. tchungense from the Bal-toceratidae, and the species probably represents a new genus ofthe Sactorthoceratidae. The Whiterockian genus Murrayocerashas a connecting ring that is typical of the Sactortoceratidae(Flower, 1964, pl. 28, figs. 1–6) and has siphuncular segmentsthat are concave in early growth stages and convex in more ma-ture growth stages. Murrayoceras is morphologically intermediatebetween ellesmeroceridans and typical sactorthoceratids (e.g.,Sactorthoceras).

Flower (1964, p. 135) regarded the Apocrinoceratidae as a fam-ily represented by slender cyrto- and orthoconic conchs that, un-like the Protocycloceratidae, have thick connecting rings that areexpanded within the chambers. This definition overlaps with theSactorthoceratidae. In fact, Apocrinoceras Teichert and Glenister,1954 is a cyrtocone with strongly recumbent septal necks, and ismore similar to discosoridans than to orthoceridans. If the Apo-crinoceratidae has any utility, then it should be restricted to formssimilar to Apocrinoceras. Thus, the problematical genus Glenis-teroceras should be referred to the Sactorthoceratidae.

Available evidence indicates the Sactorthoceratidae are limitedto a relatively short time interval within the latest Early Ordovi-cian to Middle Ordovician. The aim of the emendation of thefamily has been to include a number of forms in this time intervalinto a higher taxon because they appear to be closely related.

The Polymeridae and the Rangeroceratidae are similar to theSactorthoceratidae in several aspects, such as the narrow septalspacing, the comparatively thick, slightly expanded connectingrings and the expanded siphuncular segments. Therefore, a closephylogenetic relationship with these earliest orthoceratid familiescan be assumed.

Genus BRAULIOCERAS new genus

Type species.⎯Braulioceras sanjuanense n. sp. from the Mid-dle Ordovician of the San Juan Formation, Argentine Precordil-lera.

Diagnosis.⎯Smooth orthoconic orthocerids with very closeseptal spacing (ca. eight septa over a length comparable to conchcross section diameter). Septal necks orthochoanitic. Siphunclecentral, slightly expanded between septa and has a beaded ap-pearance. Siphuncular tube ca. 0.1 times conch cross section di-ameter. Thickness of connecting ring similar to that in ellesmer-oceridans. Endosiphuncular or cameral deposits unknown.

Comparison.⎯Sactorthoceras differs from Braulioceras inhaving suborthochoanitic septal necks and a siphuncular tube thatis constricted at the septal foramina. Murrayoceras differs by hav-ing conchs with a larger apical angle and by showing a changein septal neck shape during ontogeny.

Etymology.⎯Named for the topotype locality on Don Braulio Creek in theVillicum Range, San Juan Province, Argentina.

Occurrence.⎯Eoplacognathus suecicus Zone, Middle Ordovician; westernArgentina (San Juan Formation).

Discussion.⎯Although the siphuncular tube of Brauliocerasexpands within the chambers and is, in general, very similar toKobayashi’s (1934) ‘‘Group of Sactorthoceras tenuicurvatum,’’the new genus is significantly different. Sactorthoceras wongifor-me Kobayashi, 1934 and S. tenuicurvatum clearly have suborth-ochoanitic septal necks, and therefore belong to Sactorthocerassensu stricto. We propose Braulioceras for Sactorthoceratidaewith a smooth shell, a slightly expanded siphuncle, and short or-thochoanitic septal necks.

‘‘Michelinoceras’’ nanjingense Pan, 1986, from the Middle Or-dovician of North China may represent a Braulioceras. However,the shape of the connecting ring of this species is not presentlyknown.

BRAULIOCERAS SANJUANENSE new genus and speciesFigure 5.1–5.3

Polygrammoceras sp. ACENOLAZA AND BERESI, 2002, p. 115, pl. 2, fig. E

Diagnosis.⎯Same as for the genus.Description.⎯Holotype 36 mm long, with maximum cross section diam-

eter of 15.9 mm, minimum diameter of 14.9 mm. Conch very slightly cyr-toconic, cross section circular. Sutures are straight. Septal spacing is narrow(ca. eight chambers occur over length comparable to conch cross section di-ameter). Interseptal distance in holotype is ca. 2 mm. Septal necks are ortho-choanitic. Siphuncule is central, ca. 2 mm in diameter. Segments of siphun-cular tube are slightly expanded within chambers, each segment nests in thepreceding one. Connecting ring thickness is 0.14 mm. Connecting ring con-sists of opaque mass of calcite. Siphuncle and camera are without deposits.


FIGURE 5—Sactorthoceratidae and Proteoceratidae of the San Juan Formation, Precordilleran Argentina. 1–3, Braulioceras sanjuanense n. gen. and sp.,holotype, PI-IANIGLA No 925, Eoplacognathus suecicus Zone, San Juan Formation, Middle Ordovician; Don Braulio Creek; 1, lateral view, note narrowseptal spacing, the straight sutures, and the slightly bent conch, 1.9; 2, median section, note the thick connecting ring and the expanded siphuncular segments,6.5; 3, median section, note the orthochoanitic septal necks, 12; 4–9, Gangshanonceras villicumense n. sp.; 4, specimen PI-IANIGLA No 928 Vi5, fromEoplacognathus variabilis Zone, Darriwilian, San Juan Formation, Middle Ordovician; Gustavo Creek, Villicum Range, lateral view, 2; 5, same specimen,median section, 2; 6, holotype PI-UNSI No 691 Vi82, Darriwilian, Gustavo Creek, note adult septal crowding, 1.7; 7, Same specimen, detail of thesiphuncular tube with orthochoanitic septal necks, tubular adult siphuncle and thin connecting ring, 2.7; 8, same specimen detail of the adult siphuncle andseptal neck, 14; 9, specimen PI-UNSI No 692 Vi74, from Darriwilian, Gustavo Creek, showing episeptal cameral deposits, 3.3; 10, Gangshanoceras sp.,PI-IANIGLA No 937 Vi6, from Eoplacognathus suecicus Zone, upper member, San Juan Formation; Talacasto, Villicum Range, 1.2.


Etymology.⎯Named for occurrence of the species in the San Juan For-mation of western Argentina.

Type.⎯Holotype PI-IANIGLA No 925 Vi42, the only known specimen.Occurrence.⎯Eoplacognathus suecicus Zone, San Juan Formation, Middle

Ordovician; Don Braulio Creek, Precordilleran Argentina.

Family PROTEOCERATIDAE Flower, 1962, emend.

Emended diagnosis.⎯Orthocerids known from smooth or an-nulated, faintly cyrtoconic longicones with straight sutures. Si-phuncle central or located between conch wall and center (nevermarginal) on convex side of shell. Siphuncule narrow, tubular, orslightly expanded between septa. Septal necks cyrtochoanitic orsuborthochoanitic in juvenile stages and orthochoanitic in adult.Apex is spherical with constriction, and lacks a cicatrix. Parietalor annular endosiphuncular deposits and cameral deposits known.

Comparison.⎯The Proteoceratidae differ from the Orthocera-tidae in having suborthochoanitic septal necks in juvenile stagesand orthochoanitic necks in mature stages. The connecting ringmorphology is identical in both of these families.

Genera included.⎯Archigeisonoceras T.-E. Chen, 1984; Bay-konuroceras Barskov, 1972; Ephippiorthoceras Foerste, 1925;Euorthoceras Foerste, 1893; Gangshanoceras Zou, 1988; Gor-byoceras Shimizu and Obata, 1935; Isorthoceras Flower, 1962;Liulinoceras Zou and Shen (in J.-Y. Chen and Zou, 1984); Mes-naquaceras Flower, 1955; Metephippiorthoceras Zhuravleva,1957; Monomuchites Wilson, 1961; Orthonybyoceras Shimizuand Obata, 1936; Paraproteoceras Chen (in J.-Y. Chen et al.,1981); Proteoceras Flower, 1955; Pseudeskimoceras Shimizu andObata, 1936; Pseudoliolinoceras Zou and Shen (in J.-Y. Chen andZou, 1984); Stereospyroceras Flower, 1955; Tofangoceras Ko-bayashi, 1927; Tofangocerina Kobayashi, 1936; TreptocerasFlower, 1942; Ulmioceras Zhuravleva, 1990.

