Pseudoconocardium licharewi Zavodowsky, 1960, the latest representative of the extinct class Rostroconchia (Mollusca)

Abstract

Rostroconchs are one of the “minor classes” of the phylum Mollusca. They were especially diverse in Early and Middle Palaeozoic, but by the end of Late Palaeozoic the group’s diversity had sharply declined. This study shows that rostroconchs completely disappeared from the fossil record before the great end-Permian mass extinction event. The paper offers the first review of all Permian rostroconch localities in Northeast Russia and discusses their possible depositional settings. All records are precisely referenced to the zonal bivalve scale of Northeast Asia, calibrated and correlated with the International Chronostratigraphic Chart using SHRIMP and ID-TIMS dating, taking into account δ¹³C_org chemostratigraphic data. All Permian rostroconchs in this region are represented by different forms of preservation of the same species — Pseudoconocardium licharewi Zavodowsky, 1960 and are distributed in the interval: the second half of the Artinskian — the lower part of the Changhsingian. Thus, the species in question is the latest known representative of rostroconch in the world. The type series of Pseudoconocardium licharewi, type species of the genus Pseudoconocardium Zavodowsky, 1960, as well as specimens housed in the collection of the Borissiak Paleontological Institute of the Russian Academy of Sciences, are examined. Based on a detailed study of shell morphology, an emended generic diagnosis and a new description are presented here. It is also shown that the genus Pseudoconocardium is monotypic.