Meeting Abstract
31.2 Friday, Jan. 4 A New Iniopterygian (Class Chondrichthyes) from the Mississippian Bear Gulch Limestone of Montana and New Insights into the Early Chondrichthyan Neurocranial Design. GROGAN, E.D.*; LUND, R; Saint Joseph’s University, Philadelphia, PA; Carnegie Museum, Pittsburgh egrogan@sju.edu
The Iniopterygii are Paleozoic chondrichthyans that are poorly understood. Morphological evidence of the form code-named Iniop3 from the Bear Gulch Limestone of Montana is now presented. It displays a laterally compressed head and body and a homocercal tail. It exhibits the iniopterygian synapomorphy of pectoral fins which articulate along the dorsolateral margins of the trunk immediately behind the head and project dorsolaterally. A single aspinous dorsal fin originates at approximately the midpoint of the fish. There is virtually no caudal fin known distal to the caudal endoskeleton. Squamation is absent except along the anterior pectoral fin margins. Bony cranial plates are also absent. Of all iniopterygians, Iniop 3 is most notable for information that it presents for the neurocranium and the feeding apparatus. The neurocranium calcified as two moieties separated by an intracranial joint. The feeding apparatus is not mandibular in nature. Two pairs of upper pre-palatal parasymphysial tooth whorls and one median lower premandibular symphysial tooth whorl are present and supported by a series of labial cartilages. The lower symphysial whorl and its supporting cartilage appear to have been passively mobile. They are preserved in an extended position in specimens with depressed lower jaws. In specimens where the lower jaws have been elevated into the closed position the lower symphysial whorl is found to be rotated posterodorsally into shearing occlusion between the upper symphysial whorls. When considered in the context of early gnathostome evolution, the cranial anatomy and feeding apparatus of Iniop 3 present novel information on the diversity of early chondrichthyans and suggest greater commonalities with other gnathostome classes than has been previously acknowledged.
