Osteology of <i>Klamelisaurus gobiensis</i> (Dinosauria, Eusauropoda) and the evolutionary history of Middle–Late Jurassic Chinese sauropods

Fossil-rich deposits from the Middle and Late Jurassic of China have yielded a diverse array of sauropod dinosaurs, including numerous species referred to <i>Mamenchisaurus</i> and <i>Omeisaurus</i>. Despite an abundance of fossils and a proliferation of taxa, the anatomy of Middle–Late Jurassic Chinese sauropods remains poorly documented. Here, we comprehensively redescribe and illustrate <i>Klamelisaurus gobiensis</i> from the Middle–Late Jurassic Shishugou Formation of northwest China. Phylogenetic analyses conducted under parsimony and time-calibrated Bayesian optimality criteria consistently recover <i>Klamelisaurus</i> as a member of a predominantly Chinese radiation of exceptionally long-necked eusauropods that includes <i>Mamenchisaurus</i> spp., <i>Chuanjiesaurus</i>, <i>Qijianglong</i> and <i>Wamweracaudia</i>. In most analyses, this lineage also includes <i>Euhelopus</i>, reviving a ‘traditional’ Euhelopodidae and calling into question the macronarian affinities of <i>Euhelopus</i>. <i>Klamelisaurus</i> shares several features with <i>Euhelopus</i> that are unique to a subset of East Asian taxa or rare among sauropods, including a convex ventral margin of the prezygodiapophyseal lamina in middle–posterior cervical vertebrae, a ventrally bifurcated postzygodiapophyseal lamina in posterior cervical vertebrae, and development of a rugose projection extending anteriorly from the epipophysis into the spinodiapophyseal fossa in most cervical vertebrae. Anatomical comparisons of the cervical vertebrae of <i>Klamelisaurus</i> to several other sauropodomorphs and insights from myological studies of extant archosaurs strongly suggest that this latter structure, often considered part of an epipophyseal-prezygapophyseal lamina, is an epaxial muscle scar that is distinct from pneumatic structures of the lateral surface of the neural spine. The phylogenetic and comparative anatomical data presented here provide a foundation for future revision of the taxonomy and systematics of sauropods from the Junggar and Sichuan basins.