PHYLOGENETIC ANALYSIS OF EXTREME MORPHOLOGIES - DEEP-SEA HOLASTEROID ECHINOIDS

Because of their 'bizarre' features, some echinoids can be considered laboratories in which to analyse the origin of extreme morphologies. T he holasteroid family Urechinidae Duncan, 1889 is such a group. It is composed of 18 previously named living species in three genera: Urechi nus A. Agassiz, 1879, Plexechinus A. Agassiz, 1898, and Pilematechinus A. Agassiz, 1904. With Stereopneustes, the most closely related livin g taxon, and the calymnid-plus-pourtalesiid clade as outgroups, a phyl ogenetic analysis on 35 binary characters is produced. We focus on two major aspects of morphology: test features (apical system, basicorona ls, interambulacrum 5, test shape, ambulacra) and external appendages (pedicellariae, spines, sphaeridia). New data on plate architecture, m orphology, and appendages are illustrated, and the position of the fos sil, Chelonechinus, is discussed. We show that the Urechinidae is para phyletic: the genus Plexechinus shares more recent common ancestry wit h the calymnids and pourtalesiids than with other urechinids. In order to retain the well-circumscribed clade, Pilematechinus A. Agassiz, 19 04, the phylogenetic classification also recognizes two additional gen era: Cystechinus A. Agassiz, 1879, and Antrechinus new genus. A key to the species is provided. The production of extreme morphologies, such as internal parental care, is explored by mapping test size, the occu rrence of fascioles, and periproct position onto the phylogeny. By add ing plates early in ontogeny, Pilematechinus develops radically differ ent plate architectures from another genus of large forms, Cystechinus . In contrast, the latter gets large by exaggerating allometric trends seen in other holasteroids. Paedomorphosis and miniaturization have e volved independently in Antrechinus and some Plexechinus, but always b y truncation of allometric trajectories.