BRACHIOPODA FROM THE SOOM SHALE LAGERSTA ¨ TTE (UPPER ORDOVICIAN, SOUTH AFRICA) MICHAEL G. BASSETT, 1 LEONID E. POPOV, 1 RICHARD J. ALDRIDGE, 2 SARAH E. GABBOTT, 2 AND JOHANNES N. THERON 3 1 Department of Geology, National Museum of Wales, Cardiff, CF10 3NP, Wales, U.K., ,[email protected]., ,[email protected].; 2 Department of Geology, University of Leicester, Leicester, LE1 7RH, U.K., ,[email protected]., ,[email protected].; and 3 Department of Geology, University of Stellenbosch, Private Bag XI, Stellenbosch 7602, South Africa ABSTRACT—Within the Soom Shale Lagersta ¨ tte of South Africa (Upper Ordovician, Hirnantian), two brachiopod taxa preserve traces of organic tissue. In Trematis, presumed bands of periostracum are preserved on the flanks of the pedicle notch, and clay mineral casts of the pedicle are preserved in many specimens of Kosoidea. Both these genera are organophosphatic-shelled Linguliformea, together with a third genus identified as Plectoglossa. A fourth brachiopod taxon in the fauna belongs within the calcitic-shelled Rhynchonelliformea (Plectothyrella). Kosoidea cedarbergensis is a new species of discinoidean. INTRODUCTION T HE UPPER Ordovician (Hirnantian) Soom Shale Member of the lower Cedarberg Formation in Cape Province, South Africa is now well known as a Lagersta ¨ tte containing a diverse fossil biota in which many specimens retain varying traces of exceptional soft tissue preservation. Complex processes of replication are involved in the different fossil groups. For background details and further references to the overall stratigraphy, biodiversity and taphonomy of the Soom Shale see, for example, Aldridge and Theron (1993), Aldridge et al. (1994, figs. 1, 2; 2001), and Gabbott (1998, 1999). Brachiopods have been reported previously from the Soom Shale as isolated specimens, as clusters on cephalopod shells (Gabbott, 1999), and as fragmented specimens within the bromalites produced by durophagous predators (Aldridge et al., 2006). Brachiopods from the overlying Disa Member of the Cedarberg Formation were reviewed and described by Cocks et al. (1970) and Cocks and Fortey (1986); see also Rong and Harper (1988, p. 388). We have also studied this comparative material at the Department of Palaeontology, Natural History Museum, London (NHM). This paper addresses the biology, taxonomy, ecology and biogeography of the brachiopods in the Soom Shale fauna, comprising both originally organophosphatic- shelled and organocarbonatic-shelled stocks. These are of particular interest not only because some of the material preserves details of soft tissue but also because of their relevance to the globally widespread Upper Ordovician Hirnantia Fauna. SYSTEMATIC PALEONTOLOGY Described and figured specimens are from the Soom Shale Member of the Cedarberg Formation at Keurbos Farm, some 11.5 km south-southeast of Clanwilliam, and at Krom- rivier, 48 km southeast of Clanwilliam, Cape Province, South Africa (Aldridge et al., 2006, text-fig.1). The material is deposited in the Council of Geosciences of South Africa, Bellville. Order LINGULIDA Waagen, 1885 Superfamily LINGULOIDEA Menke, 1828 Family OBOLIDAE King, 1846 Genus PALAEOGLOSSA Cockerell, 1911 Type species.—Lingula attenuata Sowerby, 1839; Rorring- ton Formation (Llandeilo-Costonian), Rorrington, Shelve, Shropshire, England. PALAEOGLOSSA sp. Figure 1.1–1.6 Material.—One ventral (C1716A, B) and two dorsal valves (C467A, B; C2043AI, BI), plus unfigured ventral valve C2043AII, BII). All specimens from Keurbos. Remarks.—Small lingulide shells referred here to Palaeo- glossa sp. are characterized by an elongate suboval outline and a crescent-shaped undivided dorsal interarea at the level of the valve floor. They are probably juveniles and lack specifically diagnostic characters in the interiors. The shell surface is covered only by fine, closely spaced growth lines without any trace of pitting on the larval or postlarval shell. They are generally comparable with Palaeoglossa or Lingulella, but probably represent the former genus. Rowell (in Cocks et al., 1970) described four specimens of a lingulid from the Disa Member as ?Plectoglossa sp., but these specimens are more strongly biconvex and have considerably coarser concentric growth ornament than the Soom Shale specimens, and we regard them as being generically different. Superfamily DISCINOIDAE Gray, 1840 Family TREMATIDAE Schuchert, 1893 Genus TREMATIS Sharpe, 1848 Type species.—Orbicula terminalis Emmons, 1842, from the Trenton Group (Caradoc), New York, U.S.A. R Figure 1—1–6, Palaeoglossa sp.: 1, 5, C2043AI, BI; dorsal valve interior and internal mould, 315. 