Occurrence.⎯Upper Lower Ordovician–Upper Silurian; cosmopolitan.Discussion.⎯The emended diagnosis of the Proteoceratidae

largely follows Flower (1962). However, the diagnostic featuresof the family are broadened to include taxa with conchs withsuborthochoanitic septal necks. This emendation is justified by aconsideration of the minimal differences between the septal neckshape of such Middle Ordovician genera as Gangshanoceras, withsuborthochoanitic septal necks, and Liulinoceras, with short cyr-tochoanitic septal necks. This emendation requires the assignmentof Archigeisonoceras and Gangshanoceras to the Proteoceratidae.Consequently, Archigeisonoceras is the earliest representative ofthe Proteoceratidae. The apex and connecting ring characters ofArchigeisonoceras are similar to those of other known Orthocer-ida (Kroger, 2006). The occurrence of suborthochoanitic juvenileseptal necks in early representatives of the family means that areference of the Proteoceratidae to the Orthocerida is justified.Sweet (1964) and subsequent authors tentatively classified theProteoceratidae in the Pseudorthoceratinae because of their ju-venile cyrtochoanitic septal necks. This association of genera withconchs with suborthochoanitc septal necks means that the emend-ed family is a homogenous clade with clear orthoceridan affinities.

Genus GANGSHANOCERAS Zou, 1988

Type species.⎯Gangshanoceras jurongense Zou, 1988, fromthe Dawan Formation in Jiangsu, north China.

Other species.⎯Gangshanoceras densum Zou, 1988; G. gui-chinense Ying, 1989; G. villicumense n. sp.; G. wannanense Ying,1989.

Diagnosis.⎯Proteoceratids with smooth, slender cyrtoconicconchs with circular cross section. Expansion rate of conch pro-gressively decreases from juvenile to adult growth stages. Si-phuncle is excentric and on convex side of growth axis in juvenileconchs, but more central in mature growth stages. Siphuncularsegments are slightly expanded in chambers. Septal spacing isclose (more than five septa occur in length comparable to conch

cross section diameter). Septal necks are suborthochoanitic in ear-ly growth stages, orthochoanitic in mature growth stages. Parietalendosiphuncular deposits are developed (diagnosis after Zou,1988).

Comparison.⎯Archigeisonoceras differs from Gangshanocer-as in having annular endosiphuncular deposits, and in lacking ashift in siphuncle position during ontogeny. Liulinoceras andPseudoliulinoceras differ from Gangshanoceras in having a cen-tral siphuncle and cyrtochoanitic septal necks in early growthstages.

Occurrence.⎯Upper Lower–Middle Ordovician; North China and Precor-dilleran Argentina. The recovery of Gangshanoceras in the Precordillera isits first record outside of China.

GANGSHANOCERAS VILLICUMENSE n. sp.Figure 5.4–5.9

Diagnosis.⎯Gangshanoceras species with conchs with circularcross section, ornamented with fine transverse lirae; expansionrate is ca. 6�. Interseptal distance small (ca. five to six septa occurover a length comparable to conch cross section diameter). Septalcurvature is broad. Siphuncle is excentric in juvenile stages, cen-tral in mature stages. Siphuncle diameter 0.15 times conch crosssection diameter. Siphuncle slightly expanded within chambers injuvenile growth stages, tubular in mature stages. Septal necks sub-orthochoanitic in juvenile growth stages, orthochoanitic in adultstages. Endosiphuncular deposits unknown. Cameral deposits epi-septally developed.

Description.⎯Conch faintly ornamented with transverse striae, approxi-mately 20–25 per 10 mm (specimen PI-IANIGLA No 928 Vi5). Apical angleof conch highly variable, maximum 9� and minimum 2� (mean 6�, n 13).Close but highly variable septal spacing (mean septal distance 0.18 times crosssection diameter, maximum 0.32 times, minimum 0.08 times, n 20). Siphunclediameter varies between 0.23 times and 0.12 times cross section diameter(mean 0.15, n 20). Siphuncle expanded within chambers in juvenile growthstages, tubular in mature stages. Septal necks suborthochoanitic if cross sec-tion diameter is less than 13–14 mm, orthochoanitic in larger fragments. Adultseptal crowding in specimen holotype at 16 mm cross section diameter, atspecimen PI-IANIGLA No 930 Vi100 at 26 mm, at specimen INGEO-PI No

691 Vi82 at 21 mm.Etymology.⎯Named for the Sierra de Villicum in western Argentina.Type.⎯Holotype INGEO-PI No 691 Vi82, Eoplacognathus variabilis Zone,

Darriwilian, San Juan Formation, Middle Ordovician; Gustavo Creek, Villi-cum Range, Argentina.

Other material examined.⎯Twenty specimens from the San Juan Forma-tion of Precordilleran Argentina in the PI-IANIGLA collection.

Occurrence.⎯Eoplacognathus variabilis Zone, Darriwilian, San Juan For-mation, Ordovician; Don Braulio Creek and Gustavo Creek in the VillicumRange, Precordilleran Argentine.

Discussion and comparison.⎯The species is the most commonorthocerid in the San Juan Formation. The variation in septalspacing, siphuncular diameter, and adult size of the available spec-imens is relatively high. However, the material does not allowseparation of the specimens into two or more species. The ex-treme values may represent separate species or simply a relativelyvariable species. Future collecting may resolve this problem.

Gangshanoceras villicumense n. sp. is unique within the genusin having a nearly tubular siphuncle with low excentricity. Theexcentricity of its siphuncle is less than in all Chinese species ofGangshanoceras.

GANGSHANOCERAS sp.Figure 5.10

Description.⎯Specimen 97 mm long, maximum cross section diameter of18 mm, diameter at adapical end ca. 14 mm. Conch circular in cross section,outer shell not preserved. Three to four chambers present in a length com-parable to conch cross section diameter. Sutures straight. Broad curvature ofsepta, septal necks orthochoanitic throughout specimen. Siphuncle tubular,central, diameter ca. 0.12 times conch cross section diameter.

Material examined.⎯Specimen PI-IANIGLA 937 Ta6, Eoplacognathusvariabilis–Eoplacognathus suecicus zones, Darriwilian, Talacasto, Precordil-leran Argentina.


Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus Zones,Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯This specimen represents an undescribed speciesof Gangshanoceras. It differs from previously described speciesof Gangshanoceras in its combination of a central siphuncle withlow expansion rate and wide septal spacing. However, the erectionof a new species is not possible because a differential diagnosiswould require more material.

Family ORTHOCERATIDAE McCoy, 1844Diagnosis.⎯Orthocerids with orthoconic conchs with slender,

central or subcentral, tubular or moderately expanded siphuncle.Septal necks suborthochoanitic or orthochoanitic, respectively.Shell smooth or annulated, with more-or-less well developed lon-gitudinal and transverse ribs, ridges, or combination of these fea-tures. Bowl-shaped apical shell small, straight, without cicatrix,without initial constriction, smooth or ornamented with transver-sal and longitudinal elements, but without annulations. Endosi-phuncular and cameral deposits occur only in most apical cham-bers of quasi-mature specimens (diagnosis after Kroger andIsakar, 2006).

PALORTHOCERAS new genusType species.⎯Palorthoceras kayseri n. sp. from the upper

Lower Ordovician of the San Juan Formation, Precordilleran Ar-gentina.

Included species.⎯Palorthoceras kayseri n. gen. and sp.,Michelinoceras buttsi Flower, 1962.

Diagnosis.⎯Orthoceratids with smooth very slender orthocon-ic conchs with relatively widely spaced septa (ca. three septaalong a length comparable to cross section diameter). Siphuncleis central, slightly expanded between septa; septal necks are sub-orthochoanitic. Endosiphuncular deposits occur as annuli at septalperforations or as parietal endosiphuncular deposits; episeptal andhyposeptal deposits known. Cameral deposits are episeptal. En-dosiphuncular deposits occur as irregular lining, annuli, or parietaldeposits.

Etymology.⎯Palorthoceras, an old, ancient (Latin palæo) form comparedwith Orthoceras.

Occurrence.⎯Upper Lower–lower Middle Ordovician; western Argentina(San Juan Formation) and western United States (Florida Mountains Forma-tion, Wahwah Formation).

Discussion and comparison.⎯The genus is known from spec-imens that represent the oldest known orthoceratids, and occur inthe Florida Mountains Formation of New Mexico and West Texas,and the coeval Wahwah Limestone in the Ibex area, western Utah(Oepikodus evae–Paroistodus originalis Zone-equivalents), andin the lower San Juan Formation, Precordilleran Argentina (Oep-ikodus evae Zone).

