Osteology of Dashanpusaurus dongi (Sauropoda: Macronaria) and new evolutionary evidence from Middle Jurassic Chinese sauropods

The Middle Jurassic lower Shaximiao Formation in Sichuan Province of south-western China has yielded a diverse terrestrial vertebrate fauna dominated by sauropod dinosaurs. However, many of these sauropods lack detailed descriptions or explicit phylogenetic diagnoses. Here, we present a comprehensive redescription of Dashanpusaurus dongi, a species of sauropod found only in the lower Shaximiao Formation. We define the revised autapomorphies of the species as follows: neural canals are sub-square in anterior dorsal vertebrae; the presence of a thin accessory lamina that contacts the prezygodiapophyseal and paradiapophyseal laminae of the middle dorsals, forming an angle of 75° to the horizontal; and four ridges on the anterodistal edge of the humerus. Often considered part of the epipophyseal-prezygapophyseal lamina, a strut invades the spinodiapophyseal fossa in the cervical and anterior dorsal vertebrae. Anatomical comparisons indicate that this feature was widespread among early-diverging Middle Jurassic eusauropod lineages. This comparative anatomical data provides an opportunity to revisit the phylogenetic position of Dashanpusaurus and the relationships of the neosauropod clade. Recovered as a macronarian, a better understanding of Dashanpusaurus dongi will allow for clarification of the origin, early evolution, and palaeogeographical distribution of neosauropods. This study also suggests that the diversity and dispersity of the neosauropod clade occurred much earlier than previously realized.