CONSTABLE, H.; MOOI, R.*; California Academy of Sciences, San Francisco; California Academy of Sciences, San Francisco: Phylogenetic analysis of echinothurioids, deep-sea echinoids with venomous spines

Echinothurioids are strange, “regular” sea urchins having features indicative of a basal position in the phylogeny of extant echinoids. A few echinothurioids live on coral reefs, but the vast majority almost globally inhabits the deep sea to almost 5000 m. All other extant urchins have rigid tests, but that of echinothurioids is soft and deformable, being made up of loosely sutured, partially imbricated plates. Echinothurioids are armed with venomous spines sometimes known to cause serious injury in humans. There are over 50 extant species, but the fossil record is limited to 2 known species whose phylogenetic positions are poorly established. The fragility and rarity of echinothurioids in collections and strata mean that very little is known about their origins and diversification. Only a single attempt to reconstruct the phylogeny of echinothurioids presently exists, and that was based on genera whose monophyly remains untested. Here we present the first species level phylogeny of the echinothurioids in order to perform these and other tests. The present analysis clarifies previously misinterpreted morphologies, establishes the monophyly of the Echinothurioida, and pinpoints features that place it basal to the other acroechinoids. Initial findings suggest that the largest genus, Araeosoma, is monophyletic, but that at least one other genus is not only oversplit but not monophyletic. Recent field work in Antarctica turned up additional material of the rare and enigmatic Kamptosoma, an echinothurioid whose autapomorphies have obscured its true position in the evolution of the group.