DISPERSALS AND THE SHIFTING MARINE BIODIVERSITY HOTSPOT: AN EXPLORATION WITH LIVING AND FOSSIL CRINOIDS

Studies of geographical patterns in species richness have often sought explanations in terms of differences in speciation rate, but no general association between speciation and richness has been recovered. The marine species richness hotspot has shifted from the West Tethys to the Indo-West Pacific over the Cenozoic, providing a natural experiment in the generation of diversity gradients. We consider two explanations for this shift in the center of greatest richness: a “multiple hotspots” scenario driven by changing in situ diversification rate, and a “single hotspot” scenario in which the constituent lineages of the ancient hotspot dispersed to and established the new one. Among comatulid crinoids, modern richness is concentrated in the IWP, but a new fossil database shows that all Mesozoic occurrences are from around the West Tethys and Atlantic. An origin and initial diversification of comatulids in the West Tethys is corroborated by ancestral range estimation on a phylogeny of extant comatulids, with which we also recover elevated dispersal into the IWP. Taphonomic controls suggest the Mesozoic absence of comatulids from the IWP does not result from poor preservation or sampling. Moreover, phylogenetic analyses place Jurassic fossils deep in crown Comatulida, implying that the major lineages originated and diversified in the West Tethys before dispersing to the IWP. Consilient paleontological and neontological evidence thus favors a “single hotspot” scenario in which the new IWP hotspot was assembled by asymmetrical dispersal from the adjacent West Tethyan hotspot, concurrent with tectonic activity that generated shelf area in the former and closed off the latter.