Ordovician To Devonian Diversification of the Bivalvia
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Devonian
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Data from 19 localities throughout the world yielding bivalves were analysed by means of cluster analysis. Due to the uncertainty about the completeness of the fossil record, genera restricted to a single locality at each of the three selected time intervals (Early, Middle, and Late Ordovician) were considered exclusive instead of endemic. Results show very low similarity values, inclusive among localities belonging to the same biochore based on other marine benthic organisms (i.e. brachiopods). Moreover, the percentage of exclusive genera per locality is always very high at any time interval. Since the Early Palaeozoic bivalves are not biostratigraphically important, their real diversity has probably been underestimated. As a result, even if understanding of them has strongly increased in the last years, it is not still enough to give us an accurate picture of their generic distribution in space and time. However, the incompleteness of the fossil record itself does not explain this pattern of distribution. Most important factors could be related to biotic factors, such as the stenotopic nature of most of the shallow dwelling species, and the probably short-lived pelagic larval stage. The low percentage of shared genera among different localities seems to preclude the use of bivalves as relevant palaeobiogeographic indicators during the Ordovician.
Paleoecology
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A bivalve fauna of Early Ordovician (late Arenig) age is described from the Hsiangyang Formation of the eastern part of West Yunnan, China. The fauna contains elements in common with Early Ordovician faunas of southern Gondwanan areas such as the Montagne Noire and Morocco and with those of Avalonia, together with several previously undescribed taxa. The following taxa are new: Biseriodonta simplex gen. et sp. nov.; Glyptarca sinensis sp. nov.; Trigonoglyptarca magna gen. et sp. nov.; Erhaiconcha xiangyangensis gen. et sp. nov.; Fasciculodonta impressa gen. et sp. nov.; Yunnanoredonia laevis gen. et sp. nov.; Daliella gen. nov.; Goniophorina ( Goniophorina ) contracta sp. nov.; Haidongoconcha radialis gen. et sp. nov.; Eopterinea aequiconcha gen. et sp. nov. The bivalve fauna includes the most diverse glyptarcid fauna and the earliest nuculanid and paracyclid known hitherto. Eastern West Yunnan belonged to the Indochina terrane and it is concluded that the bivalve fauna represents a high‐latitude assemblage and that the Indochina terrane should be considered a component part of the Peri‐Gondwanan continent in the Ordovician; its bivalve faunas contrast with those of neighbouring terranes which have affinities with those of lower latitudes.
Baltica
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Abstract In the 30 years since publication of the bivalve Treatise , (Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology. Part N. Mollusca 6, Bivalvia , Geological Society of America and University of Kansas) important new faunas have been described from the early and mid Cambrian and from the early and mid Ordovician. These contain significant new forms, including some long-ranging intermediate groups, that indicate the relationships between the principal bivalve clades, but lack of fossils from the late Cambrian and earliest Ordovician is a major hindrance. The principal phase of bivalve diversification followed on from the evolution of the filibranch gill in the latest Cambrian or earliest Ordovician. The fundamental division of the class is into two subclasses, Protobranchia and Autolamelli branchiata; links between the two can be demonstrated in the early Ordovician. Major divisions of each subclass are recognized as superorders. Within the Protobranchia, the Nuculoida developed specialist food-gathering palps and an enlarged foot. Diverging early from the protobranch stock were other bivalves that lived symbiotically with sulphur-oxidizing chemoautotrophic bacteria; this allowed colonization of anaerobic substrates and produced two distinct stocks: the deeply infaunal anteriorly elongate Solemyoida and the shallower infaunal Nucinelloida. The Autolamellibranchiata, initially identified by strongly asymmetrical hinges, diversified in three directions, each characterized by distinctive hinges. The Trigonioida were characterized by ligamental nymphs and frequently denticulate teeth, and rapidly regained greater symmetry; the Anomalodesmata also developed a strong ligamental insertion within nymphs and largely lost their dentition, whilst the Heteroconchia, principally with a shell including a complex crossed-lamellar structure, had various combinations of cardinal and lateral teeth. Heteroconch diversifications were mainly in the Mesozoic and Cenozoic, but one Ordovician group, the Glyptarcoidea, is a good ancestor for the Pteriomorphia. The following new taxa are proposed: Cardiolarioidea superfam. nov., Eritropidae fam. nov. and Catamarcaidae fam. nov.
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A series of Cambrian morphologically intermediate forms show that the bivalved shell of pelecypods is probably derived from the pseudobivalved shell of rostroconchs, which in turn is probably derived from the univalved shell of helcionellacean monoplacophorans. In Ordovician time, pelecypods underwent their first major radiation, which produced seven major groups herein called subclasses. The subclasses Orthonotia and Lucinata are newly named herein; the subclass Actinodontia is elevated to this rank from the rank of order. The group Heterodonta may be polyphyletic, as the taxa included within it seem to be derived from various Ordovician groups, herein regarded as subclasses. It seems likely that the solemyids are descendants of Ordovician palaeotaxodonts. The duplivincular ligament may have had a polyphyletic origin, having evolved independently in the Arcacea, Myalinidae, and Pteriacea.
Polyphyly
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Study of 95 Western Atlantic bivalve mollusk species representing 29 families has demonstrated that morphologic features of the bivalve shell other than hinge type and microstructure primarily reflect life habits and habitat preferences. Many morphologic features represent potentially powerful tools for paleoecologic and evolutionary interpretation.
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Welsh
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