There seems to be a quite large variation in the developmentof the endosiphuncular deposits within the genus; more materialmay enhance the understanding of the intra- and interspecific var-iation of this character.

Orthoceras Bruguiere, 1789 differs from Palorthoceras in hav-ing a characteristic fine longitudinal and transverse ornamentationon the conch, a wider septal spacing, and orthochoanitic septalnecks. Michelinoceras differs in having a wider septal spacingand a narrower, strictly tubular siphuncle. Finally, Pleurorthocer-as Flower, 1962 differs from Palorthoceras in having an excen-tric, tubular siphuncle that is narrower and constricted at the septalforamina.

PALORTHOCERAS KAYSERI new genus and speciesFigure 6.1, 6.2, 6.5

Orthoceras sp. KAYSER, 1876, p. 14, pl. 5, fig. 5; CECIONI, 1953, p. 1; ACENO-LAZA AND BERESI, 2002, p. 8.

Michelinoceras sp. HOOK AND FLOWER, 1977, p. 44, 45, pl. 4, figs. 4–8; pl.11, figs. 6–8, pl; 17, figs. 4–7; pl. 19, figs. 4–6.

non Michelinoceras sp. HOOK AND FLOWER, 1977, p. 45, pl. 17, figs. 1–3,8–13.

Diagnosis.⎯Species of Palorthoceras n. gen. with conch witha very low apical angle (ca. 1�), ca. four chambers occur over alength comparable to conch cross section diameter, siphuncle di-ameter is 0.17 times cross section diameter of conch. Siphuncleis slightly expanded within chambers. Endosiphuncular and cam-eral deposits are unknown.

Description.⎯Holotype lacks outer shell. Holotype 43 mm long, maximumcross section diameter 12 mm, minimum diameter 11 mm, apical angle ca.1�. Shell with circular cross section. Sutures straight with 3 mm separation,interseptal distance of 0.25 times conch cross section diameter. Siphunclecentral, 2 mm diameter, very slightly expanded within chambers. Connectingring thin. Septal necks suborthochoanitic.

Etymology.⎯Species named in honor of Emanuel Kayser, the first pale-ontologist to describe fossils from Precordilleran Argentina (see Kayser 1876).

Type.⎯Holotype PI-IANIGLA No 940 Ta 5, Oepikodus evae Zone, SanJuan Formation, upper Lower Ordovician; Quebrada de Talacasto section,central Precordillera, San Juan Province.

Other material examined.⎯The species is only known from the holotype.Occurrence.⎯Oepikodus evae Zone, upper Lower Ordovician; San Juan

Formation; Precordilleran Argentina and western United States (FloridaMountains Formation).

Discussion and comparison.⎯Palorthoceras kayseri is one ofthe oldest orthocerids. It is approximately coeval with the earlyorthocerid Michelinoceras primum Flower, 1962, from Oepikodusevae Zone-equivalent strata in the Fillmore Formation of the Ibexarea, western Utah. Some of the specimens that have been de-scribed as Michelinoceras sp. from the Florida Mountains For-mation of New Mexico (Hook and Flower, 1977) are assigned tothe species as they are very similar to the fragment from thePrecordillera. Palorthoceras buttsi (Flower, 1962) differs from P.kayseri in having annular endosiphuncular deposits, a wider in-terseptal distance, and a wider siphuncle.

Genus ORTHOCERAS Bruguiere, 1789Type species.⎯Orthoceratites regularis Schlotheim, 1820, Or-

thoceratite Limestone from Tallinn, Estonia.Diagnosis.⎯‘‘Straight, orthoconic shells with longitudinal im-

pressions of the living chamber. Exterior sculptured with trans-verse lines of growth forming a banding somewhat similar to thatin Geisonoceras, but the bands are composed of densely crowdedminute longitudinal ribs, which are especially well shown on aslightly weathered surface. Apertural angle is small. Air chambersand siphuncle are of medium size; siphuncle is central or subcen-tral.’’ Siphuncle is tubular, septal necks are orthochoanitic (diag-nosis from Troedsson, 1931, p. 12).

Comparison.⎯Palorthoceras conchs differ from those of Or-thoceras in having slightly expanded siphuncular segments andsuborthochoanitic septal necks. Michelinoceras conchs have awider septal spacing and a smooth exterior surface. Finally, Pleu-rorthoceras differs from Orthoceras in having an excentric si-phuncle with characteristic constrictions at septal foramina and inhaving suborthochoanitic septal necks.

Occurrence.⎯Eoplacognathus suecicus–Pygodus anserinus zones, MiddleOrdovician; Baltoscandia, China?, Argentina?

Discussion.⎯The paleogeographic distribution of Orthocerasremains undetermined because the genus is frequently used as‘‘wastebasket’’ for poorly known, simple, straight orthocones witha central siphuncle and orthochoanitic septal necks. Moreover, thedesignation Michelinoceras is used interchangeably with Ortho-ceras for these nautiloids. Chinese Middle Ordovician forms suchas M. chaoi Chang, 1957, M. beianense Ying, 1989, M. parae-longatum Chang, 1962, and M. sinoceraforme Lai, 1960 may rep-resent species of Orthoceras, but no description of the externalconch ornamentation has been provided for these species. Ortho-ceras is not known from North America.

ORTHOCERAS? sp.Figure 5.3, 5.4, 5.6

Description.⎯Specimen PI-IANIGLA No 941 Vi70 35 mm long, maxi-mum cross section diameter 11.5 mm, diameter at adapical end ca. 8 mm.


FIGURE 6—Orthoceratidae of the San Juan Formation, Precordilleran Argentina. 1, 2, 5, Palorthoceras kayseri n. gen. and sp., holotype PI-IANIGLA No

940 Ta 5, from the Oepikodus evae Zone, San Juan Formation, upper Lower Ordovician; Quebrada de Talacasto section, central Precordillera; 1, Mediansection showing details of the thin connecting ring and suborthocoanitic septal necks, 11; 2, median section showing details of the septal neck, 20; 5,same specimen as in 1 and 2, median section of the entire fragment, 2.7. 3, 4, 6, Orthoceras? sp., PI-IANIGLA No 942 Vi70, from Darriwilian, GustavoCreek, Villicum Range, medial section showing detail of the septal neck and connecting ring, 4; 4, same specimen, medial section of entire fragment, 4.7;6, PI-IANIGLA No 942 Vi73, from Darriwilian, Gustavo Creek, Villicum Range, median section of the entire fragment, 2.5.

Conch is circular in cross section; outer shell is not preserved. Fragmentdisplays ten chambers, with ca. three chambers over a length comparable tocross section diameter. Suture lines are straight. Septal curvature is broad;septal necks are orthochoanitic; siphuncle is tubular, central, with diameterca. 0.18 times conch cross section diameter.

Specimen PI-IANIGLA No 942 Vi73 ca. 25 mm long, maximum crosssection diameter 15 mm, diameter at adapical end ca. 11.5 mm. Conch iscircular in cross section with outer shell not preserved. Fragment displaysfour chambers, with ca. 2.5 chambers over a length comparable to conch crosssection diameter. Suture lines are straight. Septal curvature is broad; septalnecks are orthochoanitic; siphuncle is tubular, central with diameter ca. 0.15times conch cross section diameter.

Material examined.⎯Two specimens PI-IANIGLA No 941 Vi70, 942 Vi73,Darriwilian, Gustavo Creek, Villicum Range, Precordilleran Argentina.

Occurrence.⎯Eoplacognathus variabilis–Eoplacognathus suecicus zones,Darriwilian, Middle Ordovician; western Argentina (San Juan Formation).

Discussion.⎯The tubular, central siphuncle, orthochoaniticseptal necks, chamber spacing, lack of endosiphuncular deposits,and the general conch shape of the specimens all allow an as-signment to Orthoceras. However, a confident generic assignmentof the specimens is impossible because the external conch orna-mentation, the adult body chamber, and the apical characters are

not preserved. For these reasons, only a tentative assignment toOrthoceras? is proposed.

Order LITUITIDA Starobogatov, 1974

Diagnosis.⎯Cephalopods with longicone orthoconic conchswith cyrtoconic or coiled apical part. Siphuncle is on convex sideof shell in earliest ontogenetic stages, on concave side in laterstages. Siphuncle is narrow; septal necks are long, orthochoaniticor hemichoanitic; connecting ring is often disaggregated and cov-ered with cameral deposits that extend to cover the septal neckand inner parts of the connecting ring. Endosiphuncular and en-docameral deposits are adnate. Cameral deposits often show lon-gitudinal lamellae or sheets that longitudinally divide chambers.Apex is subspherical without cicatrix. Narrow and deep hypon-omical sinus occurs (diagnosis largely after Dzik, 1984).

