Cretaceous Research (1996) 17 , 607 – 632

Paleobiology of two new species of the bivalve Anomia from Colombia and Venezuela and the importance of the genus in recognition of the base of the Turonian

Tomas Villamil

Department of Geological Sciences , University of Colorado , Boulder , CO 8 0 3 0 9 - 0 2 5 0 , USA ; Current address : Advance Exploration Organization , Conoco Inc . , 6 0 0 North Dairy Ashford , PO Box 2 1 9 7 , Houston , TX 7 7 2 5 2 - 2 1 9 7 , USA

Revised manuscript accepted 1 6 February 1 9 9 6

The base of the Turonian Stage in Colombia and Venezuela contains abundant small anomiid bivalves . These anomiids are well preserved , stratigraphically restricted to the Lower Turonian and are widely distributed in northern South America . The anomiids described in this paper are excellent fossils for regional correlation because of their high potential for preservation , easy recognition , wide geographical dispersion , high abundance , and restricted vertical stratigraphic distribution . Their occurrence may have been controlled by firm substrates resulting from stratigraphic condensation related to late Cenomanian – early Turonian transgressive pulses which began in the latest Cenoma- nian and culminated in a lower Turonian condensed section . Three dif ferent morphotypes are recognized : Anomia minuta n . sp ., A . colombiana n . sp ., and a species which can be compared to Anomia papyracea . Anomia minuta is a very small species that occurs at the base of the Turonian , forming bioclastic limestone beds several cm in thickness . This is a widely distributed and biostratigraphically-restricted species . The second species , Anomia colombiana occurs tens of cm to a few stratigraphic meters above A . minuta ; specimens are also very small but are less abundant . A . colombiana shows a radially-ribbed left valve . Anomia sp . cf . A . papyracea occurs together with A . minuta and A . colombiana but its last occurrence lies stratigraphically above A . colombiana in the Lower Turonian , and is a more inflated and smooth anomiid . ÷ 1996 Academic Press Limited

K E Y W O R D S : Early Turonian ; anomiids ; northern South America ; new species .

La base del Turoniense en Colombia y en Venezuela contiene abundantes bivalvos ano ́ midos ; estos ano ́ midos estan estratigra ́ ficamente restringidos al Turoniense inferior y estan ampliamente distribuidos en el norte de Sur Ame ́ rica siendo fo ́ siles excelentes para bioestratigrafı ́ a por su fa ́ cil preservacio ́ n , fa ́ cil reconocimiento , marcada abundancia y distribucio ́ n estratigra ́ fica restringida . La aparicio ́ n y abundancia de los ano ́ midos pueden estar controladas por endurecimiento del substrato causado por condensacio ́ n estratigra ́ fica relacionada con pulsos de transgresio ́ n . Estos pulsos comenzaron en el Cenomaniense ma ́ s superior y culminaron en una seccio ́ n condensada del Turoniense inferior . Se reconocieron tres morfotipos de los cuales dos son especies nuevas y uno es comparable con Anomia papyracea . Anomia minuta n . sp . es una Anomia muy pequen ̃ a de la base del Turoniense , esta Anomia forma calizas biocla ́ sticas de varios cm de espesor ; A . minuta esta ́ a ́ mpliamente distribuida y es estratigra ́ ficamente restringida . La segunda especie , Anomia colombiana n . sp . aparece decenas de cm a pocos metros estratigra ́ ficos por encima de A . minuta ; los ejemplares de A . colombiana tambie ́ n son pequen ̃ os pero se encuentran en menor abundancia . Anomia colombiana muestra costillas radiales en su valva izquierda . Anomia sp . cf . A . papyracea es una Anomia ma ́ s inflada , ma ́ s grande y ma ́ s lisa que las anteriores y se encuentra asociada con las dos especies mencionadas pero su u ́ ltima aparicio ́ n se observa en capas de la parte superior del Turoniense inferior .

0195 – 6671 / 96 / 050607 1 26 $18 . 00 / 0 ÷ 1996 Academic Press Limited

T . Villamil 608

P A L A B R A S C L A V E : Turoniense inferior ; Anomia ; norte de Sur Ame ́ rica ; dos nuevas especies .

1 . Introduction

1 . 1 . Background Cretaceous anomiid bivalves were mentioned for the first time in the Colombian or Venezuelan literature by Bu ̈ rgl (1961) , who stated : ‘‘En este piso (Cenomani- ano) encontramos ... Un Lamelibranqio muy pequen ̃ o (3 – 8 mm de largo) parece ser ide ́ ntico a Anomia subtruncata D’ORBIGNY conocida del Cenomaniano de Tu ́ nez ... ’’ . However , Bu ̈ rgl (1961) was probably referring to the pectinid bivalve Entolium gregarium when he discussed an abundant small Anomia sp . in the Cenomanian of the Upper Magdalena Valley .

I have conducted extensive field work in the same region and on the same sections that Bu ̈ rgl studied . Instead of large numbers of Anomia in the Cenomanian of the Upper Magdalena Valley these beds contain surfaces crowded with a small pectinid called the Entolium gregarium bioevent by Villamil (in press) . The E . gregarium bioevent is easily recognized , widely distributed , and can be traced throughout the Upper Magdalena Valley of Colombia and to some regions of the Eastern Cordillera . This bioevent occurs associated with non-calcareous to slightly calcareous shales approximately 100 stratigraphic meters below the anomiid bivalves described in this paper . The morphology and mode of occurrence of Entolium gregarium are superficially very similar to those of the Turonian anomiids discussed in this paper ; however , E . gregarium inhabited soft soupy sediments rather than firm hemipelagic limes with early diagenetic concretions , and specimens are generally white rather than black as the anomiids described here .

1 . 2 . Purpose The purpose of this paper is to describe two new lower Turonian species of anomiid bivalves and to discuss their importance in Cretaceous biostratigraphy and correlation . In addition , the paper discusses the conditions that may have favored wide distribution of anomiids in distal of fshore , fine-grained facies .

