Skull-less Majiashanosaurus could represent the post-crania of the skull-only Palatodonta at the base of the Placodontia

The Early Triassic skull-less Majiashanosaurus discocoracoidis (Jiang et al. 2014) was originally nested with Dianopachysaurus and Keichousaurus within the eosauropterygia. Not sure why yet, but in the large reptile tree (not yet updated) it nests at the base of the Placodontia along with the skull only Palatodonta, which we tried to guess the post-crania of earlier here.

From the abstract:
“The transverse processes of the dorsal vertebrae are not distinctively elongated. The dorsal ribs are single-headed, and the clavicles articulate on the anteromedial aspect of the scapula. The humerus is curved. These features allow assignment to a new sauropterygian taxon. The interclavicle has no posterior process, and the scapula is of typical eosauropterygian shape, with a broad and ventrally expanded glenoidal portion that is separated from a narrow posterodorsal blade by a distinct constriction. The coracoid is round and plate-like without a waist. This feature is different from that of all other known eosauropterygians, but resembles that of placodonts.”

Possible reasons for tree topology differences

  1. The large reptile tree does not include Dianopachysaurus and Keichosaurus. The former has the proportions and size of a juvenile (on a Google search). I’ll have to take a first look at the latter.
  2. The Jiang et al. tree excluded many enaliosaur taxa which would have attracted several of the included pachypleurosaurs (eosauropterygians) away from that clade.

In the large reptile tree shifting Majiashanosaurus to Paraplacodus adds only two steps. And these taxa are all sisters to basal eosauropterygians like Pachypleurosaurus). Since the number of cervicals is unknown, the number of pre sacrals is also unknown, but with only 19 dorsals and the present nesting, the number of cervicals is likely less than that of Pachypleurosaurus. This is one of the most basal taxa in which the clavicles are dorsal to the interclavicle, which has no medial posterior process.

References
Da-Yong Jiang, et al. 2014. The Early Triassic eosauropterygian Majiashanosaurus discocoracoidis, gen. et sp. nov. (Reptilia, Sauropterygia), from Chaohu, Anhui Province, People’s Republic of China, Journal of Vertebrate Paleontology, 34:5,1044-1052, DOI: 10.1080/02724634.2014.846264

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