Abstract
Clypeaster is a speciose echinoid genus with almost 50 extant and approximately 350 extinct species, encompassing a great heterogeneity of form. While some attempts to subdivide this genus have been made, none has gained widespread support, and all recent taxonomic treatments have left the genus intact. Here we report new data on internal buttress arrangement, determined from X-Ray tomography, and plate architecture, and use this to establish relationships amongst 19 extant species encompassing 8 ofthe 10 nominal subgenera that have been proposed. A cladistic analysis of these characters allows us to test the validity of previously suggested subgenera of Clypeaster. Our analysis confirms that Clypeaster is monophyletic with the clypeasteroid familyArachnoididae (as represented by Arachnoides and Ammotrophus) as its immediate sister-group. It also identifies Orihanihus as the most primitive subgenus in the family Clypeaster. However, none of the previously proposed subdivisions of Clypeaster wererecovered as clades and test architecture proves too ho-moplasous to allow a confident basis on which to subdivide the genus.
NSTL