Discussion.⎯Starobogatov (1974) defined the lituitidans as acephalopod taxon of the same rank as the Orthoceratida. However,Dzik (1984, p. 131–141) first emphasized the peculiar features ofthis group. The fragile, often disaggregated connecting ring, theadnate deposits with vertical lamellae, the long orthochoanitic


FIGURE 7—Rhynchorthoceras minor n. sp. from the San Juan Formation, Paroistodus originalis–Eoplacognathus variabilis zones, Middle Ordovician,Precordilleran Argentina. 1, INGEO-PI No 693 Vi53, from Gustavo Creek section, Villicum Range, lateral view, 2.6; 2, INGEO-PI No 694 Vi53, view ofconvex side of the conch, note the traces of transverse striation, 3.4; 3, holotype PI-IANIGLA No 943 Vi10, from from Don Braulio section, lateral view,2.6; 4, INGEO-PI No 695 Vi53, from Gustavo Creek section, Villicum Range, median section, 6.4; 5, same specimen, detail of the siphuncle showing thelong septal necks, 13; 6, INGEO-PI No 695 Vi53, from Gustavo Creek section, Villicum Range, cross section, showing the longitudinal lamella on convexside of the conch, 6.4; 7, PI-IANIGLA No 947 Vi107, from Don Braulio section, median section, showing detail of the septal necks without cameral deposits,4.7; 8, same specimen, median section, showing different development of cameral deposits in the chambers, 3; 9, same specimen, median section, showingmore apical chambers with cameral deposits cover septal necks, 7; 10, PI-IANIGLA No 943 Vi10, from Don Braulio section, median section, specimenwith relatively low expansion rate and wide septal spacing, 5.


septal necks, and the frequent occurrence of juvenile coiling dis-tinguish the group from the Orthocerida and justify the ordinallevel of the lituitidans. The Orthocerida and the Lituitida appearin the Early Paleozoic, and are the earliest cephalopods with aspherical apex and lack of a cicatrix on the conch. The two orderscompose the Neocephalopoda (Engeser, 1996). The earliest Li-tuitida are known from the lower Middle Ordovician (Volkhovian)of Baltoscandia (Dzik, 1984) and from coeval Paroistodus ori-ginalis–Eoplacognathus variabilis zones in China (Qi, 1980) and,now, Precordilleran Argentina.

Family SINOCERATIDAE Shimizu and Obata, 1935Diagnosis.⎯Lituitidans without a coiled apical shell; apex cyr-

tochoanitic; siphuncle is subcentral; hyponomic sinus is weaklydeveloped (after Dzik, 1984).

Discussion.⎯Dzik (1984) noted that the distinction betweenlituitidans and orthoceridans may seem to be arbitrary in sometaxa, particularly if the Sinoceratidae are considered. However,the presence of typical cameral and endosiphuncular depositseven in the earliest Sinoceratidae seems to represent a distinctivecriterion to distinguish the two groups. Rhynchorthoceras Remele,1882, an early representative of the Sinoceratidae, shows the char-acteristic lituitidan vertical lamella, and a probable syn vivo re-sorption of the connecting ring is documented (Sweet, 1958, p.121, 122, 139). The Precordilleran lituitidans show these featuresvery clearly (see Fig. 7.6, 7.9).

Genus RHYNCHORTHOCERAS Remele, 1882, emend.Type species.⎯Lituites breynii Boll, 1857, from Orthoceratite

Limestone of erratics, northern Germany.Emended diagnosis.⎯Lutuitidans with orthoconic longicone

conchs with slight curvature at apex. Siphuncle is tubular orslightly expanded within chambers, large (diameter one-sixth ofconch diameter), subcentral and displaced toward convex side ofshell. Septal necks are orthochoanitic, cameral deposits cover sep-tal necks in some specimens. Cameral deposits with single verticallamella on concave side of shell.

Comparison.⎯Ancistroceras Boll, 1857 differs from Rhyn-chorthoceras in having coiled juvenile growth stages. The shellshape of Sinoceras Shimizu and Obata, 1935 is generally moreelongate and slender in nearly growth stages, and Sinoceras haslonger septal necks.

Occurrence.⎯Middle Ordovician of Baltoscandia, North China, and Pre-cordillera Argentina. The genus is not known from Laurentian North America.

Discussion.⎯In Baltoscandia, Rhynchorthoceras is dominantlyrepresented by species with transversely ornamented conchs. Theoriginal diagnosis of the genus therefore included this. However,smooth species, such as Orthoceras conicum Hisinger, 1837, oc-cur which are similar to the genotype of Rhynchorthoceras in allaspects except for the ornamentation. Therefore, Dzik (1984) re-ferred O. conicum to Rhynchorthoceras. We agree with this sug-gestion, and have modified the generic diagnosis in order to cor-rect the inconsistency.

In the Chinese literature on Ordovician cephalopods, Rhyn-chorthoceras and Ancistroceras are used interchangeably. Thus,the criterion of a coiled apex is not applied to species assignedto Ancistroceras by Qi (1980), J.-Y. Chen and Zou (1984), andXu and Lai (1987). It also seems that conchs with a high expan-sion rate are more likely to be assigned to Ancistroceras in thesereports. Consequently, many of the species assigned to Ancistro-ceras in these reports may actually represent Rhynchorthocerasspecies. The entire group that includes Ancistroceras, Rhyn-chorthoceras, and Sinoceras needs revision in order to provide anunequivocal classification.

RHYNCHORTHOCERAS MINOR n. sp.Figure 7

Clinoceratidae indet. ACENOLAZA AND BERESI, 2002, p. 113, pl. 1, fig. I.Family Orthoceratidae gen. et sp. indet. ACENOLAZA AND BERESI, 2002, p.

115, pl. 2, fig. I.

Diagnosis.⎯Rhynchorthoceras species showing a combinationof narrow septal spacing (ca. 0.19 mm), compressed cross section,and apical angle of 13�. Conch is cyrtocone in early growth stag-es, nearly straight in later growth stages, weakly transversely stri-ated. Septal neck length is 0.4 times chamber height. Siphuncletubular with diameter ca. 0.14 times conch cross section diameter.

Description.⎯Outer shell is weakly transversely ornamented (INGEO-PINo 695 Vi53) with ca. 20 striae/10 mm. Striae with shallow lobe on convexside of conch. Largest specimen with 13 mm cross-cross section diameter (PI-IANIGLA No 943 Vi10). Initial 5 mm of conch is strongly cyrtocone withsiphuncle on convex side, later growth stages are very slightly cyrtoconic inopposite direction with siphuncle on concave side. Conch expansion is ini-tially strong but subsequently decreases. Apical angle 9�–16� (mean 13�, n 7).Conch cross section is compressed with short axis/long axis ratio of 0.8 (IN-GEO-PI No 695 Vi53). Septal spacing is narrow (mean 0.19 times cross sec-tion diameter, maximum 0.38 times, minimum 0.16 times). Siphuncle is tu-bular, diameter 0.14 times of conch cross section diameter. Septal necks areorthochoanitic with length ca. 0.4 times chamber height (INGEO-PI No 695Vi53). Vertical lamella are present in INGEO-PI No 695 Vi53.

Etymology.⎯From Latin minoro decreasing; all known specimens aresmall compared with other Rhynchorthoceras conchs.

Type.⎯Holotype PI-IANIGLA No 943 Vi10 from Paroistodus originalis–Eoplacognathus variabilis zones, Middle Ordovician; Don Braulio section,Villicum Range, San Juan Province.

Other material examined.⎯Eleven additional specimens are in the PI-IA-NIGLA and INGEO-PI collections from the Eoplacognathus variabilis Zone,Darriwilian, Middle Ordovician, Gustavo Creek and Cerro Viejo, Huaco area,Gustavo Creek, Villicum Range eastern Precordillera and Cerro Viejo, Huacoarea, San Juan Province, Argentina.

Occurrence.⎯Paroistodus originalis–Eoplacognathus variabilis Zones,Middle Ordovician; western Argentina (San Juan Formation).