2, 6, C1761A, B; ventral valve interior and internal mould, 315; 3, 4, C467A, 467B dorsal valve exterior and external mould, 315; 7–12, Kosoidea cedarbergensis sp. nov.: 7, C1177A, cluster of mainly articulated valves possibly representing a single generation, 32; 8, C1371A, small cluster of conjoined valves representing two different generations, 35.4; 9–11, C850B, digitally inverted view of dorsal umbonal area, 36, dorsal external mould, 32.7, detail of ornament, 36.7; 12, C934A, dorsal valve exterior, 36. All specimens are from the Upper Ordovician, Hirnantian, Soom Shale Member, Cedarberg Formation, South Africa; 1–6, 9–12, from Keurbos; 7–8, from Kromrivier. J. Paleont., 83(4), 2009, pp. 614–623 Copyright ’ 2009, The Paleontological Society 0022-3360/09/0083–0614$03.00 614
Transcript
BRACHIOPODA FROM THE SOOM SHALE LAGERSTATTE(UPPER ORDOVICIAN, SOUTH AFRICA)
MICHAEL G. BASSETT,1 LEONID E. POPOV,1 RICHARD J. ALDRIDGE,2 SARAH E. GABBOTT,2 AND
JOHANNES N. THERON3
1Department of Geology, National Museum of Wales,Cardiff, CF10 3NP, Wales, U.K., ,[email protected]., ,[email protected].; 2Department of Geology,
University of Leicester, Leicester, LE1 7RH, U.K., ,[email protected]., ,[email protected].; and 3Department of Geology, University of Stellenbosch,Private Bag XI, Stellenbosch 7602, South Africa
ABSTRACT—Within the Soom Shale Lagerstatte of South Africa (Upper Ordovician, Hirnantian), two brachiopodtaxa preserve traces of organic tissue. In Trematis, presumed bands of periostracum are preserved on the flanks ofthe pedicle notch, and clay mineral casts of the pedicle are preserved in many specimens of Kosoidea. Both thesegenera are organophosphatic-shelled Linguliformea, together with a third genus identified as Plectoglossa. A fourthbrachiopod taxon in the fauna belongs within the calcitic-shelled Rhynchonelliformea (Plectothyrella). Kosoideacedarbergensis is a new species of discinoidean.
INTRODUCTION
THE UPPER Ordovician (Hirnantian) Soom Shale Memberof the lower Cedarberg Formation in Cape Province,
South Africa is now well known as a Lagerstatte containing adiverse fossil biota in which many specimens retain varyingtraces of exceptional soft tissue preservation. Complexprocesses of replication are involved in the different fossilgroups. For background details and further references to theoverall stratigraphy, biodiversity and taphonomy of the SoomShale see, for example, Aldridge and Theron (1993), Aldridgeet al. (1994, figs. 1, 2; 2001), and Gabbott (1998, 1999).Brachiopods have been reported previously from the SoomShale as isolated specimens, as clusters on cephalopod shells(Gabbott, 1999), and as fragmented specimens within thebromalites produced by durophagous predators (Aldridge etal., 2006). Brachiopods from the overlying Disa Member ofthe Cedarberg Formation were reviewed and described byCocks et al. (1970) and Cocks and Fortey (1986); see alsoRong and Harper (1988, p. 388). We have also studied thiscomparative material at the Department of Palaeontology,Natural History Museum, London (NHM).
This paper addresses the biology, taxonomy, ecologyand biogeography of the brachiopods in the Soom Shalefauna, comprising both originally organophosphatic-shelled and organocarbonatic-shelled stocks. These are ofparticular interest not only because some of the materialpreserves details of soft tissue but also because of theirrelevance to the globally widespread Upper OrdovicianHirnantia Fauna.
SYSTEMATIC PALEONTOLOGY
Described and figured specimens are from the SoomShale Member of the Cedarberg Formation at Keurbos Farm,some 11.5 km south-southeast of Clanwilliam, and at Krom-rivier, 48 km southeast of Clanwilliam, Cape Province, SouthAfrica (Aldridge et al., 2006, text-fig.1). The material is
deposited in the Council of Geosciences of South Africa,Bellville.
Order LINGULIDA Waagen, 1885Superfamily LINGULOIDEA Menke, 1828
Family OBOLIDAE King, 1846Genus PALAEOGLOSSA Cockerell, 1911
Material.—One ventral (C1716A, B) and two dorsal valves(C467A, B; C2043AI, BI), plus unfigured ventral valveC2043AII, BII). All specimens from Keurbos.
Remarks.—Small lingulide shells referred here to Palaeo-glossa sp. are characterized by an elongate suboval outline anda crescent-shaped undivided dorsal interarea at the level of thevalve floor. They are probably juveniles and lack specificallydiagnostic characters in the interiors. The shell surface iscovered only by fine, closely spaced growth lines without anytrace of pitting on the larval or postlarval shell. They aregenerally comparable with Palaeoglossa or Lingulella, butprobably represent the former genus.
