Meeting Abstract

71.1  Tuesday, Jan. 6  Heads, Jaws, and Feeding: In Search of the Basal Chondrichthyan Condition. GROGAN, E.D.*; LUND, R; Saint Joseph’s University; Carnegie Museum egrogan@sju.edu

Chondrichthyans, represented today by sharks, skates, rays, and chimaeras, are gnathostomes with an evolutionary history spanning 400-500 million years. They are often invoked to exemplify the primitive jawed vertebrate condition by virtue of their cartilaginous skeleton, a neurocranium presenting as a continuous unit, and in the forms of jaw and jaw suspension that they exhibit. Yet, new observations and discoveries of Paleozoic chondrichthyans reveal much greater variation in chondrocranial design, suspensorial design, and feeding mechanisms than previously acknowledged. The data suggest that most Carboniferous chondrichthyans were operculate and characterized by jaws suspended directly from the cranium, without the intervention of the hyoid arch. This autodiastylic condition has previously been proposed as the condition from which all other chondrichthyan suspensorial states may arise. With regard to chondrocranial design, Devonian and Carboniferous finds have confirmed early forms with a partial to complete intracranial fissure. A more basal condition is suggested by taxa from the Bear Gulch Limestone of Montana (Serpuhkovian, Namurian E2b) which provide evidence of a neurocranium in three parts, generated by an intracranial joint and a complete oticooccipital fissure. Despite some variation within these forms, features of the suspensorium and feeding mechanism of these taxa reveal a premandibular component to the feeding mechanism in chondrichthyans. Taken collectively, the anatomical details of these forms reveal that early chondrichthyans exhibited features previously attributed only to other gnathostomes. This provides insight into the basal chondrichthyan condition and, so, a better resolution of the relationship of chondrichthyans to other gnathostomes