Discussion and comparison.⎯Rhynchorthoceras minor conchsare unique in the genus by their combination of narrow septalspacing, compressed cross section, and relatively low apical an-gle. Rhynchorthoceras minor also differs from many species ofthe genus in having an only slightly ornamented shell. Rhyn-chorthoceras conicum (Hisinger, 1837) is smooth, but differs inhaving a circular cross section and a more slender conch. Rhyn-chorthoceras subcurvatum (Qi, 1980) differs in having a siphun-cle that is expanded within the chambers, and R. densum (Qi,1980) differs in having a narrower siphuncle and a larger apicalangle.

No adult specimen of Rhynchorthoceras minor has been found.However, larger specimens show a reduced expansion rate of theconch, and suggest a relatively small adult size of the specieswith a conch reaching approximately 20 mm in cross section di-ameter. This is smaller than in any of the previously describedspecies of Rhynchorthoceras.

Rhynchorthoceras densum and R. subcurvatum occur in the Da-wan Formation of Anhui, North China, in strata approximatelycoeval with the occurrence of R. minor in the San Juan Formation(i.e., Paroistodus originalis–Eoplacognathus variabilis Zones).These three species represent the earliest known lituitidans.

ACKNOWLEDGMENTS

The sections were prepared by J. Suarez. This work is part of DFG projectKR 2095-2 (BK principal investigator). MSB is indebted to B. Baldis, whoaccompanied her for the first time to the Don Braulio section, where ortho-cerid nautiloids were collected during the Ph.D. field work. She is grateful toS. and N. Peralta, who accompanied her to the Gustavo Creek (VillicumRange) and Cerro Viejo (Huaco area), Argentine Precordillera collecting nau-tiloids. The systematic synthesis benefited from the general help and supportof D. Korn, Museum fur Naturkunde, Berlin. We are grateful for the carefulreviews of H. Mutvei, Stockholm, and an anonymous reviewer. This work isa contribution to IGCP Project 503: Ordovician Paleogeography and Paleo-climate.

REFERENCES

ACENOLAZA, F. G. AND M. S. BERESI. 2002. Ordovician nautiloids of Argen-tina, p. 107–120. In F. G. Acenolaza (ed.), Aspects of the Ordovician Sys-tem in Argentina, INSUGEO, Serie de Correlacion Geologica, 16. Univ-ersitad Nacional Tucuman, Tucuman.


ACENOLAZA, F. G. AND A. TOSELLI. 1999. Argentine Precordillera: Allo-chthonous or autochthonous Gondwanic? Zentralblatt fur Geologie Palaon-tologie, Teil I, 7/8:743–756.

ACENOLAZA, F. G., F. R. DURAND, AND R. D. TADDEI. 1977. Nautiloideosordovıcicos de la Precordillera Argentina. Fauna de Huaco, Provincia deSan Juan. Acta Geologica Lilloana, 13:219–242.

ACENOLAZA, F. G., H. MILLER, AND A. J. TOSELLI. 2002. Proterozoic–EarlyPaleozoic evolution in western South America—a discussion. Tectono-physics, 354:121–137.

ALBANESI, G. L. AND G. ORTEGA. 2002. Advances on conodont-graptolitebiostratigraphy, p. 146–165. In F. G. Acenolaza (ed.), Aspects of the Or-dovician System in Argentina, INSUGEO, Serie de Correlacion Geologica,16. Universitad Nacional Tucuman, Tucuman.

ALBANESI, G. L., G. ORTEGA, C. R. BARNES, AND M. A. HUNICKEN. 1999.Conodont-graptolite biostratigraphy of the Gualcamayo Formation (MiddleOrdovician) in the Gualcamayo–Guandacol rivers area, Argentina Precor-dillera. Acta Univesitatis Carolinae, Geologica, 43(1/2):45–48.

ASTINI, R. A., J. L. BENEDETTO, AND N. E. VACCARI. 1995. The Early Pa-leozoic evolution of the Argentine Precordillera as a Laurentian rifted, drift-ed, and collided terrane; a geodynamic model. Geological Society of Amer-ica Bulletin, 107:253–273.

BALASHOV, Z. G. 1953. Stratigraficheskoe rasprotranenie nautiloidea v or-dovike Pribaltiki. Trudy Vsesoyuznogo Neftyanogo Nauchno-Issledovatel-skogo Geologo-Razvednoknogo Instituta (VNIGRI), 78:197–216. (In Rus-sian)

BALASHOV, Z. G. 1955. Semeistvo Cochlioceratidae nom. nov. Voprosy Pa-leontologii, 2:55–60. (In Russian)

BALASHOV, Z. G. 1957. Protoconch drevnepaleozoiskogo predstavitelia rodaOrthoceras. Doklady Akademii Nauk SSSR, 116(5):855–857. (In Russian)

BALASHOV, Z. G. 1962. Nautiloidei Ordovika Sibirskoi platformy. Isdat’elstvoLeningradskogo. Universiteta, Leningrad/St. Petersburg, 205 p. (In Russian)

BALASHOV, Z. G. AND F. A. ZHURAVLEVA. 1962. Ortriad Orthoceratida. p.82–92. In V. E. Ruzhencev (ed.), Osnovy Paleontologii. Isdat’ielstvo Aka-demii Nauk SSSR, Moskva. (In Russian)

BALDIS, B. AND M. BERESI. 1981. Biofacies de culminacion del ciclo depos-icional calcareo del Arenigiano en el Oeste argentino. II Congreso Lati-noamericano de Paleontologıa I, Porto Alegre, Brazil: 11–17.

BALDIS, B., O. BORDONARO, C. ARMELLA, M. BERESI, N. CABALERI, S. H.PERALTA, AND H. BASTIAS. 1989. La cuenca Paleozoica inferior de la Pre-cordillera Argentina. Correlacion Geologica, 6:101–121.

BARNES, C. R. 2004. Ordovician oceans and climate, p. 72–76. In B. D.Webby, F. Paris, M. L. Droser, and I. G. Percival (eds.), The Great Ordo-vician Biodiversification Event. Columbia University Press, New York.

BARSKOV, I. S. 1972. Pozdneordovikskie i Silurijskie Golovonogie MollyuskiKazachstana i Srednei Azii. Isdat’ielstvo Nauka, Moscow, 109 p. (In Rus-sian)

BENEDETTO, J. L. 1998. Early Palaeozoic brachiopods and associated shellyfaunas from western Gondwana: Their bearing on the geodynamic historyof the pre-Andean margin. Geological Society of London Special Publi-cation, 179:87–102.

BENEDETTO, J. L., T. M. SANCHEZ, M. G. CARRERA, E. D. BRUSSA, AND M.J. SALAS. 1999. Paleontological constraints on successive paleogeographicpositions of Precordillera terrane during the early Paleozoic, p. 21–42. InV. A. Ramos and J. D. Keppie (eds.), Laurentia–Gondwana Connectionsbefore Pangea, Geological Society of America, Special Paper 336.

BERESI, M. S. 1981. Fauna y ambiente en los sedimentos carbonaticos aren-igianos de Talacasto, San Juan, Argentina. VIII Congreso Geologico Ar-gentino, V. 2:399–417.

BERESI, M. S. 1986. Paleoecologıa y biofacies de la Formacion San Juan alsur del paralelo de 30� S, Precordillera de San Juan. Unpublished disser-tation, Ciencias Exactas, Fısicas y Naturales, Universidad Nacional de SanJuan, 400 p.

BERESI, M. S. 1992. Ordovician events in the Precordillera Argentina, p. 337–344. In B. D. Webby, and J. R. Laurie (eds.), Global Perspectives on Or-dovician Geology. Balkema, Rotterdam.

BERESI, M. S. AND J. K. RIGBY. 1993. The Lower Ordovician sponges of SanJuan, Argentina. Brigham Young University, Geology Studies, 39:1–64.

BOLL, E. 1857. Beitrag zur Kenntnis der silurischen Cephalopoden im nord-deutschen Diluvium und den anstehenden Lagern Schwedens. Archiv desVereins der Freunde der Naturgeschichte in Mecklenburg, 11:58–95.

BORRELLO, A.V. AND P. G. GARECA. 1951. Sobre la presencia de Nemagrap-tus gracilis (Hall) en el Ordovıcico del norte de San Juan. AsociacionGeologica Argentina, 6(3):187–993.

BRUGUIERE, J. G. 1789. Histoire Naturelle des vers. Vol. 1. Pt. 1. Encyclo-pedie methodique, 6, Panckoucke, Paris, 757 p.

