Phylogenetic analysis of skates, a morphologically conservative Clade of Elasmobranchs (Chondrichthyes: Rajidae)

Skates (Rajidae) are unique among chondrichthyans for their high species diversity and morphological conservatism. To elucidate phylogenetic interrelationships within this taxon, we surveyed a wide range of morphological character complexes under the premise that characters within different character complexes are constrained in different ways and would thus reveal a robust phylogeny. Maximumparsimony analysis employing 31 taxa, including three outgroups, and 55 characters produced 20 equally parsimonious trees of 160 steps (consistency index = 0.681, retention index = 0.850, homoplasy index = 0.462, and rescaled consistency index = 0.579). The strict consensus tree divided rajids into two major clades. The first, Rajinae, consisted of two partially resolved clades and one fully resolved clade and was defined by three ambiguous character states: (1) scapulocoracoids that lack an anterior bridge; (2) claspers that are distally expandible; and (3) claspers that posses the component rhipidion. The taxon comprises three tribes, 15 genera (seven elevated from subgenera of Raja and Gurgesiella), and 149 species. The second major clade, Arhynchobatinae, was nearly fully resolved and was defined by two unambiguous character states: (1) basihyal cartilages 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 evolution within rajids. Some parallelisms, such as reduction of the rostral cartilage and concomitant forward extension of pectoral radials and muscles and enlarged nasal capsules, appear to be adaptations for deep-sea 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 subsequently rajids made multiple radiations into the deep sea and developed flexible snouts for grubbing in soft substrates and enlarged nasal rosettes for increased chemosensistivty in regions with little light.