Rowell (in Cocks et al., 1970) described four specimens of alingulid from the Disa Member as ?Plectoglossa sp., but thesespecimens are more strongly biconvex and have considerablycoarser concentric growth ornament than the Soom Shalespecimens, and we regard them as being generically different.
1970 Trematis taljaardi ROWELL in COCKS ET AL., p. 589, pl.39, figs. 3–9.
Holotype.—NHM BB31504, partly exfoliated ventral valve,with a margin of associated dorsal valve; Upper Ordovician,Ashgill, Hirnantian, Cedarberg Formation, Langvlei, Pam-poenfontein farm near Porterville, South Africa.
Material.—Four dorsal valves, three ventral valves (speci-mens C411A; C412A, C764A, B; C975A, B; C1371C, D;C1629A, B; C1703C; C1994), plus several tens of unfiguredspecimens.
Description.—Shell convexoplane, subcircular in outline,about 80% as long as wide. Ventral valve almost flat, witha depressed area posterior to the umbo around the pediclenotch; this area lacks characteristic pitted micro-ornamentand has only growth lines preserved. Umbo situated about20–25% of length from the posterior margin. Pedicle notchslightly elongate, subtriangular, up to 5 mm long in the largestspecimens; it is covered partly by a minute listrium andnarrow, inclined, smooth periostracal bands attached to thelateral sides of the notch (Fig. 2.1–2.2).
Lateral profile of the dorsal valve gently convex withmaximum height about one quarter of valve length from themarginal umbo. Shell surface covered by fine, bifurcating ribsvarying from 12 to 16 per 3 mm along the anterior margin.Interspaces between the ribs occupied by rows of fine,hemispherical pits about 100 to 150 mm across, slightlyelongated in the umbonal area, gradually becoming transversedistally. Concentric ornament of fine growth lines visible onsmooth parts of the shell posterior to the umbo, together withrare concentric lamellae.
Ventral interior lacking distinctive features. Dorsal interiorwith a small visceral area occupying posterior quarter of the
valve. Within the musculature only the attachments of centraland oblique lateral muscles can be recognized (Fig. 3.12).
Dimensions (in mm):
Remarks.—Notwithstanding their significantly larger size,which exceeds up to three or four times the size of the shellsdescribed by Rowell (in Cocks et al., 1970) there is no doubtthat the specimens in our collection are conspecific with theholotype because they show similar ornament, position of theventral umbo and characters of the pedicle notch. Pittedornament of both valves comprises radial rows of pits betweenfine ribs and can be referred to the type ‘‘b’’ of Wright (1963, fig.5b), but in the parts of the shell repaired after damageof the mantle margin, it may be transformed into a‘‘honeycomb’’ pattern (type ‘‘d’’). Size of pits increases withdistance from the umbo and they become increasinglytransverse in outline. Affinities of Trematis taljaardi werediscussed in detail by Rowell (in Cocks et al., 1970). The closestspecies to Trematis taljaardi, and possibly the only contempo-raneous species of the genus, is Trematis norvegica Cocks, 1982,from the Hirnantian Langøyene Formation of the Oslo Regionin Norway. Both species are among the largest known for thegenus; they have a similar type of pitted ornament between fineribs, and a pedicle notch of similar shape and size surroundedby bands of the shell lacking pitted ornament. The onlydifferences are the strongly depressed area of the shell posteriorto the umbo and finer radial ornament in Trematis taljaardi.
FIGURE 2—1–3, Trematis taljaardi Rowell. C1994, from Keurbos, external mould of ventral valve showing variations in the character of pittedornament, 34.5; umbonal area showing pedicle notch, 314; enlarged shell surface showing pitted ornament, 335.
Figure 3—1–12, 14, Trematis taljaardi Rowell. 1, 4, C764B, C764A, dorsal valve exterior and external mould, 32.5; 3, 5, 6, C1703C, ventral valve; 2,cast of pitted microornament in the middle part of the valve, 332; 3, details of pedicle notch showing periostracal bands (pb), 355; 5, external mould; 6,details of pitted microornament in distal part of the valve, 338; 7, C975A, dorsal valve exterior, 33, pitted microornament, 312; 8, C412A, inverteddorsal external mould, 32.5; 9, 12, C411A, dorsal internal mould, 32.7, details of umbonal area showing position of posterolateral muscle fields (plm),central muscle scars (cm), oblique lateral muscle scars (ol), 37.3; 10, 11, 14, C1629B, C1629A, ventral valve external mould and counterpart exterior, 33,digitally inverted photograph of umbonal area of ventral internal mould showing ornament and periostracal bands partly enclosing pedicle notch, 37;13, Plectothyrella sp. C1127B, dorsal external mould, 37. All specimens are from the Upper Ordovician, Hirnantian, Soom Shale Member,CedarbergFormation, South Africa. Figs 1–12, 14 from Keurbos; 13 from Kromrivier.
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Family DISCINIDAE Gray, 1840Subfamily ORBICULOIDEINAE Schuchert in Schuchert and
LeVene, 1929Genus KOSOIDEA Havlıcek and Mergl, 1988
Type species.—Kosoidea fissurella HAVLICEK AND MERGL,1988, Silurian, Ludlow, Kopanina Formation, Bohemia.