CANAS, E. L. 1995. Early Ordovician carbonate platform facies of the Ar-gentine Precordillera: Restricted shelf to open platform evolution, p. 221–224. In J. D. Cooper, M. Droser, and S. Finney (eds.), Short Papers for theSeventh International Symposium of the Ordovician System, Las Vegas,SEPM, Book 77.

CANAS, E. L. AND M. G. CARRERA. 1993. Early Ordovician microbial sponge-receptaculitid bioherms of the Precordillera, Western Argentina. Facies, 29:169–178.

CARRERA, M. 2001. Analisis de la distribucion y composicion de las biofaciesde la Formacion San Juan (Ordovıcico temprano), Precordillera Argentina.Ameghiniana, 38:169–184.

CECIONI, G. 1953. Contribucion al conocimiento de los Nautiloideos-Eopa-leozoicos Argentinos. Parte I: Protocycloceratidae-Cyclostomiceratidae.Boletin del Museo Nacional de Historia Natural, 26(2):56–110.

CECIONI, G. 1965. Contribucion al conocimiento de los Nautiloideos-Eopa-leozoicos Argentinos. Parte II: Robsonoceratidae, Ellesmeroceratidae, Pro-terocameroceratidae, Baltoceratidae. Boletin del Museo Nacional de His-toria Natural, 29(1965–1971):1–24.

CHANG, ZHI-DUN. 1957. Fossil nautiloid Cephalopoda in the Yangtzeela poloiZone of the Middle Ordovician of Chanyang, Hupei. Acta PalaeontologicaSinica, 5:33–61. (In Chinese)

CHANG, ZHI-DUN. 1962. Some species of nautiloids from the Middle Ordo-vician of the Kuansyan region. Acta Palaeontologica Sinica, 10:514–523.(In Chinese)

CHEN, JUN-YUAN. 1974. Ordovician Nautiloidea, p. 138–143. In A Handbookof the Stratigraphy and Paleontology in Southwest China, Science Press,Beijing. (In Chinese)

CHEN, JUN-YUAN. 1976. Advances in the Ordovician stratigraphy of NorthChina with a brief description of nautiloid fossils. Acta PalaeontologicaSinica, 15:55–74. (In Chinese)

CHEN, JUN-YUAN, GENG-WU LIU, AND TING-EN CHEN. 1981. Silurian nauti-loid faunas of central and southwestern China. Memoirs of the NanjingInstitute of Geology and Palaeontology, 13:1–104. (In Chinese)

CHEN, JUN-YUAN AND ZOU XI-PENG. 1984. Ordovician cephalopods from theOrdos area, China. Memoirs of the Nanjing Institute, Geology Palaeontol-ogy, 20:33–111. (In Chinese)

CHEN, TING-EN. 1984. The Ordovician cephalopod fauna and the subdivisionof Ordovician from southern Xizang (Tibet). Acta Palaeontologica Pinica,23:452–468. (In Chinese with English abstract)

CHEN, TING-EN. 1987. Ordovician nautiloids from Xainza, northern Xizang.Bulletin of Nanjing Institute of Geology and Palaeontology, Academia Sin-ica, 11:133–191. (In Chinese)

COCKS, L. R. M. AND T. H. TORSVIK. 2004. Major terranes in the Ordovicia,p. 61–67. In B. D. Webby, F. Paris, M. L. Droser, and I. G. Percival (eds.),The Great Ordovician Biodiversification Event. Columbia University Press,New York.

CRICK, R. E. 1990. Cambro–Devonian biogeography of nautiloid cephalo-pods, p. 147–161. In W. S. McKerrow and C. R. Scotese (eds.), Palaeozoicpalaeogeography and biogeography. The Geological Society, Memoir 12.

DZIK, J. 1982. Origin of the Cephalopoda. Acta Palaeontologica Polonica, 26:161–191.

DZIK, J. 1984. Phylogeny of the Nautiloidea. Palaeontologia Polonica, 45:3–203.

EDGECOMBE, G. E., B. D. E. CHATTERTON, N. E. VACCARI, AND B. G. WEIS-FELD. 1999. Ordovician pliomerid and prosopiscid trilobites from Argen-tina. Journal of Paleontology, 73:1155–1175.

EICHWALD, E., DE. 1860. Lethaea Rossica ou Paleontologie de la Russie.Stuttgart, Schweizerbart, 1654 p.

ENGESER, T. 1996. The position of the Ammonoidea within the Cephalopoda,p. 3–23. In N. H. Landman, K. Tanabe, and R. A. Davis (eds.), AmmonoidPaleobiology. Plenum Press, New York.

EVANS, D. H. 2005. The Lower and Middle Ordovician cephalopod faunasof England and Wales. of the Palaeontographical Society Monograph, 158,81 p.

FANNING, C. M., R. PANKHURST, C. RAPELA, E. BALDO, C. CASQUET, AND

C. GALINDO. 2004. K-bentonites in the Argentine Precordillera contem-poraneous with rhyolite volcanism in the Famatinian Arc. Journal of theGeological Society, London, 161:747–756.

FINNEY, S., J. GLEASON, G. GEHRELS, S. PERALTA, AND G. ACENOLAZA.2003a. Early Gondwanan connection for the Argentine Precordillera ter-rane. Earth and Planetary Science Letters, 205:349–359.

FINNEY, S., J. GLEASON, G. GEHRELS, S. PERALTA, AND J. D. VERVOORT.2003b. U/Pb geochronology of detrital zircons from Upper Ordovician LasVacas, La Cantera, and Empozada formations, NW Argentina, p. 191–196.In G. L. Albanesi, M. S. Beresi, and S. H. Peralta (eds.), Ordovician fromthe Andes, INSUGEO, Serie Correlacion Geologica, 17.

FLOWER, R. H. 1942. An Arctic cephalopod faunule from the Cynthiana ofKentucky. Bulletins of American Paleontology, 27(103):1–41.

FLOWER, R. H. 1946. Ordovician cephalopods from the Cincinnati region, Pt.1. Bulletins of American Paleontology, 29(116):3–547.

FLOWER, R. H. 1955. New Chazyan orthocones. Journal of Paleontology, 29:808–830.

FLOWER, R. H. 1962. I. Revision of Buttsoceras. II. Notes on the Micheli-noceratida. New Mexico Institute of Mining and Technology, State Bureauof Mines and Mineral Resources, Memoir 10, 40 p.


FLOWER, R. H. 1964. The nautiloid order Ellesmeroceratida (Cephalopoda).New Mexico Institute of Mining and Technology, State Bureau of Minesand Mineral Resources, Memoir 121, 164 p.

FLOWER, R. H. AND C. TEICHERT. 1957. The cephalopod order Discosorida.University of Kansas Paleontological Contributions, Mollusca, 6, 144 p.

FOERSTE, A. F. 1893. Remarks on specific characters in Orthoceras. TheAmerican Geologist, 12:232–236.

FOERSTE, A. F. 1925. Cephalopoda of Lake Timiskaming area and certainrelated species. Canada Department of Mines, Geological Survey, Memoir,145:64–93.

FOERSTE, A. F. 1926. Actinosiphonate, Trochoceroid and other cephalopods.Journal of the Scientific Laboratories of Denison University, 21:285–383.

FOERSTE, A. F. 1929. The Ordovician and Silurian of American Arctic andSubarctic regions. Journal of the Scientific Laboratories of Denison Uni-versity, 24:27–79.

FOERSTE, A. F. 1930. The color patterns of fossil cephalopods and brachio-pods, with notes on gastropods and pelecypods. University of MichiganContribution of the Museum of Paleontology, 3(6):109–150.

FOERSTE, A. F. 1932. Black River and other cephalopods from Minnesota,Wisconsin, Michigan, and Ontario, (Pt. 1). Journal of Scientific Laborato-ries of Denison University, 27:47–136.

FOORD, A. H. 1888. Catalogue of the fossil Cephalopoda in the British Mu-seum (Natural History), Pt. 1. London, British Museum, 344 p.

FORTEY, R. A. AND L. R. M. COCKS. 2003. Palaeontological evidence onglobal Ordovician–Silurian continental reconstructions. Earth-Science Re-views, 61:245–307.

FREY, R. C. 1995. Middle and Upper Ordovician cephalopods of the Cincin-nati region of Kentucky, Indiana, and Ohio. U.S. Geological Survey Pro-fessional Paper 1066P:P1–P119.

