Meeting Abstract
5.4 Saturday, Jan. 4 08:45 The Role of Guitarfish in the Origin of Skates and Rays CLAESON, KM*; ASCHLIMAN, NC; UNDERWOOD, CJ; PCOM; St. Ambrose University; Birkbeck College kerincl@pcom.edu
Recent phylogenetic analyses based on molecular data, paleontological data, or extant morphology consistently recovered a paraphyletic “Rhinobatiformes” (guitarfishes) but disagreed on the position of their close relatives. This prompted our investigation into the evolutionary processes that shaped early batoid diversity. Specifically, we sought to determine the extent to which patterns and rates of phenotypic evolution varied among the “rhinobatiform” taxa indicated to be ancestral to more derived lineages of Batoidea. We examined phenotypic trajectories using morphometrics of the synarcual, a well-supported synapomorphy of Batoidea, across a concatenated time-calibrated tree based on modern and fossil data. We found evidence of repeated similar phenotypic trajectories among disparate batoid groups. For instance, synarcual hyaline and tessellated cartilages became longer and were perforated by more and more spinal nerve foramina (i.e., the synarcual is short and incipient in Jurassic taxa but more massive and complex in derived batoids). At the same time, the relative position of the first free vertebral centrum is found more posterior along the synarcual. Previously demonstrated for skates, this is now noted in electric rays, stingrays, and “guitarfish” independently. These changes coincide with modifications in lifestyle from an early (Jurassic) batoid rhinobenthic habitus of bottom dwelling/feeding plus caudal propulsion, to parallel adaptations for less caudal forms of locomotion and additional pectoral specializations. Similarities between Jurassic batoids and modern guitarfishes are superficial; therefore modern guitarfish paraphyly may indicate rapid diversification after the appearance of taxa restricted to the Jurassic. Similarities between skates and rays are convergent occurring most recently in rays.