1 . 3 . Location During extensive field work from 1990 – 1992 in the Upper Magdalena Valley of Colombia (Figure 1) abundant anomiid bivalves were found at one specific stratigraphic level of Early Turonian age . This level was subsequently traced throughout the Upper Magdalena Valley , where it occurs sub-parallel to marker bentonite beds and parallel to ammonite first occurrences . Subsequently , the anomiid-rich level was found at two localities in the Eastern Cordillera of Colombia , one near Bogota ́ on the road to Mesitas del Colegio (Figure 1) and another one near the town of Villa de Leiva , a classic locality for the Cretaceous of central Colombia . The anomiid-rich level was also discovered near the Sierra Nevada del Cocuy , western Venezuela , and in eastern Venezuela (Figure 1) . This bioevent interval has now been found in all documented stratigraphic sections that contain Lower Turonian strata (Figures 1 – 4) .

The anomiid beds are restricted to black shales , levels rich in calcareous

Paleobiology of two new species of the bivalve Anomia 609

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Paleobiology of two new species of the bivalve Anomia 613

concretions , and hemipelagic limestones with few coarse-grained clastics . In some localities anomiids form bioclastic limestones up to 5 cm thick and occur in high-abundance horizons . In some limestones and calcareous concretions the number of bivalves can exceed 50 specimens / cm 2 (Figure 5) and therefore , once the anomiid-rich level is found , specimens can be collected by the thousands in a very short period of field time . The geographic and stratigraphic distribution of the anomiid level suggests that the high abundance of anomiids in the earliest Turonian was in response to regional , short-lived environmental changes such as hardening of the substrate caused by stratigraphic condensation associated with vigorous upwelling . These bivalves can be used for regional correlation and aid in the recognition of the Cenomanian – Turonian boundary interval in northern South America .

2 . Stratigraphy

The stratigraphic nomenclature of the anomiid-bearing portion of the Cretaceous of Colombia and Venezuela has been complicated by the use of dif ferent names for the same facies and the same name for dif ferent stratigraphic levels (Figures 2 – 4) . Early Turonian facies , however , show remarkable lateral continuity from southern Colombia to eastern Venezuela . Early Turonian facies of the Upper Magdalena Valley and western flank of the Eastern Cordillera near Bogota ́ belong to the base of the La Frontera Formation (Figure 3) within the Villeta Group (Villamil & Arango , in press) . On the eastern side of the Eastern Cordillera these facies belong to the Chipaque Formation . Very similar and coeval facies in the Eastern Cordillera near Villa de Leiva are part of the lower San Rafael Formation (Figure 4B) . More to the north , in the Cocuy region , similar facies of the same age belong to the Capacho Formation , whereas in western Venezuela they have been placed in the La Luna and the Navay Formations . In some regions of western Venezuela , e . g ., near the city of Me ́ rida , the base of the Turonian coincides with the base of the La Luna Formation (as in the type section of the La Luna) . However , to the north of Me ́ rida , the base of the La Luna is much older . South of Me ́ rida in the Barinas-Apure Basin , facies coeval to the La Luna loose their black coloration and are replaced by the white cherty beds of the Navay Formation . To further complicate nomenclature issues , in eastern Venezuela the same lithologies of the same age belong to the lower portions of the Querecual Formation (Figure 4A) .

Distal of fshore stratigraphic sections (e . g ., the Olinı ́ , Yaguara ́ , Mesitas , and Querecual sections , Figures 2 , 3 , and 4A) consist of highly calcareous , finely-laminated , black , organic-rich shales interbedded with hemipelagic forami- niferal limestones and calcareous concretions . These facies are also associated with numerous discrete mm-scale phosphatic-rich lags with abundant fish debris . Stratigraphic sections proximal to the shore line (e . g ., near Villa de Leiva and near Chipaque ; Figures 1 , 4B) contain more clastics , fewer concretion intervals and hemipelagic limestones , probably because of increased sediment supply combined with a slight decrease in paleoproductivity caused by proximity to the shoreline . Anomiid bivalves are less concentrated but still quite abundant in these shales . In the most distal stratigraphic sections (e . g ., in the Yaguara ́ and Olinı ́ sections ; Figures 1 , 2A , 3A) Anomia minuta n . sp ., A . colombiana n . sp ., and A . sp . cf . A . papyracea are found in great abundance , forming coquinoid limestones and comprising a large volume of calcareous concretions (Figures 2A , 3 , 4) .

T . Villamil 614

Figure 5 . Photograph of the top of one Anomia -rich bed from the Yaguara ́ River stratigraphic section (A) and a concretion level from the Olini stratigraphic section (B) associated with juvenile Watinoceras , Mammites , and Benueites (see Figure 1 for location and Figures 2 and 3 for stratigraphy) . Anomiids in this interval form relatively deep water bioclastic limestones entirely composed of bivalves . The particular anomiid bed shown in A is about 5 cm thick and can be traced throughout the Upper Magdalena Valley of Colombia ; note the tiny Anomia minuta .

Paleobiology of two new species of the bivalve Anomia 615

The occurrence of these anomiids was regulated by sequence stratigraphic relationships and dynamics in oceanographic and sea bottom paleoenvironments . Upper Cenomanian facies were deposited in the last portion of a prograding lowstand systems tract (prograding complex) . These facies comprise sandstones or siltstones with shallow-water ostreid bivalves such as Exogyra spp . and some gastropods . Upper Cenomanian shallow-water facies are separated from the latest Cenomanian – Lower Turonian condensed section by a marked transgressive surface (Figure 6 , TS in Figure 7) characterized by an abrupt lithological change from shallow-water sandstones or siltstones to hemipelagic highly calcareous shales . This surface is regionally extensive and forms the boundary between several lithostratigraphic units (e . g ., the boundary between the Churuvita and the San Rafael Formations in the Villa de Leiva region , the base of the La Frontera Formation in the Bogota-Apulo-Mesitas del Colegio region , and the base of the La Luna Formation in the Andes of Me ́ rida region ; Figures 2 , 4) . In many regions of northern South America , this transgressive surface separates proximal siltstones or sandstones from hemipelagic limestones and organic-rich shales containing calcareous concretions (e . g ., Figures 2 , 7) . A few beds above the transgressive surface and within the early part of the transgressive systems tract lies the Cenomanian – Turonian boundary , which , based on ammonites , planktic foraminifers and geochemistry , coincides with the first appearance of abundant Anomia minuta . Anomiids are concentrated in association with sediment starva- tion surfaces and thin transgressive lags formed during transgressive pulses and shoreward trapping of sediment . Anomiids reach their peak in abundance just below the Early Turonian maximum flooding surface , a major , regionally- distributed condensed section (Figures 5 , 6 , and 7A , dark shaded interval) . This condensed interval is characterized by a regionally extensive concretion level that is extremely rich in micro- and macrofossils (Villamil & Arango , in press ; Villamil , in press) . The highstand systems tract that overlies this condensed section contains Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea . Most of the anomiids discussed here become rare up-section and disappear just below the early – middle Turonian boundary (Figures 2 – 4) . Anomiid-bearing facies are interpreted as having been formed on a distal of fshore sea floor with little current energy acting on the bottom (Figure 7) , high amounts of organic carbon , and dysoxic to anoxic conditions at and near the sediment – water interface .

