REGA, E.A.; NORIEGA, K.C.; SUMIDA, S.S.; LEE, A.H.; Western University of Health Sciences; California State University San Bernardino; California State University San Bernardino; University of California, Berkeley: Histological analysis of traumatic injury to multiple neural spines of an associated skeleton of Dimetrodon: Implications for healing response, dorsal sail morphology and age-at-death in a Lower Permian synapsid

Histological studies of fossil bone allow us to test hypotheses of growth, repair, and soft tissue reconstruction in extinct taxa. Most studies use non-pathologic bones to address those hypotheses. In the current study however, we examine the histological microstructure of pathologic neural spines from the pelycosaur, Dimetrodon giganhomogenes (FMNH � UC 1134). The most common form of pathology in Dimetrodon is healed fracture of axial structures. We test whether (1) repair in Dimetrodon neural spines is similar to that in living mammalian bone and (2) a soft-tissue sail was present, which would help to align the broken spine fragments. Sections from the hard callus area of the fracture site show highly vascularized lamellar bony tissue. There is no evidence of calcified fibrocartilage or a loose network of cancellous bone, which are deposited in the repair response of living mammalian bone. Bony tissue is present through the middle of the hard callus, which suggests that a soft-tissue sail was present. That sail would provide non-rigid fixation to the fracture site. The outer cortical tissue of the fracture site also lacks a dense network of vascular canals, so it is doubtful that pelycosaur sails were thermoregulatory.