KOSOIDEA CEDARBERGENSIS new speciesFigures 1.7–1.12; 4.1–4.6; 5; 6.1–6.8
1970 Orbiculoidea sp.; ROWELL, p. 591, pl. 39, figs. 11–161998 Orbiculoids; GABBOTT, p. 6431999 Orbiculoids; GABBOTT, p. 137, pl. 4, figs. 4, 5
Derivation of name.—After the Cedarberg Formation wherethis species is one of the most common fossils.
Holotype.—C1723A and C1723B, ventral valve and coun-terpart external mould, from Keurbos.
Paratypes.—Clusters C1177A (Kromrivier); C1371A-D(Kromrivier); C1390 A, B (Kromrivier); isolated specimensC850IB (Keurbos); C934A (Keurbos). Plus several hundredspecimens, isolated and in clusters; mostly conjoined valves.
Diagnosis.—Convexoplane subcircular shell. Ventral umboex-centric, at about one-third of valve length from theposterior margin; pedicle notch elongated, semielliptical witha small, umbonal listrium; ventral posterior margin withincision between a pair of ridges diverging from the umbo atsome distance from the lateral sides of the notch. Dorsal valveweakly convex with submarginal umbo. Concentric ornamentof strong, ridge-like rugellae separated by wide interspaces andinterrupted between ridges posterior to the ventral umbo.
Description.—Shell convexoplane, almost circular in out-line, about 90 percent as long as wide. Ventral valve almostflat, with excentrically placed umbo at about one third of valvelength from the posterior margin. Pedicle notch stronglyelongated, semielliptical, covered umbonally by a very short,concave listrium, with subparallel lateral margins convergingdistally, but not merging together. Posterior margin with acharacteristic subtriangular incision between two fine ridgesdiverging from the umbo. The region between the ridges is anarea of shell without concentric ornament, which presumablyrepresents casts of unmineralised bands of periostracum.
Dorsal valve weakly convex with submarginal umbocorresponding to the highest point of the valve. Surfaceornament of strong, ridge-like concentric rugellae separated bywide interspaces that bear very fine growth lines.
Ventral interior with a large, subtriangular, anterior musclefield situated in the middle of the valve anterior to the pedicleopening. Musculature comprises mainly anterior adductorscars combined centrally with small scars of oblique internalmuscles. Ventral oblique posterior muscles comprising a pairof small, rounded scars in the posterolateral corners of theanterior ventral muscle field. Ventral posterior adductor scarssituated on both sides of the pedicle foramen. Ventral obliquelateral muscle scars poorly defined, possibly situated laterallyof posterior adductor scars (Fig. 5).
Dorsal interior with weakly impressed, elongate subovalvisceral area; anterolateral areas of visceral region occupied bylarge, elongate suboval anterior adductor muscle scarscombined with anterior lateral scars. Posterior periphery ofthe dorsal visceral area occupied by oblique internal andoblique posterior muscle scars situated on both sides ofsomewhat larger posterior adductor scars (Fig. 5).
Remarks.—This is the most abundant brachiopod taxon inthe Soom Shale, numbering over some hundreds of specimensin the collections. It has been referred to previously asOrbiculoidea (from the Disa Member) and as ‘Orbiculoids’ (seesynonymy). There is no doubt that it is a discinid related toOrbiculoidea, but it differs in significant ways. First, Orbicu-loidea species are generally more strongly biconvex withsubcentral umbones and with relatively strong concentric fila.The pedicle notch of Kosoidea is strongly elongated, extendingto the posterior margin, and in Orbiculoidea the foramen isrelatively short, but is continued internally as a tube thatopens close to the posterior margin.
Kosoidea has been recorded previously only from upperSilurian (Ludlow) rocks in Bohemia (Havlicek and Mergl1988; Mergl 2001), but the Soom Shale Ordovician specimensare sufficiently similar as to warrant confidence in the genericassignment. This species differs from Kosoidea fissurellaHavlıcek and Mergl, 1988 in having a very short listriumand a pair of ridges that originate at the umbo and divergelateral to the pedicle notch. Unlike the Bohemian species, theridges enclose parts of the shell completely lacking concentricornament, and the Soom Shale shells also have a moremarginal position of the dorsal umbo.
The new South African species is somewhat similar toSchizotretina euxina Havlıcek, 1994, from the upper AshgillKosov Formation of Bohemia in having a submarginaldorsal umbo, as well as in characters of concentric ornament,lateral profile and outline of both valves, but it is readilydistinguished in having a pedicle notch that is openposteriorly, with a smooth shell between the two ridgesposterior to the ventral umbo.