3 . Taphonomy and paleoecology

3 . 1 . General The small anomiids discussed in this paper are paper thin and are found in very large numbers restricted to particular stratigraphic intervals . Hasenmueller & Hattin (1990) , in their description of similar anomiids of the Greenhorn Limestone of Kansas , interpreted their occurrence as the consequence of unusual taphonomic and diagenetic conditions , which they did not mention . The preservation of northern South American anomiids may reflect exceptional taphonomic and diagenetic conditions , particularly regional consolidation of the substrate and development of widespread firmgrounds . Explaining the occur- rence of anomiids only with preservation factors neglects the paleoenvironmental conditions that prevailed during those times , and which favored anomiid colonization of consolidated bottoms . Prevailing paleoenvironmental conditions that favored presence and growth of anomiids are quite complicated to evaluate ;

T . Villamil 616

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Paleobiology of two new species of the bivalve Anomia 617

Figure 7 . Simplified models showing conditions that favored accumulation and preservation of described anomiid bivalves . A , sequence stratigraphic model for the Cenomanian – Turonian boundary interval with the position of anomiid-rich levels ; LST (PC) 5 lowstand systems tract , prograding complex ; TST 5 transgressive systems tract ; HST 5 highstand systems tract ; SB 5 sequence boundary ; TS 5 transgressive surface . B , paleoceanographic conditions that , combined with low sedimentation rate , favored substrate hardening . Wind direction is into figure , Ekman transport perpendicular to wind ; SL 5 sea level . C , schematic possible attachment surfaces for Anomia spp . on the sea bottom , anomiids attached to ammonites and firm-hard bottoms .

however , it is proposed that hardened bottoms were developed during trans- gressive pulses and associated distal of fshore starvation surfaces . Hardening of the substrate is suggested by the large amounts of silica in the system combined with time of non-deposition (evidenced by abundant , well-preserved radiolarians and scattered chert beds) and abundant phosphatized remains (abundant phosphatic lags) . Silica and phosphates suggest vigorous upwelling and paleo- productivity during times of deposition of anomiid-rich stratigraphy . Villamil (1995) suggested that Ekman transport caused by westward blowing winds could have established the high paleoproductivity regime (Figure 7B) .

Periods of non-deposition and early diagenetic induration by silica , phosphates and possibly carbonates are necessary to have hardened mudstones and calca- reous shales . The discussed stratigraphic interval is one of the most marked

T . Villamil 618

condensed sections of the Cretaceous in northern South America being actually the maximum flooding of the Phanerozoic in the region . Lack of deposition combined with enough calcite , silica , and phosphate in the water and in sedimentary pores could have indurated the sea bottom and made it favorable for colonization by anomiids . This scenario could also probably apply to the Western Interior anomiids described by Hasenmueller & Hattin (1990) and probably to the French anomiids discussed by Marlie ̀ re (1964) .

Diagenetic and taphonomic modification of sediments , the formation of calcareous concretions , and compaction of shales persisted throughout the Turonian and Coniacian of northern South America , but strata younger than Early Turonian do not contain anomiids . In addition , Middle Turonian – Upper Coniacian strata contain delicate fauna such as juvenile Sergipia and very small ammonites ( Harleites , Subprionotropis and others) but do not contain anomiid bivalves , supporting the idea that anomiids were preserved in a restricted stratigraphic interval because of paleoenvironmental rather than post-depositional conditions .

Anomiids from northern South America are well preserved in-the-round when found in calcareous concretions or in anomiid limestones . However , only the external characteristics of left valves of these anomiids are observable ; therefore , the description and systematics are based uniquely on the exposed features . Ammonites ( Mammites , Watinoceras , Pseudaspidoceras , Vascoceras , Hoplitoides and many others) , petrified logs , and other bivalves associated with the small anomiids in calcareous concretions are similarly preserved and show no compac- tion . A similar type of preservation was reported for the Kansas anomiids by Hasenmueller & Hattin (1990) . South American organic-rich shales , however , also contain abundant small anomiids that appear as gray specks and are completely crushed and wrinkled by compaction . No recognizable ammonites were found associated with the crushed anomiids in the shales , probably due to early diagenetic dissolution of their aragonitic shells . Inoceramid bivalves ( Mytiloides ) were found in calcareous concretions , hemipelagic limestones and in shales , generally associated with anomiids .

Northern South American anomiids occur in large numbers and are restricted to particular stratigraphic levels (e . g ., Figures 2 – 5 and 7C) . Size – class distribu- tion of dif ferent levels rich in anomiids , such as the one illustrated in Figure 5 , show how specimens of A . minuta and A colombiana peak in abundance at a restricted size class , generally a single cohort . Similar population structures observed in inoceramid bivalves and other flat clams are termed event com- munities (Kauf fman , 1978 ; Kauf fman & Sageman , 1990 ; Sageman , 1989 ; Kauf fman et al ., 1992) . Very small ( , 2 mm) juvenile anomiids were found associated with concretion intervals perhaps because this lithology is the only one that allows preservation of the paper-thin specimens or perhaps represents mass mortality surfaces or preserved spat-falls .

