Evolutionary and biogeographic patterns in the deep-sea echinoid families Pourtalesiidae Agassiz 1881 and Ceratophysidae fam. nov. (Echinoidea)

The sea urchin family Pourtalesiidae is primarily an abyssal taxon, exhibiting extremely modified morphologies that have emerged as an adaptation for burrowing in soft sediment. Here, we present the first detailed molecular phylogeny of the family Pourtalesiidae. Both morphological and molecular evidence support the establishment of the family Ceratophysidae fam. nov. to accommodate seven former pourtalesiid genera. The name Pourtalesiidae is retained for the group of genera Cystocrepis and Pourtalesia. Based on morphological data, the fossil genus Galeaster is transferred to the monotypic family Galeasteridae fam. nov.. Families Pourtalesiidae and Ceratophysidae are each characterized by a unique pattern of plastron plating. Reconstructions indicate that elongated tests, as well as other adaptations for burrowing, evolved independently in these families. The evolutionary histories of pourtalesiids and ceratophysids are complex and include several instances of increase and decrease in this specialization. Molecular data support the Antarctic origin of Pourtalesiidae and Ceratophysidae, although for the latter the support is limited. The initial dispersal of pourtalesiids from the Antarctic occurred in the northward direction and included distribution of the least specialized forms throughout the eastern Pacific. Broad radiation of the more specialized forms apparently occurred later. The biogeographic history of the family Ceratophysidae is more complex and at present cannot be reconstructed reliably.