PHYLOGENETIC ANALYSIS OF SKATES, A MORPHOLOGICALLY CONSERVATIVE CLADEOF ELASMOBRANCHS (CHONDRICHTHYES, RAJIDAE)

Skates (Rajidae) are unique among chondrichthyans for their high speci es diversity and morphological conservatism. To elucidate phylogenetic interrelationships within this taxon, we surveyed a wide range of mor phological character complexes under the premise that characters withi n different character complexes are constrained in different ways and would thus reveal a robust phylogeny. Maximum-parsimony analysis emplo ying 31 taxa, including three outgroups, and 55 characters produced 20 equally parsimonious trees of 160 steps (consistency index = 0.681, r etention index = 0.850, homoplasy index = 0.462, and rescaled consiste ncy index 0.579). The strict consensus tree divided rajids into two ma jor clades. The first, Rajinae, consisted of two partially resolved cl ades and one fully resolved clade and was defined by three ambiguous c haracter states: (I) scapulocoracoids that lack an anterior bridge; (2 ) claspers that are distally expandible; and (3) claspers that possess the component rhipidion, The taxon comprises three tribes, 15 genera (seven elevated from subgenera of Raja and Gurgesiella), and 149 speci es. The second major clade, Arhynchobatinae, was nearly fully resolved and was defined by two unambiguous character states: (1) basihyal car tilages that possess lateral extensions; and (2) claspers that possess the component projection. The taxon comprises two tribes, 11 genera, two genera elevated from subgenera of Raja, and 79 species. The strict consensus tree revealed considerable parallelisms in morphological ev olution within rajids. Some parallelisms, such as reduction of the ros tral cartilage and concomitant forward extension of pectoral radials a nd muscles and enlarged nasal capsules, appear to be adaptations for d eepsea benthic habitats. The closest relatives of rajids, Rhinobatos, Zapteryx and Trygonorrhina, are limited to shallow water. Thus, it is likely that ancestral rajids had a similar habitat and that subsequent ly rajids made multiple radiations into the deep sea and developed fle xible snouts for grubbing in soft substrates and enlarged nasal rosett es for increased chemosensistivty in regions with little light.