Based on shell concavity , Hasenmueller & Hattin (1990) proposed that Western Interior anomiids were epizoic on baculitid conchs ; however , they pointed out the lack of direct evidence of such attachment . In contrast , northern South American anomiids show direct evidence of epizoic attachment on ammonites (Figures 7C , 8) . Figure 8 shows two specimens of Anomia minuta attached to a specimen of the ammonite Pseudaspidoceras armatum ; Riccardi (1980) illustrated a similar example for the Maastrichtian of Argentina . The anomiids are attached to the upper surface of the ammonite , as determined by

Paleobiology of two new species of the bivalve Anomia 619

Figure 8 . Photographs of the ammonite Pseudaspidoceras armatum with two specimens of Anomia minuta attached to the external part of the cast ; A , un-coated specimen and B , specimen coated with ammonium chloride . The sutures of the ammonite can be clearly seen in A and therefore this specimen represents an internal mold , however , there is calcitic cast separating the attached anomiids from the internal mold ; see enlargement in C . Attached anomiids prevented the cast from destruction . Anomiid attachment can be observed only in the half of the ammonite that was exposed to the sea water rather than the side touching the sediment ; this is known because the stratigraphic top of the sample was recorded , and because the geopetal fill of the ammonite is restricted only to the side shown . Anomiid attachment to other ammonites was observed only in the side exposed to the water .

geopetal orientation (note dif ferent color in Figure 8) suggesting that anomiids attached to the specimen once it was dead and lay flat on the bottom . The specimen of Pseudaspidoceras with attached Anomia minuta shows internal sutures (Figure 8A) , however , the internal mold of the ammonite is separated from the

T . Villamil 620

attached Anomia by a thin calcite cast of replaced original ammonite shell . Even though the external shell of the ammonite is missing from most of the specimen , there is some shell preserved under the attached epibionts . Several examples of epizoan attachment of anomiids to ammonites were observed in the South American samples . However , in no case does the anomiid attachment suggest that they were epizoic during life of the host . The small anomiids were never found attached to the commonly associated shells of inoceramid bivalves ( Mytiloides ) . This may be explained by two alternative hypotheses : (1) The periostracum of inoceramid bivalves chemically and / or physically discouraged settlement by anomiid larvae , or (2) anomiid attachment was not favored by the ornamentation of Mytiloides ; i . e ., rugae and growth lines were too convex and / or inter-rugae spaces too deep . The second hypothesis is favored because post- mortem decay of the periostracum would allow attachment of anomiids onto relatively inert calcite shells .

3 . 2 . Comparative ecology Living Anomia spp . inhabit shallow subtidal settings with moderate current flow . They attach principally to pebbles , cobbles and shell fragments in very gravelly coarse sand sediment (Stanley , 1970 ; Hadfield & Anderson , 1988) . The shape of Anomia is highly variable and reflects the type of substrate to which it attaches . Right valves of specimens that are byssally attached to small pebbles grow slowly ; the left valves have to grow faster to accommodate body size increase . The resulting shape may be irregular and very wide . In contrast , specimens that attach to large particles exhibit a dif ferent shape : larger right valves and less concave left valves .

Anomiid bivalves have developed special adaptations to inhabit hard substrates , i . e ., a large byssus passes through a hole in the lower valve , and therefore the presence of typical anomiids in situ in the fossil record suggests that the substrate was firm or hard . Additionally , the shape of ancient anomiids may give clues concerning the topography of the substrate they originally attached . Most living anomiids inhabit relatively high energy environments ; however , the northern South American anomiids discussed here inhabited relatively deep , firm to hard substrates under the influence of very low energy conditions . Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea occur in close association with dark , organic-rich shales (up to 16% TOC) interbedded with sparse concretion levels and hemipelagic limestone beds (Figures 2 – 4) . Anomiid bivalves collected from laminated shales were not studied because they are extremely crushed by compaction ; in contrast , bivalves from concretion levels exhibit exquisite 3-D preservation .

Deeper water Turonian anomiids , might be expected to exhibit dif ferent shape , size , and adaptive strategy in comparison to shallow water forms like living Anomia simplex . The shape of Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea shows less ecophenotypic variation in comparison to the living species A . simplex because of the uniform topography of the bottom . Ornamentation of the small fossil anomiids consists of small and subtle growth lines , with a generally pronounced shell concavity and radial ribs in the case of Anomia colombiana . The size is markedly smaller than in modern species , possibly because of poor environmental conditions related primarily to widespread dysoxia and anoxia .

Paleobiology of two new species of the bivalve Anomia 621

3 . 3 . Comparison of Turonian Anomia ecology with flat clam life strategy Ecologically and morphologically , attached anomiids that inhabited poorly oxygenated environments are comparable to the ‘‘flat clam’’ life strategy of Kauf fman et al . (1992) . Flat clams generally inhabited oxygen-deficient environ- ments , were widely distributed , and had rapid evolutionary rates ; Cretaceous examples are the genera Mytiloides , Sergipia and Didymotis . The following aspects of the anomiids described herein are similar to the adaptive characters of flat clams : (1) they occur in very large numbers , and were able to survive in highly-stressed , oxygen-deficient environments ; (2) the epifaunal life position of Turonian anomiids was very similar to that of flat clams with the commissural plane parallel to the substrate ; (3) anomiids were probably opportunistic species that had the potential to rapidly and ef ficiently colonize favorable substrates ; (4) they had very thin shells , weak hinge structure , and are monomyarian ; and (5) the feeding strategy (suspension feeders) and the food resources were the same . Perhaps the main dif ference between Turonian Anomia and the flat clams is the special adaptations that Anomia has for attachment to firm or hard substrates rather than lying free on the substrate . Described anomiid bivalves could not survive on soupy substrates where other flat clams thrive .

3 . 4 . Hardening of the substrate for anomiid colonization Regional and possibly global scale paleoenvironmental conditions during the early Turonian favored the colonization of the substrate by small anomiids . The fact that remarkably similar anomiid morphotypes have been found in France , the Western Interior of the United States , and throughout northern South America in strata of the same age supports the presence of similar , widespread , paleoenvir- onmental conditions during the early Turonian (Figure 9) . The stratigraphic

Figure 9 . 90 Ma schematic paleogeographic reconstruction with localities where small Lower Turonian anomiids associated with black shales and hemipelagic limestones have been found (small white circles) . Simplified paleogeographic map modified from Kauf fman & Fagerstrom (1993) .