GNOLI, M. 2002. Northern Gondwanan Siluro–Devonian palaeogeography as-sessed by cephalopods. Palaeontologia Electronica, 5 (2), 19 p. (917MB.http://www-odp.tamu.edu/paleo/2002�2/gondwana/issue2�02.htm)

GONZALEZ, A. F., J. OTERO, A. GUERRA, R. PREGO, F. J. ROCHA, AND A. W.DALE. 2005. Distribution of common octopus and common squid paralar-vae in a wind-driven upwelling area (Ria of Vigo, northwestern Spain).Journal of Plankton Research, 27:271–277.

HERRERA, Z. A. AND J. L. BENEDETTO. 1991. Early Ordovician brachiopodfaunas from the Precordillera basin, western Argentina: Biostratigraphy andpaleobiogeographical affinities, p. 283–301. In D. I. Mac Kinnon, D. E.Lee, and J. D. Campbell (eds.), Brachiopods Through Time, Balkema, Dun-edin.

HISINGER, W. 1837. Lethaea Svecica seu Petrificata Svecicae, Iconibus etCharacteribus Illustrata. Holmiae, Norderstedt, 124 p.

HOLM, G. 1899. Palaeontologiska notiser. Om ett par Bactrites-liknande Un-tersiluriska Orthocer-former. Geologiska Foreningens i Stockholm Forhan-dlingar, 20:354–360.

HOOK, S. C. AND R. H. FLOWER. 1977. Late Canadian (Zones J, K) cepha-lopod faunas from southwestern United States. New Mexico Bureau ofMines and Mineral Resources Memoir 32, 56 p.

HUANG, BAOCHUN, ZHU RIXIANG, Y. OTOFUJI, AND YANG ZHENYU. 2000.The Early Paleozoic paleogeography of the North China block and othermajor blocks of China. Chinese Science Bulletin, 45:1057–1065.

HUFF, W., S. BERGSTROM, D. KOLATA, C. CINGOLANI, AND D. DAVIS. 1995.Middle Ordovician K-bentonites discovered in the Precordillera of Argen-tina: Geochemical and paleogeographical implications, p. 343–349. In J.Cooper, M. Droser, and S. Finney (eds.), Short Papers for the SeventhInternational Symposium of the Ordovician System, SEPM, Book 77.

HUNICKEN, M. AND G. ORTEGA. 1987. Lower Llanvirn–lower Caradoc (Or-dovician) conodonts and graptolites from the Argentine Central Precordil-lera, p. 136–145. In R. L. Austin (ed.), Conodonts: Investigative Techniquesand Applications. Ellis Horwood Limited, Chichester.

HYATT, A. 1884. Genera of fossil cephalopods. Boston Society of NaturalHistory Proceedings, 22:273–338.

HYATT, A. 1900. Cephalopoda, p. 502–592. In K. A. Zittel and C. R. East-mann (eds.), Textbook of Palaeontology, Volume 1, Macmillan, London.

KAYSER, E. 1876. Ueber Primordiale und Untersilurische Fossilien aus derArgentinischen Republik, p. 1–24. In A. Stelzner (ed.), Beitrage zur Geo-logie und Palaeontologie der Argentinischen Republik. II Palaeontologisch-er Theil. Palaeontographica Supplement III. Verlag von Theodor Fischer,Cassel.

KELLER, M. 1994. Argentine Precordillera: Sedimentary and plate tectonichistory of a Laurentian crustal fragment in South America. Geological So-ciety of America Special Paper 341, 131 p.

KELLER, M. AND E. FLUGEL. 1996. Early Ordovician reefs from Argentina:Stromatoporid vs. stromatolite origin. Facies, 34:177–192.

KING, A. 1999. A review of Volchovian and Kundan (Arenig–Llanvirn) nau-tiloids from Sweden, p. 137–159. In F. Oloriz and F. J. Rodrıguez-Tovar(eds.), Advancing Research on Living and Fossil Cephalopods. Kluwer Ac-ademic/Plenum Publishers, New York.

KOBAYASHI, T. 1927. Ordovician fossils from Korea and South Manchuria.Japanese Journal of Geology and Geography, 5(4):173–212.

KOBAYASHI, T. 1931. Studies on the stratigraphy and palaeontology of theCambro–Ordovician formation of Hua-lien-chai and Niu-hsin-tai, southManchuria. Japanese Journal of Geology and Geography, 8:131–173.

KOBAYASHI, T. 1934. The Cambro–Ordovician formations and faunas ofSouth Chosen. Palaeontology, Pt. I, Middle Ordovician faunas. Journal ofthe Faculty of Sciences of the Imperial University of Tokyo, Section II,Geology, Mineralogy, Geography, Seismology, 3:329–519.

KOBAYASHI, T. 1935. Suggestions for the natural classification and benthonicadaptation of early uncoiled nautiloids. Proceedings of the Tokyo ImperialAcademy, 12:296–298.

KOBAYASHI, T. 1936. On the Stereoplasmatidae. Japanese Journal of Geologyand Geography, 13:229–242.

KROGER, B. 2004. Revision of Middle Ordovician orthoceratacean nautiloidsfrom Baltoscandia. Acta Palaeontologica Polonica, 49:57–74.

KROGER, B. 2006. Early growth stages of Arenigian Baltoscandic Orthocerida(Mollusca, Cephalopoda). Lethaia, 39(2):129–139.

KROGER, B. AND M. ISAKAR. 2006. Revision of annulated orthoceridan ceph-alopods of the Baltoscandic Ordovician. Fossil Record, Mitteilungen ausdem Museum fur Naturkunde in Berlin, Geowissenschaftliche Reihe, 9:139–165.

KROGER, B. AND R. H. MAPES. 2005. Revision of some common Carbonif-erous genera of North American orthocerid nautiloids. Journal of Paleon-tology, 79:1002–1011.

KROGER, B. AND R. H. MAPES. 2007. Carboniferous actinoceratoid Nautilo-idea (Cephalopoda)—A new perspective. Journal of Paleontology, 81(4):714–724.

KROGER, B. AND H. MUTVEI. 2005. Nautiloids with multiple paired musclescars from Lower–Middle Ordovician of Baltoscandia. Palaeontology, 48:781–791.

KUHN, O. 1940. Palaozoologie in Tabellen. Fischer Verlag, Jena, 50 p.LAI, CAI-GEN. 1960. Nautiloid fossils of Yangtzeella poloi beds from Shensi

and Hupei. Acta Palaeontologica Sinica, 8:251–271. (In Chinese)LANDING, E. 1976. Early Ordovician (Arenigian) conodont and graptolite bio-

stratigraphy of the Taconic allochthon, eastern New York. Journal of Pa-leontology, 50:614–646.

LANDING, E. AND R. LUDVIGSEN. 1984. Classification and conodont-basedage of the Ordovician trilobite Ellsaspis (middle Arenigian, Ville Guay,Quebec). Canadian Journal of Earth Sciences, 21:1483–1490.

LEHNERT, O. 1995. The Tremadoc/Arenig transition in the Argentine Preor-dillera, p. 145–148. In J. Cooper, M. Droser, and S. Finney (eds.), ShortPapers for the Seventh International Symposium of the Ordovician System,SEPM, Book 77.

LI, JUN AND T. SERVAIS. 2002. Ordovician acritarchs of China and their utilityfor global palaeobiogeography. Bulletin Societe Geologique France, 173:399–406.

LINDSTROM, M. 1955. Conodonts from the lowermost Ordovician strata ofsouth-central Sweden. Geologiska Foreningens i Stockholm Forhandlingar,76:517–604.

MCCOY, F. 1844. A Synopsis of the Characters of the Carboniferous Lime-stone Fossils of Ireland. University Press, Dublin, 207 p.

MUTVEI, H. 2002. Nautiloid systematics based on siphuncular structure andposition of muscle scars. Abhandlungen der Geologischen Bundesanstalt,57:379–392.

NIOCAILL, C., B. A. VAN DER PLUIJM, AND R. VAN DER VOO. 1997. Ordo-vician paleogeography and the evolution of the Iapetus ocean. Geology, 25:159–162.

OTTONE, E. G., G. L. ALBANESI, G. ORTEGA, AND G. HOLFELRIGTZ. 1999.Palynomorphs, conodonts and associated graptolites from the OrdovicianLos Azules Formation, central Precordillera, Argentina. Micropaleontology,45:225–250.

