Pneumatic structures of sauropod cervical vertebrae from the Lower Cretaceous Sao Khua Formation of northeastern Thailand

Abstract

Internal structures of sauropod vertebrae are hollowed out by pneumatic diverticular systems similar to extant birds. Their vertebral columns contain extensive air sac systems that are highly prominent in the cervical regions. Here we analyze the pneumatic structures in the cervical vertebrae of two sauropod taxa: Phuwiangosaurus sirindhornae and a diplodocoid sauropod, from the Lower Cretaceous Sao Khua Formation of northeastern Thailand, using a CT-scan technique. The internal pneumatic structures of P. sirindhornae mainly consisted of camerae within the centrum and systems of camellae in the articulation regions, representing a semicamellate pattern. The diplodocoid sauropod exhibited a camerate pattern with its centrum filled with only pneumatic camerae. The semicamallate pattern of Phuwiangosaurus sirindhornae is similar to that of Brachiosaurus sp. and Giraffatitan brancai, differing from previous interpretations that suggested a close relationship to somphospondyls. Due to the more primitive internal structures of Phuwiangosaurussirindhornae, we instead propose that Phuwiangosaurus is a non-somphospondyl titanosauriform more closely related to brachiosaurids. In the Sao Khua diplodocoid sauropod, the vertebral pneumatization was much higher than in dicraeosaurids but less intense than in most neosauropods. The presence of a semicamellate pattern in Phuwiangosaurus extends the temporal range of such a condition to the Early Cretaceous, while the camerate pattern of the Sao Khua diplodocoid suggests that Cretaceous dicraeosaurids likely had a varying degree of pneumaticity, which might have evolved independently in their lineages.