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relationships between Upper Cenomanian and Lower Turonian facies in the US Western Interior and throughout northern South America suggest a marked and rapid rise in eustatic sea level determined to be about 30 m for the Colombian Cretaceous passive margin (Villamil & Arango , in press) . Facies evidence of this sea-level rise are the formational contacts between units of this age in northern South America and the Western Interior of the US . In the Western Interior Basin , the abrupt facies change is between the latest Cenomanian Hartland Shale and Bridge Creek Limestone members of the Greenhorn Formation . The stratigraphic relationship between the Hartland (shales more abundant than hemipelagic limestones and calcarenites) and the Bridge Creek (limestones more abundant than shales) suggests that deposition of the Bridge Creek in relation to the underlying Hartland was caused by terrigenous sediment starvation combined with increased surface productivity . The boundary between the Hartland and the Bridge Creek can be classified as a transgressive surface . Very similar strati- graphic relationships occur in northern South America , e . g ., the contact between the sandstones of an unnamed unit and the hemipelagic limestones of the La Frontera Formation in the Upper Magdalena Valley and the Eastern Cordillera near Bogota ́ (Figures 2 , 3) ; the contact between the sandstones of the Churuvita and the shales of the San Rafael Formation in the Villa de Leiva region of Colombia (Figure 4) ; and the contact between the Guayaca ́ n shallow-water limestones and the shales and hemipelagic limestones of the base of the La Luna Formation of western Venezuela . In eastern Venezuela this transgressive surface is expressed by the abrupt appearance of calcareous shales with concretions and hemipelagic limestones above non-calcareous shales and mudstones , all within the Querecual Formation .

Relative hardening or firming of the sea floor either by early cements such as silica , carbonates , and / or phosphates , by packing and condensation of coarse bioclastic grains and / or deposition of a new hard substrate (e . g ., ammonite shells) may have been a requirement for anomiid colonization . Latest Cenoma- nian to early Turonian pulses in sea-level rise may have favored conditions for development of firm to hard substrates and colonization by anomiids at an inter-regional scale because of the combination of stratigraphic condensation and increased paleoproductivity favoring phosphatic and / or silica precipitation and substrate induration . Anomiid morphotypes discussed in this work may be key fossils for the recognition of the base of the Turonian Stage .

4 . Biostratigraphic potential of the new species

The stratigraphic distribution of Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea is restricted to the Lower Turonian . Anomia minuta occurs within just 5 – 10 stratigraphic meters in the most condensed intervals , such as the Yaguara ́ and Olinı ́ sections , and in 5 – 18 stratigraphic meters in the sections that are not so condensed ; e . g ., the Mesitas del Colegio , Villa de Leiva , and Rio Querecual sections , and in the base of the La Luna Formation in western Venezuela (Figure 1 for location and Figures 2 – 4 for stratigraphic distribution of anomiids) . The radially-ribbed A . colombiana was found in beds above A . minuta , and also has a restricted stratigraphic distribution of just a few meters (Figures 2 – 4) . Specimens of A . colombiana were collected from all the stratigraphic sections within an interval that varies between 8 and 15 m thick . Valves of Anomia sp . cf . A .

Paleobiology of two new species of the bivalve Anomia 623

papyracea were found slightly above A . colombiana and also restricted to a few stratigraphic meters that contain facies which allow preservation of this species .

The anomiids discussed herein have a wide geographic distribution within northern South America . Specimens of Anomia minuta and A . colombiana have been collected from all the stratigraphic sections that contain Lower Turonian facies . The geographic distribution of these bivalves comprises at least Colombia and Venezuela . Anomia sp . cf . A . papyracea was collected from the Rio Querecual stratigraphic section of eastern Venezuela and the Mesitas del Colegio section in the central part of the Eastern Cordillera of Colombia (Figures 1 – 4) .

The restricted stratigraphic ranges of Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea and their wide geographic distribution makes them biostrati- graphically useful index fossils for northern South America . The correlation scheme in Figure 6 shows how first and last occurrences of anomiids are regionally sub-parallel to time lines defined by bentonites and sequence strati- graphic surfaces . The small size of the anomiids makes them good biostrati- graphic fossils for subsurface correlation as well . Well cuttings and cores may contain abundant anomiids , which are generally well preserved and easy to identify .

5 . Anomiids and the base of the Turonian Stage

Marlie ̀ re (1964) recognized the importance of anomiid bivalves , particularly Anomia papyracea , in the depiction of the base of the Turonian Stage of France . Marlie ̀ re noted that the small size and extremely high abundance of Anomia papyracea was observed by R . Dehe ́ e and proposed this event as a key to separating the Cenomanian from the Turonian of France . In Marliere’s own words : ‘‘Il existe plus haut un niveau a ̀ Anomia papyracea , remarque ́ en 1924 par Rene ́ Dehe ́ e ... imme ́ diatement sous les marnes a ̀ Inoceramus labiatus se trouve un petit lit (1 cm d’e ́ paisseur) a ̀ nombreuses valves d’ Anomia papyracea ... et , avec quelques variantes quant a l’abondance ou la fre ́ quence d’ Anomia , dans un grand nombre de sondages du Basin de Mons . ... En conside ́ ration de ces faits constants , et a ̀ de ́ faut d’autres arguments , j’ai propose ́ d’utiliser le niveau a ̀ Anomia papyracea comme sepparant le macrofaunes ce ́ nomaniennes ( In . crippsi reachensis , In . pictus ) des macrofaunes turoniennes ( In . labiatus ) . ’’

Very similar anomiid morphotypes , Anomia cobbani and A . pfeiferensis , des- cribed from the Western Interior (Kansas) by Hasenmueller & Hattin (1990) , were collected from the early Turonian Jetmore and Pfeifer Members of the Green- horn Limestone . Hasenmueller & Hattin (1990) did not discuss the importance of these bivalves in the recognition of the approximate position of the base of the Turonian because the described anomiids were found slightly above the Cenomanian – Turonian boundary (Hasenmueller & Hattin , 1990 , their Figure 1) .