Order RHYNCHONELLIDA Kuhn, 1949Superfamily RHYNCHOTREMATOIDEA Schuchert, 1913
Family RHYNCHOTREMATIDAE Schuchert, 1913Genus PLECTOTHYRELLA Temple, 1965
Type species.—Atrypa? crassicostata Dalman, 1828, p. 131;Upper Ordovician (Ashgill, Hirnantian, Dalmanitina Beds),Vastergotland, Sweden [Subjective senior synonym of Plec-tothyrella platystrophoides Temple, 1965, p. 412; UpperOrdovician (Ashgill Shales) of Hol Beck, near Ambleside,Westmorland, England].
PLECTOTHYRELLA sp.Figures 3.13; 4.7
Material.—External and internal moulds of two dorsal andtwo ventral valves (C1127A, B; C1177I-IIIA, B); all from
R
Figure 4— 1, C1371C, cluster of mainly articulated valves of Kosoidea cedarbergensis sp. nov. and large disarticulated ventral valve of Trematistaljaardi Rowell (left), 33; 2–6, Kosoidea cedarbergensis sp. nov.; 2, 4, C1723A, C1723B, holotype, ventral valve external mould and counterpart exterior,showing fossilized pedicle (p) and periostracal bands (pb) on lateral sides of pedicle notch, 35; 3, C1371A, dorsal valve internal mould (left) and dorsalvalve exterior (right) showing position of posterolateral muscle fields (plm), central muscle scars (cm), oblique lateral muscle scars (ol), vascula media(vm) and vascula lateralia (vl), 38.3; 5, C1390A, ventral valve exterior of mature specimen, 33; 6, C1390A, portion of small shell cluster of individuals ofvarious sizes, 32.3; 7, Plectothyrella sp. C1177A, group of specimens encrusting an orthocone cephalopod, representing two ventral valves (left andcentre) and dorsal external mould (right), 32.5. All specimens are from the Upper Ordovician, Hirnantian, Soom Shale Member, Cedarberg Formation,South Africa. a, c, e–g from Kromrivier, b, d from Keurbos.
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Kromrivier. Four or five additional specimens from Keurbosand Kromrivier are in the collections.
Remarks.—A few poorly preserved valves with a narrowposterior margin, uniplicate anterior commissure and radialornament of up to 30 rounded, occasionally bifurcating ribsclosely resemble Plectothyrella haughtoni Cocks and Brunton,1970, from the Cedarberg Formation of Sneeukop, nearWellington in Western Cape Province (Cocks et al., 1970).This species is the only rhynchonellide brachiopod reported todate from the upper Ashgill of South Africa. However, thelimited number of specimens and lack of interior details in theSoom Shale material make it impossible to make a precisetaxonomic discrimination.
ECOLOGY AND TAPHONOMY
Life habits.—Shells of Kosoidea cedarbergensis are the mostabundant brachiopod fossils in the Soom Shale. They
commonly form clusters encrusting shells of other brachio-pods or cephalopods, or are attached to each other.Brachiopod clusters associated with orthocone cephalopodshells were analyzed in detail by Gabbott (1999), who deducedthat orthocone shells could possibly be colonized in partduring the life of the animal, and continuing after its death.The clustering habit of discinids is common in Recent faunas,as for example in large populations of Discinisca cf. tenuis(Sowerby, 1847) along the Atlantic coast of Namibia.According to Williams (in Williams et al., 2001) such clusterscould include up to four generations of juvenile and adultanimals attached to one another (Fig. 7).
Our samples of K. cedarbergensis also include a range ofindividuals of different sizes, which suggests differentgenerations (Fig. 8). Large clusters occur on decalcifiedorthocone conchs, sometimes in association with Trematisand, rarely, with shells of Plectothyrella. Clusters also occuron and around large, usually disarticulated valves ofTrematis taljaardi, and several clusters are monospecific.These clusters most probably all represent life associations ofindividuals preserved intact or only slightly displaced fromlife position shortly after death, as in the examples ofmodern Discinisca discussed above. Only one cluster fromthe studied collection contains adult shells of K. cedarber-gensis, about 14–22 mm wide. All other clusters comprisemostly immature shells, with an average width varying from4.4 to 5.2 mm, which is three to four times below the size offull-grown individuals (standard deviation from 2.1–0.8);occasionally second generation shells are also present,usually as very young specimens about 1.6–2.0 mm wide,which possibly died soon after settlement. This suggests thatmost of the animals in a particular cluster belonged to asingle generation and that premature death was caused bysome catastrophic event, which may have occurred almostannually. The most probable explanations are a rise ofanoxia level above the sea floor, possibly during the time ofspring/summer algal blooms, or a strong influx of sedimentfrom a nearby landmass resulting possibly from seasonal iceand snow melting.
Discinoid shell preservation.—The discinoidean brachiopodshells (Trematis and Kosoidea) in the Soom Shale lack anyoriginal phosphatic fabric; shells occur as clay mineralreplacements or as moulds. As discussed by Gabbott (1998)and Gabbott et al. (2001) the possible mechanisms of such claymineral replacement most probably took place in low Phconditions below the sediment-water interface. However, theseshells do preserve a characteristic lamination, which mirrorsthe alternating mineral and organic layers within the originalshell. The morphology of the area around the pedicle notch insome shells of both genera suggests also partial fossilization ofthe periostracum. However, because of the coarse nature ofreplacement of the shell, an originally vesicular structure of theperiostracum or the presence of siliceous tablets as observed inRecent discinoids (e.g., Williams et al., 2001) cannot bediscounted.
