THE SKELETAL MORPHOLOGY OF THE LOWER CARBONIFEROUS HOLOCEPHALAN DELTOPTYCHIUS FROM BEARSDEN, SCOTLAND

Holocephalans are a group of marine, cartilaginous fishes with a fossil record extending to the Devonian Period. Because of their cartilaginous skeletons, the majority of holocephalan fossils consist of tooth plates and fin spines, which provide a limited set of characters for determining phylogenetic relationships. The Lower Carboniferous holocephalan Deltoptychius is known from several complete body fossils and has the potential to illuminate interrelationships within this group.

We obtained a CT-scan of a complete Deltoptychius specimen from Bearsden, Scotland (Serpukhovian: ~326-318 Ma), and have identified and digitally isolated parts of the skull, mandible, and pectoral girdle. Deltoptychius traditionally was diagnosed by features including a head shield made up of dermal plates and scales, supraorbital sensory line grooves on the surface of the dermal bones, and lack of frontal spines on the neurocranium roof. This specimen of Deltoptychius is more complete than previously-known individuals, and possesses numerous characters that were not discernible in less complete specimens, including the position of the orbits with respect to the neurocranium, the placement of the mandibular articulation with the quadrate, morphology of the basicranium, and details of the mandibular and palatal dentitions.

Because so few complete holocephalan body fossils are known, the present CT-scan of Deltoptychius represents an opportunity to thoroughly document the skeletal morphology of this taxon for the first time, and to incorporate it into a comprehensive analysis of holocephalan phylogenetic relationships.