58.4 Saturday, Jan. 5 Deltoptychius: investigating the roots of the chimaeroid cranial condition CRISWELL, KE*; FINARELLI, JA; FRIEDMAN, M; GARWOOD, R; COATES, MI; University of Chicago; University College Dublin; Oxford University; University of Manchester and Diamond Light Source; University of Chicago email@example.com
Chondrichthyes includes elasmobranchs and holocephalans, but little is known about the early memberships of these groups. In the early 1980s the fossil collector S. P. Wood discovered exceptional specimens of Deltoptychius, a Lower Carboniferous holocephalan, while excavating the fossil fish site at Bearsden, Scotland (Serpukhovian: ~326-318 Ma). Deltoptychius traditionally was diagnosed by features including a head shield and presence of mandibular spines. CT scanning of the Bearsden specimens revealed numerous characters that were not previously known, including details of the braincase concealed by the dermatocranium. Additional comparisons with recent chimaeras and early chondrichthyans such as Chondrenchelys, iniopterygians, and Pucapampella, allowed us to investigate character transformations that occurred during the evolutionary history of this group. Deltoptychius shares with modern holocephalans the anterior location of the jaw articulation, similar size and position of the otic capsules, the presence of tooth plates, and the presence of a dorsum sellae. However, more primitive characteristics also are present. For the first time we see cranial characters approaching the general gnathostome condition within an otherwise undeniably holocephalan taxon.