Soft tissue preservation.—In some specimens of Trematistaljaardi, traces of the presumably organic bands flanking thelateral sides of the pedicle notch are clearly visible (Fig. 3.3,
R
FIGURE 6—Kosoidea cedarbergensis sp. nov. 1, C1371C, cluster of mainly articulated valves possibly representing a single generation, arrow pointedon the specimens with preserved pedicle, 33; 2–4, C1723A, C1723B, holotype, umbonal area of the ventral valve showing longitudinal section throughfossilised pedicle, 313, 320; detail of the pedicle, 375; 5, 6, C1371B, ventral valve showing partly preserved cast of the pedicle, 38; details of the pediclenotch with pedicle, 345; 7, 8, C1371B, umbonal area of ventral valve with cast of the pedicle, 342, detail of the pedicle, 380. All specimens are from theUpper Ordovician, Hirnantian, Soom Shale Member, Cedarberg Formation, South Africa. 1, 5–8 from Kromrivier, 2–4 from Keurbos.
FIGURE 5—Schematic illustration of muscle scars and position ofmantle canals in the ventral and dorsal valves of Kosoidea cedarbergensissp. nov.
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3.14). This suggests that the pedicle notch on the mineralizedshell of fossil trematids does not necessarily correspond withthe original pedicle opening, and that instead it was partlycovered by the exclusively organic shell or, more probably, byperiostracal bands as in Recent Discinisca (Holmer andPopov, 2000). Similar presumably periostracal bands are alsopreserved on the ventral valve of some specimens of K.cedarbergensis (Fig. 4.2, 4,4).
There is no evidence of fossilization of muscle tissue,lophophores or setae in the studied specimens of K.cedarbergensis. However, up to 40 specimens have acharacteristic cylindrical stalk preserved (Fig. 4.1), interpret-ed here as part of the three-dimensionally preserveddiscinoidean pedicle. This stalk is attached to the shell insidethe umbonal area of the ventral valve and is up to 4.4 mmlong and up to 0.65 mm wide in mature specimens more than20 mm wide; it completely fills the pedicle opening inimmature shells, but it is somewhat shorter and about twiceas narrow as the pedicle notch in the largest specimens. Themould of the pedicle comprises an outer layer with a smoothouter surface and an indistinctly annulated surface composedof light-coloured clay about 135–140 mm thick, with itsinternal filling usually preserved as dark grey or black claymatrix. Under high magnification in some specimens it isevident that the outer layer built of clay mineral platelets isaligned perpendicular to the outer and inner surface, and isdistinct in texture from the surrounding clay matrix(Fig. 4.4).
There is no trace of fossilized muscles and other kinds ofsoft tissue inside the shell of K. cedarbergensis. The pedicle ispreserved via fossilization of the external cuticle, which inRecent lingulids contains randomly oriented chitinous fibersmore resistant to post-mortem degradation (Williams et al.,1997). The outer epithelium and the underlying layer ofdermal muscles are not fossilized, but the indistinctlyannulated inner surface of the outer layer of the pediclemould may represent a cast of the muscular layer built ofcircumferential fibers, which suggest that a layer of collage-nous connective tissue separating the dermal muscles from theouter epithelium could also be fossilized.
The general anatomy of the pedicle in K. cedarbergensis isthus without significant modification from the patternobserved in Recent lingulids and consisted of a muscularbody wall enclosing a coelomic cavity and covered bychitinous cuticle. It was much shorter than the pedicle ofRecent lingulids and of Cambrian Lingulellotreta malongensis(Jin et al., 1993) but comparable in size to the pedicle ofRecent discinoideans. However, the available material doesnot allow us to verify the presence of pedicle muscles inside thecoelomic cavity, which is a characteristic feature of RecentDiscinisca (Williams et al., 1997).
The only other possible evidence of soft part preservation inPalaeozoic discinoids was reported by Sudkamp (1997) for theEarly Devonian Orbiculoidea? mediorhenana Fuchs, 1915.However, the interpretation of the observed structure as adiscinoid pedicle was refuted subsequently by Otto (2000),who reinterpreted the supposed structure in the Devonian‘‘Hunruckschifer’’ specimen as a preservational artefact. Thusthe three-dimensional preservation of the pedicle in the SoomShale K. cedarbergensis is the earliest documented and possiblythe only record of this kind for Palaeozoic discinoidbrachiopods.
