ONTOGENETIC AND TAXONOMIC INSIGHTS INTO DUNKLEOSTEUS TERRELLI FROM A NEARLY COMPLETE SUBADULT SPECIMEN FROM THE CLEVELAND SHALE OF OHIO

The stem-gnathostome Dunkleosteus terrelli was recognized as a distinct species 150 years ago and is known from many nearly complete and articulated specimens in both North America and Africa. A flurry of studies in the late 19^th to early 20^th century established the species as a morphological standard of comparison among stem-gnathostomes. Ironically, these early studies appear to have been done so well at the time that publications on the species slowed significantly up to the modern day. This has left many questions about the specie’s biology and ecology unanswered, such as how the organism changed during growth. A recently prepared specimen of a small D. terrelli from the Cleveland Shale of Ohio has a nearly complete head shield, gnathal elements, partial thoracic shield, and a partial articulated vertebral column which provide baseline morphological data of a subadult D. terrelli for the first time. Overall, the midline plates of the subadult head shield have greater length/width ratios in comparison to large individuals while the lateral plates of the head shield are of similar proportions. Internally, the anterior and lateral thickenings of the neurocranium typical of D. terrelli are absent or poorly developed. These, along with other observations of morphology, allow for a reevaluation of the African species Dunkleosteus marsaisi and suggest that it is not a distinct species, but a subadult D. terrelli. All seven of the autapomorphies of D. marsaisi are found to be taphonomic artifacts, within the range of intraspecific variation in D. terrelli, or ontogenetic differences.