Comparison and biomechanical interpretations of the vertebrae and osteoderms of Cacops aspidephorus and Dissorophus multicinctus (Temnospondyli, Dissorophidae)

ABSTRACT

New anatomical data are presented for the vertebrae and osteoderms of Dissorophus multicinctus Cope, 1895 and a previously unexamined specimen of Cacops aspidephorus Williston, 1910a from the Early Permian of Texas. The transverse axis of the zygapophyses of vertebrae in Cacops without osteoderms is closer to the horizontal plane than vertebrae with osteoderms. Ontogenetic fusion of internal osteoderms in Cacops to their respective neural spines begins with the anterior vertebrae and proceeds posteriorly. Midventral flanges appear on the external osteoderms after formation of the internal and external series, but prior to fusion of internal osteoderms to vertebrae. The midventral flange initially has a low round shape, but later assumes an adult blade-like morphology. Zygapophyses in Dissorophus are closer to the midline than in Cacops. Their transverse axis is at an angle of approximately 45° to the horizontal plane and the parasagittal axis is approximately 25° to the horizontal plane throughout the presacrals. A flange on each of the four internal osteoderms of Dissorophus immediately posterior to the enlarged first osteoderm inserts into a deep notch along the anterior, dorsal, and posterior sides of the neural spine. These flanges largely immobilized the vertebrae. Flanges of more posterior internal osteoderms in Dissorophus insert between neural spines. A pronounced anterior inclination of these posterior flanges and close proximity of neighboring internal osteoderms suggests either contact or a small gap between a flange and the next anterior internal osteoderm. Interosteodermal ligaments likely bound the osteoderms and limited lateral flexion.