Causes and Consequences of Extinction and Survival in Fossil Marine Invertebrates with a Special Focus on the Crinoidea (Phylum Echinodermata).

Abstract

In the geologic past, certain traits increased the chance of survival of some marine invertebrate taxa, which means that extinction did not occur randomly. However, it has been claimed that these traits buffer less against extinction at mass extinction events. Herein, a method for detecting selective extinction shows that during both background and mass extinction times, extinction of marine invertebrate genera was non-random for most of the Phanerozoic Eon. The two best-known mass extinctions, the Cretaceous-Paleogene (K-Pg), and the Permian-Triassic (P-Tr), appear to have been highly selective. It is shown that extinction will appear random at the genus level with respect to the number of species in a genus when extinction is highly selective at the species level.

A phylogenetic analysis of 51 crinoid species (Phylum Echinodermata: Class Crinoidea) addresses major, unresolved questions in crinoid evolutionary history: how many lineages survived the P-Tr extinction event? Did extinction selectivity affect crinoids during this event? The phylogeny supports a single surviving lineage of crinoids at the P-Tr extinction event. However, crinoids were likely reduced in diversity before the P-Tr boundary, which brings into question whether they experienced extinction selectivity at the P-Tr extinction event.

Lastly, crinoid biodiversity since the P-Tr extinction event is investigated. Modern crinoids are dominated by the stalkless comatulids (Order Comatulida), notable for their ability to crawl and swim. It has been claimed that these abilities are antipredatory adaptations that aided comatulid survival in the face of increased predation pressure during the Mesozoic marine revolution (MMR). Recently, relatively rapid crawling has been shown in stalked crinoids, which means that only swimming is unique to comatulids. A biomechanical model and functional morphology shows that stalk loss and other traits are required for swimming. The swimming capability of extinct comatulids is considered, revealing that the earliest comatulids may have been swimmers. Swimming may have evolved in response to benthic predation pressure. Comatulid species that can only crawl likely represent lineages that lost the ability to swim. The possibility that the MMR and the number of reef sites may have affected the diversity of crawling and swimming comatulids is considered.