Article: First record of a heterodont bivalve (Mollusca) from the Early Triassic: palaeoecological significance and implications for the Lazarus problem

The heterodont bivalve Sinbadiella pygmaea gen. et sp. nov. is described from the Sinbad Limestone Member (Olenekian, Smithian) of the Lower Triassic Moenkopi Formation of Utah (USA). The new taxon is tentatively assigned to the Lucinidae on the grounds of its external morphology, larval shell morphology and hinge characters. It is the only definite representative of the Superorder Heterodonta in the Olenekian, removing this major clade of bivalves from the long list of Early Triassic Lazarus taxa. Although at least locally very abundant, the new taxon has been overlooked by earlier workers, probably due to its minute size. Dwarfism is a widespread phenomenon in Early Triassic faunas, which has been attributed to ongoing reduction of primary production. If S. pygmaea was a lucinid, a chemosymbiotic feeding mode can be inferred, which might explain why it was able to thrive in conditions that suppressed filter feeding heterodonts.