Small , abundant anomiid bivalves in northern South America are consistently associated with Lower Turonian fossils such as Wrightoceras munieri , Mytiloides kossmati , Vascoceras propium , Mammites powelli , Hoplitoides spp ., and others and therefore can aid in recognition of the Cenomanian – Turonian boundary and serve as important biostratigraphic fossils for the division of the Turonian Stage in Colombia , Venezuela and probably adjacent countries such as Ecuador , Trinidad , and Guyana . There are multiple advantages of using small anomiids as

T . Villamil 624

fossils for recognition of the base of the Turonian in northern South America : (1) the described and discussed northern South American anomiid bivalves are generally well preserved , they are widely distributed , and occur in large numbers ; anomiids are therefore hard to miss when studying a stratigraphic section ; and (2) these anomiids are small enough that they can be recovered in large numbers from well cuttings or cores , and therefore can be used as microfossils for the recognition of the base of the Turonian in the subsurface .

6 . Systematic paleontology

Order Pterioidea Newell , 1965 Suborder Pteriina Newell , 1965 Superfamily Anomiacea Rafinesque , 1815 Family Anomiidae Rafinesque , 1815 Genus Anomia Linne ́ , 1758

Anomia minuta n . sp . Figure 10A – E

Diagnosis . Left valve very small , very thin , equilateral ; outline ranges from nearly circular to slightly elliptical ; dorsal margin slightly arched to arched , half the shell length ; posterodorsal , ventral , and anterior margins form a nearly uniform curve ; left valve moderately convex , ornamented only with fine , equally spaced growth lines ; beak orthogyrous , ranging from slightly prosogyrous to slightly opisthogyrous .

Description . Small to very small for genus (1 – 4 mm in diameter , Figures 10 , 11) , outline nearly circular with slight posterior elongation , nearly equilateral . Dorsal margin straight to slightly arched , other margins convex , nearly continuous semicircular curve . Left valve moderately convex ; width to length ratio 1 : 5 ; maximum point of convexity in mid-shell . Beak slightly prosogyrous to slightly opisthogyrous , generally orthogyrous , projecting perpendicular to hinge only fractions of mm . Valve height varies from 1 – 5 mm (average 3 . 2 mm , Figure 11) ; length varies from 1 . 3 – 6 mm ; valve width , measured from the plane of the commissure to the point of maximum inflation , varies between 1 – 2 mm . Dorsal margin nearly straight ; beak subcentral on hinge line ; anterior , ventral , and posterior margins form a continuous , uninterrupted curve . Plane of commissure flat or nearly flat , reflecting curvature of substrate vs . anomiid size . Surface of left valve marked only by equally spaced , faint , regular growth lines . Hinge structure , muscle scars unknown . Shell of laminated calcite , parallel or sub parallel to shell surface .

The holotype of Anomia minuta (Figure 10C) is the largest specimen found , it measures 6 mm in length , 5 mm in height , and 1 . 9 mm in width . The holotype is rounded and very smooth with only minor and faint concentric growth rings ornamenting the shell .

Comparison with similar species . Anomia minuta dif fers from Anomia papyracea in being smaller , less concave and ornamented with fine growth lines . It dif fers from A . colombiana in lacking strong radial striae and in being smaller and more elongated with a higher length to height ratio . It dif fers from A . pfeiferensis and A . cobbani Hasenmueller & Hattin , 1990 in being smaller and ornamented with evenly-spaced , fine , and faint growth lines ; the shell is paper thin ; and A . minuta

Figure 10 . Anomia minuta n . sp . Photographs illustrating the shell characteristics and the manner of occurrence ; all specimens are left valves . A , nearly complete specimen , paratype UCM-32479 of the Fossil Invertebrate Collection , University of Colorado ; B , paratype UCM-32482a ; C , holotype UCM-32482 . D , Anomia minuta associated with A . sp . cf . A . papyracea , A . colombiana and juvenile inoceramids of the genus Mytiloides . E , Anomia minuta associated with juvenile smooth ammonites .

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Figure 11 . Graph of height vs . length of the bivalves Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea compared to the Western Interior species A . cobbani and A . pfeiferensis of Hasenmueler & Hattin (1990) . The graph represents only the dif ferences in size of the five species of Anomia . Dif ferences in ornamentation and in ratios of length : height and length : width cannot be detected from this diagram but are discussed in the text .

is a less convex morphotype . A . minuta dif fers from A . subquadrata Stanton , 1893 , A . micronema Meek , 1875 , A . aurea Warren & Stelck , 1940 , and A . gryphorhynchus Meek , 1872 from the Cretaceous of the Western Interior of the US (see Meek , 1872 , 1875) in that A . minuta is a much smaller species ; A . micronema and A . gryphorhynchus also have faint radial striae . Figure 11 shows the dif ferences in size , particularly height and length , of Anomia minuta compared to similar species .

Type locality . The basal Turonian strata of the Yaguara ́ River stratigraphic section (2 8 53 9 30 . 81 0 N , 5 8 40 9 32 . 43 0 W) (Figure 3a) comprise the type locality for this small anomiid bivalve . In this stratigraphic section , A . minuta is found forming relatively deep-water , gray – silver , bioclastic limestones composed entirely of anomiids .

Etymology . The trivial name refers to the very small size of these anomiid bivalves .

Material . The collection of A . minuta is composed of hundreds to thousands of miniature specimens that form a 2 . 8-cm-thick bioclastic limestone from the Yaguara ́ River stratigraphic section , hundreds of specimens collected from calcareous concretions in the Olini section , and scattered specimens collected from other regions of Colombia and Venezuela . The holotype and paratypes illustrated here are part of that of fshore bioclastic limestone bed from the Yaguara ́ section . Many more specimens of A . minuta were studied from other regions of Colombia and Venezuela shown in Figures 1 – 4 .

Stratigraphic range and associated taxa . Anomia minuta is restricted to the initial

Paleobiology of two new species of the bivalve Anomia 627

stages of the early Turonian as evidenced by its association with the following macro- and microfauna : Pseudaspidoceras nodosum , Watinoceras reesidei , Watinoceras coloradoense , Mammites powelli , Glyptoxoceras sp ., Vascoceras proprium , Mytiloides mytiloides , Mytiloides kossmati , abundant Heterohelix globulosa , and Whiteinella archaeocretacea .

