ORIGINAL PAPER Bruno David á Rich Mooi A new species of subantarctic Plexechinus and its phylogenetic position within the Holasteroida (Echinodermata: Echinoidea) Accepted: 11 September 1999 Abstract A seventh species of Plexechinus, Plexechinus sulcatus sp. nov., is described from material collected at 585 m from an R/V ``Marion Dufresne'' station north- west of the Kerguelen Islands. It diers most markedly from its congeners in possessing a distinctive aboral sulcus in the anterior ambulacrum, which is unique in the Plexechinidae. There is strong phylogenetic evidence that P. sulcatus is the sister group to a clade containing P. cinctus and P. hirsutus. The implications of this placement are discussed in the light of previous work on the evolutionary biology of holasteroids. Introduction Holasteroids are irregular sea urchins that are well represented in the fossil record. They appeared in the Early Cretaceous, but the main diversi®cation of the group occurred during the Upper Cretaceous, during which holasteroids came to represent a signi®cant portion of the echinoid fauna. Holasteroids are consid- erably less common today, and are primarily inhabitants of deep-sea and Antarctic waters. In spite of this decline in their contribution to the modern echinoid fauna, holasteroids are extremely important in understanding major events in the evolution of the Irregularia because of the dramatic disparities in overall test shape, plate architecture, and external appendages between holas- teroids and other echinoids (Mooi and David 1996). Concomitantly, features of many of the Recent holas- teroids can be seen as the result of strongly expressed peramorphosis (David 1990). The ontogenetic trajecto- ries in such taxa result in strongly divergent morpholo- gies that make each of the clades relatively easy to identify. Therefore, attribution of a given specimen to a genus, and most of the time to a species, is generally unambiguous even when the material is fragmentary. Unfortunately, given the fragility of the test and the depths from which the material must be retrieved, much of the systematics of holasteroids is based on broken specimens. Under these circumstances, the advantage to systematists of strong morphological divergence among terminal taxa is obvious. However, these same diver- gences can occasionally pose a challenge to the deter- mination of homologies, particularly given the present gaps in our knowledge of deep-sea holasteroids. Many such gaps represent underestimates of the taxonomic diversity of these forms. For this reason alone, the dis- covery and description of new extant holasteroid taxa are of particular importance. The genus Plexechinus A. Agassiz, 1898 currently includes six nominal taxa (Mooi and David 1996). All are deep-sea species. Plexechinus cinctus A. Agassiz, 1898 is known from the Californian region, P. hirsutus Mortensen, 1905 from the north Atlantic, and P. spec- tabilis Mortensen, 1948 occurs in seas around the Phil- ippines. Three other species, P. aoteanus (McKnight 1974), P. parvus (Mironov 1978) and P. planus (Mironov 1978), are barely distinguishable and probably represent a single circumantarctic species distributed from sub- antarctic temperate waters to the Weddell Sea (Mooi and David 1996). Collections made in 1974 by the R/V ``Marion Dufresne'' were previously reviewed by De Ridder et al. (1992). In that paper, they brie¯y described a single, broken, dry specimen from near the Kerguelen Archi- pelago as ``Plexechinus a. cinctus''. We have been able to re-examine the lot to which this specimen belongs, and have discovered that there is a second, smaller specimen in the same container. Analysis of both Polar Biol (2000) 23: 166±172 Ó Springer-Verlag 2000 B. David (&) UMR CNRS 5561, University of Bourgogne, 6, bd. Gabriel, F-21000 Dijon, France e-mail: bruno.david@u-bourgogne.fr Tel.: +33-3-80396371, Fax: +33-3-80396387 R. Mooi Department of Invertebrate Zoology and Geology, California Academy of Sciences, Golden Gate Park, San Francisco, CA 94118-4599, USA