''Pojetaia'' is an
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of early
bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
s,
one of two genera in the extinct family
Fordillidae
Fordillidae is an extinct family of early bivalves and one of two families in the extinct superfamily Fordilloidea. The family is known from fossils of early to middle Cambrian age found in North America, Greenland, Europe, the Middle East, A ...
.
The genus is known solely from
Early to Middle Cambrian fossils found in North America, Greenland, Europe, North Africa, Asia, and Australia.
[The Paleobiology Database ''Pojetaia'' entry](_blank)
accessed 4 January 2012 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''.
Description
''Pojetaia'' are small bivalves with
valves
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...
that are subequal in size and suboval in shape. In size ''Pojetaia'' specimens reach a total shell length of less than and a height of .
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.
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.
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.
The structuring is similar to shell layering found in the extinct genera ''
Anabarella
''Anabarella'' is a species of bilaterally-flattened monoplacophoran mollusc, with a morphological similarity to the rostroconchs. Its shell preserves evidence of three mineralogical textures on its outer surface: it is polygonal near the crest ...
'' and ''
Watsonella
''Watsonella'' is a genus of 'mollusc' known from early (Terreneuvian) Cambrian strata.
The genus is closely related to '' Anabarella'', with which it bears many morphological similarities, including a laminar internal shell microstructure said ...
'', which is thought to suggest that members of the phylum
Mollusca
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
developed
nacre
Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent.
Nacre is f ...
independently several times.
Due to the size and age of the fossil specimens, ''Pojetaia'' are included as part of the Turkish
Small shelly fauna
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambri ...
.
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.
Species
''P. runnegari''
The
type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
for ''Pojetaia'', ''P. runnegari'' was first described by the Australian
paleontologist
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
Peter A. Jell in 1980.
The description was based on fossils found in early Cambrian Parara Limestone sediments exposed in South Australia.
The species has since been found in Cambrian strata of Greenland, Germany,
Transbaikalia
Transbaikal, Trans-Baikal, Transbaikalia ( rus, Забайка́лье, r=Zabaykalye, p=zəbɐjˈkalʲjɪ), or Dauria (, ''Dauriya'') is a mountainous region to the east of or "beyond" (trans-) Lake Baikal in Far Eastern Russia.
The steppe and ...
, 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''.
The species ''P. ostseensis'' was described in 1995 from two partial fossils found on the island of
Bornholm
Bornholm () is a Danish island in the Baltic Sea, to the east of the rest of Denmark, south of Sweden, northeast of Germany and north of Poland.
Strategically located, Bornholm has been fought over for centuries. It has usually been ruled by ...
. 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
Henan (; or ; ; alternatively Honan) is a landlocked province of China, in the central part of the country. Henan is often referred to as Zhongyuan or Zhongzhou (), which literally means "central plain" or "midland", although the name is al ...
, 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.
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
''Fordilla'' is an extinct genus of early bivalves, one of two genera in the extinct family Fordillidae. The genus is known solely from Early Cambrian fossils found in North America, Greenland, Europe, the Middle East, and Asia.[Bruce Runnegar
Bruce Norman Runnegar (born 2 February 1941, Brisbane) is an Australian-born paleontologist and professor at UCLA. His research centers on using the fossil record to determine how, where, and when life originated and evolved. He has published on ...]
and Pojeta 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''.
''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
The Anti-Atlas ( ar, الأطلس الصغير, shi, Aṭlas Mẓẓiyn), also known as Lesser Atlas or Little Atlas is a mountain range in Morocco, a part of the Atlas Mountains in the northwest of Africa. The Anti-Atlas extends from the Atlanti ...
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''.
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
The Rostroconchia is a class of extinct molluscs dating from the early Cambrian to the Late Permian. They were initially thought to be bivalves, but were later given their own class. They have a single shell in their larval stage, and the adult t ...
from ''Watsonella'' or it may be a bivalve. Thus the positioning of the species is currently ''Watsonella? terranovica'' but with doubt.
References
{{Taxonbar, from=Q7208535
Cambrian molluscs
Prehistoric bivalve genera
Cambrian animals of Africa
Cambrian animals of Asia
Cambrian animals of Europe
Cambrian animals of North America
Paleozoic molluscs of North America
Paleozoic life of Newfoundland and Labrador
Paleozoic life of Quebec
Cambrian genus extinctions