BIOGEOGRAPHY
Although restricted to only four currently known taxa, thebrachiopod fauna of the Soom Shale places it firmly within thedistinctive world-wide late Ordovician Hirnantia Fauna. Thismatter has been discussed in relation to the closely overlyingCedarberg Formation, and the Soom Shale faunas describedhere fit well into the overall pattern. The Soom assemblagelacks some of the most characteristic genera of the Hirnantiafauna, such as Hirnantia itself, Dalmanella and Eostropheo-
FIGURE 7—Discinisca cf. tenuis (Sowerby). National Museum of Wales2008.24G.1. Recent, Namibia, Atlantic shore at Swakopmund, collectedby Alwyn Williams; clusters of up to four generations of juvenile and adultanimals attached to one another in storm-beach deposits, 33.5.
FIGURE 8—Scatter diagram for length plotted against width of the shellof three brachiopod clusters and full grown specimens of Kosoideacedarbergensis sp. nov., and histogram showing size distribution of themeasured specimens (X, average; S, standard deviation; N, number ofmeasured specimens).
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donta, but it is incorporated in what Sutcliffe et al. (2001, p. 5)termed an atypipcal Hirnuntia fauna. For further discussion ofCedarberg (including Soom) assemblages as relatively coldwater faunas developed in conditions of slow eustatic fallrelated to growing late Ordovician ice sheets on Gondwana,see Cocks et al. 1970), Cocks and Fortey (1986), Rong andHarper (1988), Villas et al. (1999), and Sutcliffe et al. (2001).These accounts are consistent in the interpretation of theSouth African faunas as components of a relatively highlatitude environment in southern Gondwana.
ACKNOWLEDGMENTS
Research on the Soom Shale has been funded by researchgrants from NERC (GR3/10177 to RJA) and from theNational Geographic Society (6715-00 to RJA), and by theUniversity of Leicester. Mr. and Mrs. J. N. Nieuwoudt(Keurbos Farm), and Mr. and Mrs. R. Nieuwoudt (Kromri-vier), kindly allowed access to fossil localities. The Council forGeoscience, South Africa, provided logistical support. Exca-vation permits for this work were issued by the ProvincialHeritage Authority. The South African Heritage ResourcesAgency (SAHRA, National Heritage Authority) was respon-sible for authorizing the export permit to allow the study ofthe fossil material in the U.K.
REFERENCES
ALDRIDGE, R. J., S. E. GABBOTT, L. J. SIVETER, AND J. N. THERON. 2006.Bromalites from the Soom Shale Lagerstatte (Upper Ordovician) ofSouth Africa: Palaeoecological and palaeobiological implications.Palaeontology, 49:857–871.
ALDRIDGE, R. J., S. E. GABBOTT, AND J. N. THERON. 2001. The soomshale, p. 32–95. In D. E. G. Briggs and P. R. Crowther (eds.),Palaeobiology II. Blackwell Science, Oxford.
ALDRIDGE, R. J. AND J. N. THERON. 1993. Conodonts with preserved softtissue from a new Upper Ordovician Konservat-Lagerstatte. Journal ofMicropalaeontology, 12:113–117.
ALDRIDGE, R. J., J. N. THERON, AND S. E. GABBOTT. 1994. The SoomShale: a unique Ordovician fossil horizon in South Africa. GeologyToday, 10: 218–221.
COCKERELL, T. D. A. 1911. The name Glossina. Nautilus, 25:96.COCKS, L. R. M. 1982. The commoner brachiopods of the latest
Ordovician of the Oslo–Asker District, Norway. Palaeontology,25:755–781.
COCKS, L. R. M., C. H. C. BRUNTON, A. J. ROWELL, AND I. C. RUST.1970 (dated 1969). The first Lower Palaeozoic fauna proved from SouthAfrica. Quarterly Journal of the Geological Society of London,125:583–603.
COCKS, L. R. M. AND R. A. FORTEY. 1986. New evidence on the SouthAfrican Lower Palaeozoic: age and fossils reviewed. GeologicalMagazine, 123:437–444.
DALMAN, J. W. 1828. Uppstallning och beskrifning af de i Sverige funneterebratuliter. Kongliga Svenska Vetenskaps-Akademiens, Handlingar,3:85–155.
EMMONS, E. 1842. Geology of New York, Part II. Comprising the surveyof the second geological district. Natural History of New York: x + 437pp., 17 pls.
FUCHS, A. 1915. Der Hunsruckschiefer und die Unterkoblenzschichtenam Mittelrhein (Loreleigegend). Abhandlungen der preußisch geolo-gischen Landes-Anstalt, N.F., 79:1–79.
GABBOTT, S. E. 1998. The taphonomy of the Ordovician Soom ShaleLagerstatte, an example of soft tissue preservation in clay minerals.Palaeontology, 41:631–667.
GABBOTT, S. E. 1999. Orthoconic cephalopods and associated fauna fromthe Late Ordovician Soom Shale Lagerstatte, South Africa. Palaeontol-ogy, 42:123–148.
GABBOTT, S. E., M. J. NORRY, R. J. ALDRIDGE, AND J. N. THERON. 2001.Preservation of fossils in clay minerals, a unique example from theUpper Ordovician Soom Shale, South Africa. Proceedings of YorkshireGeological Society, 53:237–244.
GRAY, R. E. 1840. Synopsis of the contents of the British Museum (Forty-second) edition. London. 370 p.
