Morphology of the Craniovertebral Joint in Psittacosaurus sibiricus (Ornithischia: Ceratopsia)

Abstract

The joint of the occipital skull region and two first cervical vertebrae of Psittacosaurus sibiricus from the Shestakovo locality (Barremian–Aptian), Kemerovo Region, Russia is described. The morphological differences in the atlas–axis complex between the infraorders Psittacosauria and Neoceratopsia connected with their functional features are revealed. The atlas–axis complex of P. sibiricus consists of two unfused vertebrae. The atlas includes the centrum, neural arch, and intercentrum. The axis consists of fused centrum, neural arch, and intercentrum, which is fused with its centrum. The cranial processes of the neural arch of the atlas are well developed, could have indicated the presence of a proatlas. The shape of the occipital condyle and anterior articular surface of the atlas are evidence that the occiput–atlas joint was only slightly flexible in the vertical plane. The atlas–axis complex and occipital condyle of P. sibiricus are similar in morphology to that of ornithopods.