Field of Science

Postcranial Anatomy and Phylogenetic Relationships of the Sauropodomorph Mussaurus patagonicus

Otero, A. and D. Pol. 2013. Postcranial anatomy and phylogenetic relationships of Mussaurus patagonicus (Dinosauria: Sauropodomorpha). Journal of Vertebrate Paleontology 33:1138-1168.

Abstract - The transition from basal sauropodomorphs to sauropods is one of the most dramatic evolutionary transformations in the history of dinosaurs. Constituent taxa of this transition were recorded mainly in South Africa and South America, and to a lesser extent in North America. We describe here the postcranial anatomy of four specimens of basal sauropodomorphs from the Late Triassic of Patagonia, Argentina, and identify them as adult individuals of Mussaurus patagonicus. The material is composed of one subadult and three adult specimens and was originally identified as Plateosaurus. The completeness of the material provides more complete knowledge of this taxon and allows us to introduce aspects of basal sauropodomorph anatomy that were poorly understood until now, such as the configuration and arrangement of the distal carpal elements. The phylogenetic relationships of Mussaurus patagonicus are tested through a cladistic analysis of basal sauropodomorphs based on the anatomy of these specimens rather than on the post-hatchling and juvenile specimens previously known for this taxon. Mussaurus is recovered as a non-sauropod anchisaurian, being the sister group of Aardonyx plus more derived sauropodomorphs and is depicted outside the ‘quadrupedal clade,’ given the presence of plesiomorphic features such as a humerus/femur ratio <0.8, a curved femoral shaft in lateral view, and a nearly circular femoral midshaft cross-section. Mussaurus patagonicus adds new and valuable information that helps to clarify the core of the basal sauropodomorph-sauropod transition.

5 comments:

  1. So, Coloradisaurus is not the adult form after all, huh?

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    1. If you've looked at the cladogram in Otero and Pol, Coloradisaurus is recovered as a massospondylid and does not form a clade with Mussaurus to the exclusion of other basal sauropodomorphs. Otero and Pol used adult characters in their cladistic analysis because the juvenile characters in the holotype of Mussaurus would give the sauropodomorph workers the false impression that Mussaurus is a very primitive sauropodomorph. Since the specimens described by Otero and Pol share characters with the Mussaurus type specimen but not Coloradisaurus, the hypothesis that Mussaurus is a juvenile of Coloradisaurus is no longer tenable.

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  2. And 36 years later, Casamiquela's Plateosaurus sp. remains are finally described. Excellent paper though. One consequence of this paper that the authors don't mention the significance of is that it lets us use Sereno's taxon Sauropodiformes more effectively, as it was defined as Mussaurus + Saltasaurus. Previously, Mussaurus' affinities were poorly known so that the clade could go in several different positions.

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    1. In the cladistic analysis of Eodromaeus, Martinez, Sereno, and colleagues use Sauropodiformes for all saurodomorphs more derived than Guaibasauridae. Although Sereno uses Mussaurus as an internal specifier for Sauropodiformes but doesn't say whether the non-anchisaurian sauropodomorphs are part of his definition of Sauropodiformes, McPhee's 2013 thesis (see http://wiredspace.wits.ac.za/bitstream/handle/10539/12967/Masters_Blair%20McPhee_final.pdf?sequence=1) defines Sauropodiformes as the most inclusive clade containing Yunnanosaurus and Saltasaurus but not Massospondylus, in which case Sauropodiformes would be used for all massopods more derived than Massospondylus or Riojasaurus. To avoid future confusion, Anchisauria should be used to accomodate Anchisaurus, Mussaurus, and Sauropoda, while Sauropodiformes should be used for the Mussaurus+Aardonyx+Sauropoda clade.

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    2. Sauropodiformes was named by Sereno on his TaxonSearch website in 2005 but first published in his 2007 Ammosaurus paper with the definition "The least inclusive clade containing Mussaurus patagonicus Bonaparte and Vince, 1979 and Saltasaurus loricatus Bonaparte and Powell, 1980". The Eodromaeus paper contains an identical definition on page 210, so does specify exactly which sauropodomorphs don't belong.

      Thanks for linking to McPhee's thesis, as I had not seen it yet. But you got his definition wrong, as he defines Sauropodiformes as (Saltasaurus <- Massospondylus), which is more inclusive than Sereno's definition. I don't think this is helpful.

      Basically, Yates, Upchurch and Sereno each had a different opinion on which near-sauropod clade to define. So now we have Massopoda for (Saltasaurus <- Plateosaurus), Anchisauria for (Anchisaurus + Saltasaurus) and Sauropodiformes for (Mussaurus + Saltasaurus).

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