Meeting Abstract
57.2 Tuesday, Jan. 6 Synarcual Variation in the Purportedly Invariable Clade, Rajidae CLAESON, Kerin M.; The University of Texas at Austin kclaeson@mail.utexas.edu
Batoid fishes (electric rays, sawfishes, skates, guitarfishes, and stingrays) are united by a suite of characters including the presence of the synarcual, a tube-like skeletal element hypothesized to be the fused anterior-most vertebrae of the axial skeleton. The morphology of the synarcual is highly variable among the batoid subgroups and appears linked to the evolution of disparate locomotory styles associated among these groups. Until now, synarcual characters have been underutilized in phylogenetic analyses, especially of non-stingray taxa. For this study, I examined the ontogenetic and systematic variation of synarcual morphology in fossil and extant skates and rajid-guitarfishes, which are known to possess a morphologically conservative body plan. Fifteen fossil and extant species were prepared and studied using camera lucida, histology, traditional dissection and skeletonization, x-radiography, and 3D reconstruction from CT-scanning. New data permit me to describe previously undocumented synarcual characters, specify those suitable for use in phylogenetic analysis, and shed light on the development of the synarcual over geologic time. For instance, the number of spinal nerve foramina are too variable to be used within a rajid-specific analysis, however, when considered alongside guitarfish outgroups, foramen number increases among derived skates. Additionally, the relative position of the first vertebral centrum in the synarcual base varies only slightly within genera and in fossil representatives the first centrum is comparatively more anterior than in extant taxa. Thus within Rajidae, although the total length of the synarcual remains the same relative to that of the vertebral column, the number of true centra surrounded by the posterior flanges of the synarcual decreases systematically, effectively becoming a more massive skeletal element.
