RECOVERY OF BRACHIOPOD AND AMMONOID FAUNAS FOLLOWING THE END-PERMIAN CRISIS: ADDITIONAL EVIDENCE FROM THE LOWER TRIASSIC OF THE RUSSIAN FAR EAST AND KAZAKHSTAN

The present paper provides information on recovery of brachiopod and ammonoid faunas following end-Permian crisis, based on original evidence from the southern Russian Far East and Kazakhstan (Middle Asia) as well as the published data on Eurasia and North America. After the end-Permian mass extinction, ammonoids reached levels of taxonomic diversity higher than in the Changhsingian by the Dienerian substage of the Induan (early Early Triassic). However, brachiopods exhibit a prolonged delay in recovery, and their taxonomic diversity had not recovered to Late Permian levels even by the Olenekian (late Early Triassic). 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 (‘Lilliput effect’) 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 vulnerabilities associated with their larval stage: brachiopod larvae had a short-duration motile stage of development 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. This work is a contribution to UNESCO-IUGS IGCP Project 572 and financially supported by the Russian grants RFBR (11-05-98538-R_vostok_a and 11-05-000785-a).