Vertebral Pathology in an Ornithopod Dinosaur: A Hemivertebra in Dysalotosaurus lettowvorbecki from the Jurassic of Tanzania

A vertebral fragment of the Late Jurassic ornithopod dinosaur Dysalotosaurus lettowvorbecki from Tanzania is described. It consists of a hemivertebra that is co-ossified with a complete vertebral centrum. The hemivertebra causes a hyperkyphotic posture of the vertebral column with an angle of approximately 35 degrees between the end plates of the vertebra, that is, a dorsal bending of the vertebral column. Associated with this is a 15 degrees lateral bending, which suggests a scoliosis. Micro-CT scans reveal thickening of the cortical bone in the hemivertebra and the complete vertebral centrum as compared to a "normal" vertebra. This can be interpreted as a reaction of the bone to the abnormal direction of forces arising from the defective configuration of the vertebral column. No signs of vertebral fracture are present. The arrangement of the Foramina venosa and the trapezoidal outline of the complete centrum that is co-ossified with the hemivertebra indicate that the hemivertebra in Dysalotosaurus developed early in embryogenesis probably by "hemimetameric segmental shift", that is, a defect of the fusion of the paired vertebral anlagen. This finding illustrates that hemivertebrae represent a fundamental defect of the vertebrate ontogenetic program.