Repository . Material used for the description of Anomia minuta is deposited at the University of Colorado Museum (UCM) Fossil Invertebrate Collection , Boulder , Colorado .

Anomia colombiana n . sp . Figure 12A – E

Diagnosis . Left valve small and very thin , equilateral ; outline nearly circular to slightly elliptical elongated parallel to the beak ; dorsal margin slightly arched to straight , more than half the shell length ; posterodorsal , ventral , and anterior margins define a nearly uniform curve that forms a tear drop ; left valve slightly convex , ornamented with fine equally-spaced growth lines and radial ribs ; beak orthogyre .

Description . Small for the genus (4 – 8 mm in maximum length , Figures 11 , 12) , outline nearly circular , slightly elongated in the dorsal – ventral sense , almost equilateral . Dorsal margin straight ; other margins convex forming a semicircle or a nearly-continuous curve with a tear-drop shape . Left valve low to moderately convex , width to length ratio of 1 : 7 with the maximum point of convexity slightly above mid-shell height . Beak nearly orthogyre , extending perpendicular to the hinge structure only fractions of mm . Valve height between 3 – 7 mm (average 4 mm) , length between 3 – 6 mm (Figure 11) , valve width , from commisural plane to point of maximum inflation , is between 1-3 mm . Dorsal margin nearly straight with beak located near hinge line center ; anterior , ventral , and posterior margins form a continuous , uninterrupted curve . Plane of commissure flat . Surface of left valve ornamented with equally-spaced , faint , regular growth lines and radial striae . Hinge structure and muscle scars unknown .

The holotype of Anomia colombiana (Figure 12C) is 6 . 2 mm in length , 7 . 1 mm in height , and 2 . 4 mm in width from the commissure to the point of maximum inflation . The holotype exhibits robust and broad radial ribs and concentric rugae superimposed on very fine and faint concentric growth rings (Figure 12C) .

Comparison with similar species . Anomia colombiana dif fers from A . papyracea in being less concave and ornamented with well defined radial striae . It dif fers from A . pfeiferesis and A . cobbani Hasenmueller & Hattin , 1990 in being smaller and ornamented with well defined radial striae ; it is less elongated and less convex than the species from Kansas . A . colombiana dif fers from A . subquadrata Stanton , 1893 , A . micronema Meek , 1875 , A . aurea Warren & Stelck , 1940 , and A . gryphorhynchus Meek , 1872 from the Cretaceous of the Western Interior of the US in being smaller , and is ornamented with well defined radial striae . Figure 11 shows a height-length graph with Anomia colombiana compared to all other similar species .

Type locality . The basal Turonian strata of the Mesitas del Colegio stratigraphic section (4 8 32 9 25 . 95 0 N , 74 8 26 9 45 . 41 0 W) (Figures 1 , 3B) are the type locality for this small anomiid bivalve , which occurs there in large numbers and is generally well preserved .

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Figure 12 . Specimens and mode of occurrence of the bivalve Anomia colombiana n . sp . A , large paratype UCM-32480 from the Olini stratigraphic section ; B , D , E , occurrence of A . colombiana in shales (B) and calcareous concretions (D , E) , and with A . sp . cf . A . papyracea (E) . C , holotype UCM-32481 .

Paleobiology of two new species of the bivalve Anomia 629

Etymology . The trivial name is derived from the wide distribution of this species in the early Turonian of Colombia .

Material . Anomia colombiana is an abundant species in all of northern South America . It is easy to collect thousands of A . colombiana from any given section in Colombia and Venezuela .

Stratigraphic range and associated taxa . Anomia colombiana is restricted to the early Turonian as evidenced by its association with the following macro- and microfauna : Fagesia spp ., Pseudaspidoceras nodosum , Watinoceras reesidei , Watinoceras coloradoense , Mammites powelli , Glyptoxoceras sp ., Vascoceras proprium , Mytiloides mytiloides , Mytiloides labiatus , Mytiloides kossmati , abundant Heterohelix globulosa , and Whiteinella archaeocretacea .

Repository . Material used for the description of Anomia colombiana is deposited at the University of Colorado Museum (UCM) , Fossil Invertebrate Collection , Boulder , Colorado .

Anomia sp . cf . A . papyracea (Dehee ́ , 1924?) Figure 13A – D

Remarks . Anomia sp . cf . A . papyracea is an abundant anomiid bivalve in the lower and middle Turonian of Colombia and Venezuela ; it generally occurs in hemipelagic limestones or in calcareous concretions and may also be an indirect paleoenvironmental indicator of firm substrates . It is relatively well preserved in highly calcareous shales (Figure 13D) in comparison with A . minuta and A . colombiana , which are generally very dif ficult to recognize . Anomia sp . cf . A . papyracea is a larger shell than A . minuta and A . colombiana (Figure 11) ; it is also more elongated than A . minuta and does not show the characteristic radial ornamentation of A . colombiana .

7 . Conclusions

The new species of anomiid bivalves described herein are of biostratigraphic importance in the recognition of the Lower Turonian Stage in northern South America . Anomia minuta , A . colombiana , and A . sp . cf . A . papyracea are widely distributed species that have a narrow biostratigraphic range , are well preserved , easy to identify , and are found in large numbers . The small size of these anomiids allows recognition and identification of these bivalves in cores and well cuttings .

The widespread occurrence of anomiid bivalves in the Lower Turonian of Colombia and Venezuela was probably caused by special paleoenvironmental conditions . The most reasonable explanation could have been the development of regionally-distributed firm marine substrates associated with sediment starvation and stratigraphic condensation during a rapid sea-level rise . Increased paleo- productivity in the upper water column contributed to firmer sea bottom surfaces as of f the Peruvian continental margin today and probably hardening associated with early silica , phosphate , and / or calcite cementation . Stratigraphic , chemical , paleobiological , and lithological evidence of condensation and oceanic paleo- productivity was presented for the Cenomanian – Turonian boundary interval in Colombia by Villamil & Arango (in press) . Condensation and associated hardening of the substrate was caused by the late Cenomanian sea-level rise which occurred stratigraphically as a series of abrupt transgressive pulses leaving vast regions of the sea bottom starved of sediment for long periods of time . Long

T . Villamil 630

Figure 13 . Specimens and mode of occurrence of the bivalve Anomia sp . cf . A . papyracea . A , B , typical specimens . C , D , mode of occurrence in calcareous concretions and calcareous shales , respectively .

