{{Short description|Extinct genus of bivalves}} {{Automatic taxobox | fossil_range = {{Fossil range|Tommotian|early Middle Cambrian|Upper Tommotian – lower Middle Cambrian}} | taxon = Pojetaia | authority = Jell, 1980 | subdivision_ranks = Species | subdivision = See text. }}
'''''Pojetaia''''' is an extinct genus of early bivalves,<ref name="Carter2011">{{cite journal |last1=Carter |first1=J.G. |year=2011 |title=A synoptical classification of the Bivalvia (Mollusca) |journal=Paleontological Contributions |volume=4 |pages=1–47 |url=https://repository.naturalis.nl/pub/408278 |display-authors=etal |hdl=1808/8287 |doi=10.17161/PC.1808.8287 |doi-access=free |hdl-access=free }}</ref> one of two genera in the extinct family Fordillidae.<ref name="Elicki2009">{{cite journal |last1=Elicki |first1=O. |last2=Gürsu |first2=S. |year=2009 |title=First record of ''Pojetaia runnegari'' Jell, 1980 and ''Fordilla'' Barrande, 1881 from the Middle East (Taurus Mountains, Turkey) and critical review of Cambrian bivalves |journal=Paläontologische Zeitschrift |volume=83 |issue=2 |pages=267–291 |doi=10.1007/s12542-009-0021-9|bibcode=2009PalZ...83..267E |s2cid=49380913 | url=http://www.geo.tu-freiberg.de/~elicki/ELICKI_GUERSU_2009_PalZ_CambrianBivalves.pdf}}</ref> The genus is known solely from Early to Middle Cambrian fossils found in North America, Greenland, Europe, North Africa, Asia, and Australia.<ref name="Elicki2009"/><ref name="PBDBPojetaia">[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=16103 The Paleobiology Database ''Pojetaia'' entry] accessed 4 January 2012</ref> The genus currently contains two accepted species, '''''Pojetaia runnegari''''', the type species, and '''''Pojetaia sarhroensis''''', though up to seven species have been proposed. The genera '''''Buluniella''''', '''''Jellia''''', and '''''Oryzoconcha''''' are all considered synonyms of ''Pojetaia''.<ref name="Elicki2009"/>
==Description== ''Pojetaia'' are small bivalves with valves that are subequal in size and suboval in shape. In size ''Pojetaia'' specimens reach a total shell length of less than {{convert|2|mm|in}} and a height of {{convert|25|mm|in}}.<ref name="Elicki2009"/><ref name="Pojeta1975">{{cite journal |last1=Pojeta |first1=J. |year=1975 |title=''Fordilla troyensis'' Barrande and early pelecypod phylogeny |journal=Bulletins of American Paleontology |volume=67 |pages=363–384 |url=https://archive.org/details/bulletinsofameri287pale}}</ref> The shells are compressed laterally and the back edge is slightly broadened. The rear adductor is more developed and larger than the front adductor, with the pallial muscles arranged in partially connected series along the valve margins.<ref name="Elicki2009"/> The ligament is usually straight with an umbo which is central to subcentral and each valve possessing between one and two teeth, with a total of no more than three teeth on a specimen. The exterior of the shells shows faint ribbing and fine comarginal growth lines.<ref name="Elicki2009"/> The inner shell layers of ''Fordilla'' and ''Pojetaia'' species both consist of layers of carbonate which is akin to the laminar aragonite layer found in extant monoplacophora.<ref name="Vendrasco2011">{{cite journal |last1=Vendrasco |first1=M.J. |last2=Checa |first2=A.G. |last3=Kouchinsky |first3=A.V. |year=2011 |title=Shell microstructure of the early bivalve ''Pojetaia'' and the independent origin of nacre within the Mollusca |journal=Palaeontology |volume=54 |issue=4 |pages=825–850 |doi=10.1111/j.1475-4983.2011.01056.x|doi-access=free |bibcode=2011Palgy..54..825V }}</ref> The structuring is similar to shell layering found in the extinct genera ''Anabarella'' and ''Watsonella'', which is thought to suggest that members of the phylum Mollusca developed nacre independently several times.<ref name="Vendrasco2011"/> Due to the size and age of the fossil specimens, ''Pojetaia'' are included as part of the Turkish Small shelly fauna.<ref name="Elicki2009"/>
== Affinity== ''Pojetaia'' and ''Fordilla'' appear to form a clade, but whether this is in the stem or crown group of Bivalvia is unconstrained by the available evidence.<!--This is reflected by the open nomenclature in the systematic section.--><ref name="Elicki2009">{{cite journal |last1=Elicki |first1=O. |last2=Gürsu |first2=S. |year=2009 |title=First record of ''Pojetaia runnegari'' Jell, 1980 and ''Fordilla'' Barrande, 1881 from the Middle East (Taurus Mountains, Turkey) and critical review of Cambrian bivalves |journal=Paläontologische Zeitschrift |volume=83 |issue=2 |pages=267–291 |doi=10.1007/s12542-009-0021-9|bibcode=2009PalZ...83..267E |s2cid=49380913 | url=http://www.geo.tu-freiberg.de/~elicki/ELICKI_GUERSU_2009_PalZ_CambrianBivalves.pdf}}</ref>