PAN, ZHENG-QIN. 1986. Late Early Ordovician cephalopods from Nanjinghills, Jiangsu. Acta Palaeontologica Sinica, 25:312–325. (In Chinese)

PERALTA, S. AND M. S. BERESI. 1999. A new K-bentonite interval, from earlyLlanvirn, Villicum Range, Precordillera Western Argentina. 8th Interna-tional Symposium on the Ordovician System, Czech Republic, 1999. ActaUnivesitatis Carolinae, Geologica, 43:456–459.

QI, DUN-LUN. 1980. Ordovician cephalopods from Wuwei of Anhui and theirstratigraphical significance. Acta Palaeontologica Sinica, 19:245–260. (InChinese with English abstract)

REMELE, A. 1882. Ueber einige gekrummte untersilurische Cephalopoden.Zeitschrift der Deutschen Geologischen Gesellschaft, 34:16–138.

RISTEDT, H. 1968. Zur Revision der Orthoceratidae. Akademie der Wissen-schaften und Literatur in Mainz, Abhandlungen der mathematisch-natur-wissenschaftlichen Klasse, 1968:211–287.

ROCHA, F., R. P. GUERRA, AND U. PIATKOWSKI. 1999. Cephalopod paralarvaeand upwelling conditions off Galician waters (NW Spain). Journal of Plank-ton Research, 21:21–33.


SALADZHIUS, V. 1966. Fauna molliuskov siluriisikh otloshenii iushnoi Pribal-tiki. Paleontology and Stratigraphy of the Baltic and the Byelorussia, 1:31–73. (In Russian)

SANCHEZ, T., M. CARRERA, AND B. WAISFELD. 1996. Ordovician faunal turn-over in the Argentina Precordillera. Acta Universitatis Caronlinae Geolo-gica, 43:479–481.

SARMIENTO, G. N. 1985. La Biozona de Amorphognathus variabilis–Eopla-cognathus pseudoplanus (Conodonta), Llanvirniano inferior, en el flancooriental de la sierra de Villicum. Primeras Jornadas sobre Geologıa de laPrecordillera, Acta, 1:119–123.

SERPAGLI, E. 1974. Lower Ordovician conodonts from Precordilleran Argen-tina (Province of San Juan). Bolletino della Societa Paleontologica Italiana,13:17–98.

SCHLOTHEIM, E. F. FREIHERR, VON. 1820. Die Petrefactenkunde auf IhremJetzigen Standpunkte, Durch die Beschreibung Seiner Sammlung Verstei-nerter und Fossiler Uberreste des Tier- und Planzenreichs der Vorwelt Er-lautert. Becker Verlag, Gotha, 437 p.

SHIMIZU, S. AND T. OBATA. 1935. On a new Ordovician nautiloid genus Sin-oceras. Proceedings of the Imperial Academy Tokyo, 11:324–325.

SHIMIZU, S. AND T. OBATA. 1936. On some new genera of Ordovician nau-tiloids from East Asia. The Journal of the Shanghai Science Institute, Sec-tion 2, 2:11–25.

STAROBOGATOV, Y. I. 1974. Ksenoknkhii ikh znachenie dlia filogenii i sis-temy nekotorykh klassov molliuskov. Paleontologicesky Zhurnal, 1974:3–18. (In Russian)

STELZNER, A. 1885. Beitrage zur Geologie und Palaontologie der Argentin-ischen Republik. I Geologisher Theil. Palaeontographica. Beitrage zur Na-turgeschichte der Vorzeit. Supplement 3, Fischer Verlag, Cassel, 329 p.

SWEENEY, M. J. AND C. F. E. ROPER. 1998. Classification, type localities andtype repositories of Recent Cephalopoda. Smithsonian Contributions to Zo-ology, 586:561–594.

SWEET, W. C. 1958. The Middle Ordovician of the Oslo region of Norway.10. Nautiloid cephalopods. Norsk Geologiske Tidsskrift, 31:1–178.

SWEET, W. C. 1964. General features, p. K4–K13. In C. Teichert, B. Kummel,W. C. Sweet, H. B. Stenzel, W. M. Furnish, B. F. Glenister, H. K. Erben,R. C. Moore, and D. E. N. Zeller (eds.), Treatise on Invertebrate Paleon-tology, Pt. K, Mollusca 3, Cephalopoda. Geological Society of Americaand University of Kansas Press, Boulder, Colorado.

TEICHERT, C. AND B. F. GLENISTER. 1954. Early Ordovician cephalopod faunafrom northwestern Australia. Bulletin of American Paleontology, 35(150),94 p.

THOMAS, W. A. AND R. A. ASTINI. 1996. The Argentine Precordillera: Atraveller from the Ouachita embayment of North American Laurentia. Sci-ence, 273:752–757.

THOMAS, W. A., AND R. A. ASTINI. 2003. Ordovician accretion of the Ar-gentine Precordillera terrane to Gondwana: A review. Journal of SouthAmerican Earth Sciences, 16:67–79.

TROEDSSON, G. T. 1931. Studies on Baltic fossil cephalopods. I. On the nau-tiloid genus Orthoceras. Lund Universitets Arsskrift N. F., Avhandling. 2,27(16):1–36.

TYCHSEN, A. AND D. A. T. HARPER. 2004. Ordovician–Silurian distributionof Orthida (Palaeozoic Brachiopoda) in the greater Iapetus Ocean region.Palaeontologica Electronica 7 (3), 15 p. (706KB; http://palaeo-electronica.org/palaeo/2004�1/orthidaissue1�04.htm)

ULRICH, E. O. AND A. F. FOERSTE. 1936. New genera of Ozarkian and Ca-nadian cephalopods. Journal of the Scientific Laboratories of Denison Uni-versity, 30:259–290.

ULRICH, E. O., A. F. FOERSTE, A. K. MILLER, AND A. G. UNKLESBAY. 1944.Ozarkian and Canadian Cephalopods, Pt. III, Longicones and summary.Geological Society of America, Special Paper 58, 226 p.

WESTERMANN, G. E. G. 1999. Life habits of nautiloids, p. 263–298. In E.Savazzi (ed.), Functional Morphology of the Invertebrate Skeleton. JohnWiley and Sons, New York.

WILSON, A. E. 1961. Cephalopoda of the Ottawa Formation of the Ottawa–St. Lawrence Lowland. Geological Survey of Canada, Bulletin 67, 106 p.

XU, G. AND LAI CAI-GEN. 1987. Cephalopods, p. 245–353. In Biostratigraphyof the Yangtze Gorge area, 2. Early Palaezoic Area. Geological PublishingHouse, Beijing. (In Chinese)

YANG, ZHENYU., YO-ICHIRO OTOFUJI, ZHIMING SUN, AND BAOCHUN HUANG.2002. Magnetostratigraphic constraints on the Gondwanan origin of NorthChina: Cambrian/Ordovician boundary results. Geophysical Journal Inter-national, 151:1–10.

YING, ZHONG-E. 1989. Late Early and Middle Ordovician cephalopods fromGuichi, Anhui. Acta Palaeontologica Sinica, 28:617–633. (In Chinese)

ZHEN, YONG-YI AND I. G. PERCIVAL. 2003. Ordovician conodont biogeog-raphy reconsidered. Lethaia, 36:357–370.

ZHURAVLEVA, F. A. 1957. On the family Pseudorthoceratidae Flower et Cast-er, 1935. Doklady Akademii Nauk SSSR, 116:677–680. (In Russian)

ZHURAVLEVA, F. A. 1964. New Ordovician and Silurian cephalopods fromthe Siberian platform. Paleontologicheskii Zhurnal, 1964(4):85–100. (InRussian)

ZHURAVLEVA, F. A. 1990. New Palaeozoic cephalopods from Mongolia. Pa-leontological Journal, 1990(2):37–46.

ZOU, XI-PING. 1987. Ordovician nautiloid faunas of Yuhang and Lin’an, Zhe-jiang Province. Bulletin of the Nanjing Institute of Geology and Palaeon-tology, Academia Sinica, 12:231–295. (In Chinese)

ZOU, XI-PING. 1988. Ordovician nautiloid faunas from Lunshan, Jurong,Jiangsu. Acta Palaeontologica Sinica, 27:309–330. (In Chinese)

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EARLY ORTHOCERATOID CEPHALOPODS FROM THE ARGENTINE PRECORDILLERA (LOWER–MIDDLE ORDOVICIAN)

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