Paleobiology of two new species of the bivalve Anomia 631

periods of exposed , firm sea bottom allowed colonization and accumulation of large numbers of anomiid bivalves and permitted the formation of relatively deep-water coquinoid bioclastic limestones in the Turonian of northern South America . Similar conditions could be invoked to explain the presence of similar anomiid morphotypes in other regions of the world , such as coeval facies in the Bridge Creek Limestone of Kansas and the base of the Turonian in France .

Acknowledgments

Claudia Arango provided all the information on microfossils and contributed helpful discussions . Field work was supported by BHP Utah International , through B . Bolton and P . Okita , and by Chevron Overseas Petroleum Inc ., through C . Mann . Reviews by Erle G . Kauf fman and Jared Morrow markedly improved the quality of the original manuscript . Two anonymous reviewers and the editor of the journal also provided helpful comments .

References

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Kauf fman , E . G . & Sageman , B . B . 1990 . Biological sensing of benthic environments in dark shales and related oxygen-restricted facies . In Cretaceous resources , events and rhythms (eds Ginsburg , N . R . & Beaudoin , B . ) pp . 121 – 138 (Kluwer , Dordrecht) .

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Kauf fman , E . G ., Villamil , T ., Harries , P . J ., Meyer , C . A . & Sageman , B . B . 1992 . The flat clam controversy : where did they come from? Where did they go? Fifth North American Paleontological Convention , Paleontological Society Special Publication 6 , 159 .

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Meek , F . B . 1872 . Preliminary list of fossils collected by Dr . Hayden’s exploring expedition of 1871 , in Utah and Wyoming territories , with descriptions of a few new species . In Preliminary report of the United States Geological Survey of Montana and portions of adjacent territories , United States Geological Survey of the Territories , 5 th Annual Report of Progress (ed . Hayden , F . V) , pp . 373 – 377 .

Meek , F . B . 1875 . Notes on some fossils from near the eastern base of the Rocky Mountains , west of Greeley and Evans , Colorado , and others from about two hundred miles farther eastward , with descriptions of a few new species . Bulletin of the United States Geological and Geographical Survey of the Territories , 2 nd Series 1 , 39 – 47 .

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Villamil , T . & Arango , C . (in press) . High-resolution analysis of the Cenomanian – Turonian boundary in Colombia : evidence for sea-level rise , condensation , and upwelling . In Mesozoic – Cenozoic stratigraphy and tectonic evolution of the Caribbean region / northern South America : implications for eustasy from exposed sections of a Cretaceous – Eocene passive margin setting (eds Pindell , J . L . & Drake , C . ) , SEPM Special Publication .

Villamil , T . (in press) . Relative sea level , chronology , and a new sequence stratigraphic model for distal of fshore facies , Albian to Santonian , Colombia . In Mesozoic – Cenozoic stratigraphy and

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tectonic evolution of the Caribbean region / northern South America : implications for eustasy from exposed sections of a Cretaceous – Eocene passive margin setting (eds Pindell , J . & Drake , C . ) , SEPM Special Publication .

Villamil , T . 1995 . Un modelo tecto ́ nico-paleoceanogra ́ fico y estratigra ́ fico secuencial para el intervalo de roca generadora del Creta ́ cico del norte de Sur Ame ́ rica . Memorias VI Congreso Colombiano del Petro ́ leo 1 , 53 – 56 .

Appendix : register of localities

Stratigraphic sections were measured in the Cretaceous outcrop belt of central Colombia and in the Eastern Basin of Venezuela (Figure 1) . Distal facies were measured in the southern part of the Upper Magdalena Valley near the towns of Yaguara ́ and Chaparral as well as near the town of Bergantı ́ n in the Eastern Basin of Venezuela . The Yaguara ́ section (2 8 53 9 30 . 81 0 N , 5 8 40 9 32 . 43 0 W) was measured in the Yaguara ́ River , upstream towards the confluence of the Iquira River (Department of Huila) . The Olini reference section (3 8 40 9 32 . 43 0 N , 75 8 32 9 25 . 95 0 W) was measured in Olini Creek half way between Chaparral and Coyaima (Department of Tolima) . The Querecual section was measured in its type locality the Querecual River ( < 10 . 28 o N , < 63 . 81 8 W) . The interval that contains the anomiid bivalves discussed here is located approximately 135 m above the base of this section . Two sections were measured in the Eastern Cordillera in the Department of Cundinamarca : one near the town of Mesitas del Cole ́ gio on the western flank of the Eastern Cordillera and one near Chipa ́ que , on the eastern flank of the Eastern Cordillera . The Mesitas del Colegio section (4 8 32 9 25 . 95 0 N , 74 8 26 9 45 . 41 0 W) was measured at km 28 on the road leading from Bogota ́ to Mesitas del Cole ́ gio , along an unnamed creek . The Chipa ́ que section (4 8 26 9 45 . 41 0 N , 74 8 4 9 3 . 24 0 W) was measured in another unnamed creek , the first encountered to the south of Chipa ́ que .

Sections representing proximal facies were also measured within the Eastern Cordillera but to the north of the general area described above . The Villa de Leiva section (5 8 34 9 3 . 24 0 N and 73 8 26 9 45 . 41 0 W) was measured between Villa de Leiva and Samaca ́ , near the intersection of the main road Tunja-Villa de Leiva and the road leading to Samaca ́ (Department of Boyaca ́ ) , in the type locality for the San Rafael Formation which is located near the ‘Chircal San Rafael’ (San Rafael brick kiln) . Further north , the Cocuy section (6 8 31 9 37 . 3 0 N , 72 8 42 9 9 . 73 0 W) was measured between the towns of Capitanejo and Ma ́ laga (Department of Santander) , in an ephemeral stream east of the road . Localities where the sections were measured or visited are illustrated in Figure 1 .