==Species== ===''P. runnegari''=== The type species for ''Pojetaia'', ''P. runnegari'' was first described by the Australian paleontologist Peter A. Jell in 1980.<ref name="Elicki2009"/> The description was based on fossils found in early Cambrian Parara Limestone sediments exposed in South Australia.<ref name="Elicki2009"/> The species has since been found in Cambrian strata of Greenland, Germany, Transbaikalia, Turkey, Northern China, and Mongolia. Fossils tentatively placed in ''Fordilla'' from the Browns Pond Formation of New York and Anse Maranda Formation of Quebec have been tentatively reassigned to ''Pojetaia runnegari''.<ref name="Skovsted2007">{{cite journal |last1=Skovsted |first1=C.B. |last2=Peel |first2=J.S. |year=2007 |title=Small shelly fossils from the argillaceous facies of the Lower Cambrian Forteau Formation of western Newfoundland |journal=Acta Palaeontologica Polonica |volume=52 |issue=4 |pages=729–748 |url=https://www.app.pan.pl/archive/published/app52/app52-729.pdf }}</ref> The species ''P. ostseensis'' was described in 1995 from two partial fossils found on the island of Bornholm. In his description of the species, Hinz-Schallreuter suggested the presence of three teeth on the left valve and the slightly larger size of the valves as reason for the erection of the new species. Other researchers, such as Elicki, have noted these differences fall into the accepted range of variation for ''P. runnegari'' and as such consider ''P. ostseensis'' as a synonym rather than valid. The genus ''Oryzoconcha'', with the single species ''O. prisca'', was described in 1985 by He and Pei from fossils found in Henan province, China. Further study by researches has resulted in genus and species being rejected, and the fossils assigned to ''P. runnegari''. Similarly the genus ''Jellia'' and its two species ''J. elliptica'' and ''J. ovata'', described by Li and Zhou in 1986, are also considered synonyms of ''P. runnegari'' as they fall into the rage of variation described for the species.<ref name="Elicki2009"/> The cryptic genus ''Buluniella'' and species ''B. borealis'' was described in 1986 by V. Jermak from three fossils found in Northern Siberia. The two right and one left disarticulated valves known show a slightly convexity of the hinge, central umbo and lack of a row of muscle scars were used to the genus from ''Fordilla''. The less distinct umbones were suggested as reason to separate ''Buluniella'' from ''Pojetaia''. Due to the high variation in characters of Cambrian bivalve species the validity of ''Buluniella'' as a separate genus and species has been questioned several times. In 1992 Bruce Runnegar and {{ill|John Pojeta|de}} recommended ''Buluniella'' belonged to ''Fordilla'' and suggested the species be treated as ''Fordilla borealis''. Further examination of the fossils has resulted in both the genus and the species being currently treated as a synonym of ''P. runnegari''.<ref name="Elicki2009"/>
===''P. sarhroensis''=== ''P. sarhroensis'' was named by Geyer and Streng in 1998 and is now known from a series of 49 fossils from early middle Cambrian sediments exposed in the Anti-Atlas mountains of Morocco. In the type description the major differences between ''P. sarhroensis'' and ''P. runnegari'' were noted to be fairly minor. The most distinct features of ''P. sarhroensis'' include occasional specimens with up to four teeth, a larger auricle angle, and a posterior tooth which is larger than that in ''P. runnegari''.<ref name="Elicki2009"/>
==Other fossils== It has been suggested at times that the problematic species ''Watsonella? terranovica'' may belong to ''Pojetaia'' as ''Pojetaia? terranovica''. Reexamination of the species in 1991 by Landing resulted in the conclusion that the species may be a rostroconch from ''Watsonella'' or it may be a bivalve. Thus the positioning of the species is currently ''Watsonella? terranovica'' but with doubt.<ref name="Elicki2009"/>
==References== {{Reflist}}
{{Taxonbar|from=Q7208535}} Category:Paleozoic molluscs of Europe Category:Cambrian molluscs Category:Prehistoric bivalve genera Category:Cambrian animals of Africa Category:Cambrian animals of Asia Category:Cambrian animals of Europe Category:Cambrian molluscs of North America Category:Paleozoic molluscs of North America Category:Paleozoic life of Newfoundland and Labrador Category:Paleozoic life of Quebec Category:Cambrian genus extinctions