Recovery of Brachiopod and Ammonoid Faunas Following the End-Permian Crisis: Additional Evidence from the Lower Triassic of the Russian Far East and Kazakhstan

After the End-Permian mass extinction, ammonoids reached levels of taxonomic diversity higher than in the Changhsingian by the Dienerian Substage of the Induan. However, brachiopods exhibit a prolonged delay in recovery, and their taxonomic diversity had not recovered to Late Permian levels even by the Olenekian. The differential patterns of recovery between these two clades may reflect fundamental differences in physiology and behavior. Brachiopods were benthic organisms that were dependent on specific trophic sources, and their general reduction in size during the Early Triassic may have been a response to a relative paucity of food resources. In contrast, ammonoids were sluggish-nektic organisms that utilized a wider range of trophic sources and that suffered no comparable size decrease during the Early Triassic. Brachiopods may have been at a disadvantage also due to vulner-abilities associated with their larval stage, during which they had to locate a suitable substrate for settlement. In contrast, ammonoids had no larval stage and juveniles may have been dispersed widely into favorable habitats. These factors may account for differences in the relative success of ammonoids and brachiopods at high-latitude regions following the End-Permian mass extinction: ammonoids successfully recolonized the Boreal region during the Early Triassic whereas brachiopods did not.