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.
A new data on the geological structure, conditions of formation and faunal composition of the Early Cretaceous site of the terrestrial vertebrates by Shestakovo village (Kemerovo Region, Western Siberia) has been presented.The consolidated geological section has been constructed along the line Shestakovo-1 — Shestakovo-4 — Shestakovo-3 in which five lithologic members have been identified. A distribution of the vertebrates fauna taxons has been carried out along the selected lithologic members composing the Shestakovo series of the Ilek formation. A new data obtained during fieldwork in 2017 has made it possible to distinguish two main bony levels (lithologic members 3 and 5), which contain whole skeletons of reptiles. Lithofacies analysis has shown that the formation of the sites occurred under the conditions of the fluviolacustrine plain, where the channel, delta and floodplain facies were replaced by lake and lake-marshy facies, forming a series of sedimentation cycles. The latter are the evidence of the increase in the aridization of the climate upwards along the section. The given data calls into question previously expressed point of view about the coastal-marine or lagoon genesis of the Shestakovo series.
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