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Anatomy, Ecology and Affinities of the Australian Early Cambrian Bivalve Pojetaia runnegari Jell

Bruce Runnegar and Christopher Bentley
Journal of Paleontology
Vol. 57, No. 1 (Jan., 1983), pp. 73-92
Stable URL: http://www.jstor.org/stable/1304610
Page Count: 20
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Anatomy, Ecology and Affinities of the Australian Early Cambrian Bivalve Pojetaia runnegari Jell
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Abstract

Muscle scars on internal molds of the Early Cambrian bivalve Pojetaia runnegari help confirm P. A. Jell's suggestion that it is closely related to Ordovician and younger palaeotaxodonts. Characters which differentiate Pojetaia from Ordovician palaeotaxodonts and which have been used to unite it with the only other known Cambrian bivalve, Fordilla, probably result from its very small size (ca. 1 mm). The bivalve order Fordilloida, proposed initially for Fordilla and subsequently expanded to include Pojetaia, is no longer useful, since the latter genus can be referred to the Family Praenuculidae of the palaeotaxodont order Nuculoida, and the former genus seems to be a modiomorphid mytiloid. Both Pojetaia and Fordilla have long stratigraphic ranges: Pojetaia from late Tommotian to the end of the Early Cambrian, and Fordilla from about early Atdabanian to late Early Cambrian. Pojetaia was a tropical animal of the northern hemisphere which burrowed in carbonate muds and appears to have been an 'inhalent deposit feeder.' It grew to a mean size of 1.2 mm, probably in less than a year, and its larval development seems to have been lecithotrophic but not direct. Although only phosphatic replicas of the outer and inner shell surfaces are recovered from acid residues, there is good evidence that Pojetaia had a shell composed of a single layer of spherulitic aragonite prisms which was covered by the periostracum. In this and other respects, Pojetaia is a very primitive mollusc. It and Fordilla are suitable ancestors for most of the known higher taxa of bivalves.

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