HAVLICEK, V. 1994. Kosovian inarticulate brachiopods and theirancestors (Late Ordovician, Prague Basin). Ustredni Ustav Geologicky,Vestnik, 69:59–67.
HAVLICEK AND M. MERGL. 1988, Two new discinid genera (Brachiopoda)from the Silurian and Devonian of the Prague Basin, Czechoslovakia.Ustredni Ustav Geologicky, Vestnik, 63:169–172.
HOLMER, L. E. AND L. E. POPOV. 2000. Lingulida, p. 32–95. In R. L.Kaesler (ed.), Brachiopoda. Revised. Treatise on Invertebrate Paleon-tology, Geological Society of America, New York, and University ofKansas, Lawrence, Pt. H, Vol. 2.
JIN YU-GAN, XIAN-GUANG HOU AND HUA-YU WANG. 1993. LowerCambrian pediculate lingulids from Yunnan, China. Journal ofPaleontology, 67:788–798.
KING, W. 1846. Remarks on certain genera belonging to the classPalliobranchiata. Annals and Magazine of Natural History, Series 1,18:26–42.
KUHN, O. 1949, Lehrbuch der Palaozoologie. E. Schweizerbart, Stuttgart:v + 326 p.
MENKE, C. T. 1828. Synopsis methodica Molluscorum generum omnium etspecierum earum, quae in Museo Menkeano Adservantur. Pyrmonti. 91 p.
OTTO, M. 2000. Supposed soft tissue preservation in the Hunsruckschifer(Lower Devonian, Rheinisches Schiefergebridge), the example ofbrachiopods. Palaontologische Zeitschrift, 74:79–89.
RONG JIA-YU AND D. A. T. HARPER. 1988. A global synthesis of the latestOrdovician Hirnantian brachiopod faunas. Transactions of the RoyalSociety of Edinburgh, 79:383–402.
SCHUCHERT, C. 1893. A classification of the Brachiopoda. AmericanGeologist, 11(3):141–167.
SCHUCHERT, C. 1913. Class 2. Brachiopoda, p. 355–420. In K. A. vonZittel, Textbook of Palaeontology. 1 (second edition). London.
SCHUCHERT, C. AND C. M. LEVENE. 1929. Brachiopoda (Generum etgenotyporum index et bibliographia). Peabody Museum NaturalHistory, Memoirs, 4(1):1–270.
SHARPE, D. 1848. On Trematis, a new genus belonging to the family ofbrachiopodous Mollusca. Quarterly Journal of the Geological Societyof London, 4:66–69.
SOWERBY, J. DE C. 1839. Shells, p. 589–712, pls 1–27. In R. I. Murchison(ed.), The Silurian System. John Murray, London.
SOWERBY, G. B. 1846, 1847. The Recent Brachiopoda, p. 221–406, pls 67–73. In Thesaurus Conchyliorum, Pt. 6, 7. London.
SUDKAMP, W. 1997. Discovery of soft parts of fossil brachiopod in the‘Hunsruckschifer’ (Lower Devonian, Germany). PalaontologischeZeitschrift, 71:91–95.
SUTCLIFFE, O., E. D. A. T. HARPER, A. A. SALEM, R. J. WHITTINGTON,AND J. CRAIG. 2001. The development of an atypical Hirnantia-brachiopod Fauna and the onset of glaciation in the late Ordovician ofGondwana. Transactions of the Royal Society of Edinburgh: EarthSciences, 92: 1–14.
TEMPLE, J. T. 1965. Upper Ordovician brachiopods from Poland andBritain. Acta Palaeontologica Polonica, 10:279–427.
VILLAS, E., S. LORENZO, AND J. C. GUTIERREZ-MARCO. 1999 (for 1998).First record of a Hirnantia Fauna from Spain, and its contribution tothe Late Ordovician palaeogeography of northern Gondwana. Trans-actions of the Royal Society of Edinburgh: Earth Sciences, 89: 187–197.
WAAGEN, W. 1885. Salt Range fossils. I. Productus-Limestone fossils.Memoir of the Geological Survey of India, Palaeontologia Indica, Series13, 4:729–770.
WALCOTT, C. D. 1888. A fossil Lingula preserving the cast of thepeduncle. United States National Museum Proceedings, 11:480.
WILLIAMS, A. 1997. Shell structure, p. 267–320. In A. Williams, C. H. C.Brunton, S. J. Carlson, et al. (eds.) (44 others). Treatise on InvertebratePaleontology, Volume H, Brachiopoda (revised). Geological Society ofAmerica and University of Kansas Press, Boulder and Lawrence.
WILLIAMS, A., C. LUTER, AND M. CUSACK, 2001. The nature of siliceousmosaics forming the first shell of the brachiopod Discinisca. Journal ofStructural Biology, 134:25–34
WRIGHT, A. D. 1963. The fauna of the Portrane Limestone. 1. Theinarticulate brachiopods. British Museum (Natural History), Bulletin(Geology), 5:221–254.
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