DISPERSAL, ENDEMISM AND EXTINCTION SHAPED THE CASSIDULOID ECHINOID’S EVOLUTIONARY HISTORY

Cassiduloid echinoids (Echinodermata) are rare irregular echinoids that live buried in the sediment, where they feed on small organic particles. Cassiduloids most likely originated in the Early Cretaceous and despite their high diversity in the Paleogene, only seven species survive to the present. The cassiduloid evolutionary history has been dominated by high levels of homoplasy and a dearth of novel traits, with a concomitantly poorly resolved phylogeny. To better resolve the cassiduloid phylogeny I used micro-computed tomography (mCT) images of cassiduloid tests to increase the number of phylogenetically informative morphological characters. The inferred phylogeny revealed a new cassiduloid family, the Eurhodiidae, and showed that three of the seven living species are relicts of lineages dating back to the Eocene. The temporal and spatial diversification of two cassiduloid clades, the Cassidulidae and the Eurhodiidae, was then analyzed using a time-calibrated phylogeny and a time-stratified dispersal-extinction-cladogenesis (DEC) likelihood biogeographic model with range constraints. Eight discrete biogeographic areas were identified based on the occurrences and distribution of taxa in the selected clades. Although the cassiduloids are thought to have poor dispersal capabilities because they brood and because most species have highly restricted geographic distributions, living cassiduloids occur worldwide and none of the living species are sympatric. My analysis revealed that the cassidulids and eurhodiids had a south Tethyan and northwest Atlantic origin. Their current distribution patterns were established via initial range expansion (dispersal), followed by vicariance, then geographically restricted extinction. The most important time of diversification occurred within the northwest Atlantic from the Late Paleocene to the Early Eocene.