Sunday, January 21, 2018

[Paleontology • 2018] A Revised Cranial Description of Massospondylus carinatus Owen (Dinosauria: Sauropodomorpha) based on Computed Tomographic Scans and A Review of Cranial Characters for Basal Sauropodomorpha


Massospondylus carinatus  Owen, 1854

in Chapelle & Choiniere, 2018
   DOI: 10.7717/peerj.4224 

Abstract

Massospondylus carinatus is a basal sauropodomorph dinosaur from the early Jurassic Elliot Formation of South Africa. It is one of the best-represented fossil dinosaur taxa, known from hundreds of specimens including at least 13 complete or nearly complete skulls. Surprisingly, the internal cranial anatomy of M. carinatus has never been described using computed tomography (CT) methods. Using CT scans and 3D digital representations, we digitally reconstruct the bones of the facial skeleton, braincase, and palate of a complete, undistorted cranium of M. carinatus (BP/1/5241). We describe the anatomical features of the cranial bones, and compare them to other closely related sauropodomorph taxa such as Plateosaurus erlenbergiensis, Lufengosaurus huenei, Sarahsaurus aurifontanalis and Efraasia minor. We identify a suite of character states of the skull and braincase for M. carinatus that sets it apart from other taxa, but these remain tentative due to the lack of comparative sauropodomorph braincase descriptions in the literature. Furthermore, we hypothesize 27 new cranial characters useful for determining relationships in non-sauropodan Sauropodomorpha, delete five pre-existing characters and revise the scores of several existing cranial characters to make more explicit homology statements. All the characters that we hypothesized or revised are illustrated. Using parsimony as an optimality criterion, we then test the relationships of M. carinatus (using BP/1/5241 as a specimen-level exemplar) in our revised phylogenetic data matrix.


Figure 1: Photographs of the skull of BP/1/5241. (A) Left lateral view. (B) Right lateral view. (C) Dorsal view.

Figure 2: Reconstructed skull of BP/1/5241.
(A) Left lateral view. (B) Right lateral view.

 aof, antorbital fenestra; aofs, antorbital fossa; bo, basioccipital; ecpt, ectopterygoid; eo, exoccipital; fr, frontal; itf, infratemporal fenestra; j, jugal; la, lacrimal; mx, maxilla; n, nasal; nf, narial fenestra; obt, orbit; pa, parietal; pf, prefrontal; pmx, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sq, squamosal.

SYSTEMATIC PALEONTOLOGY
Dinosauria Owen, 1842
Saurischia Seeley, 1887
Sauropodomorpha Huene, 1932

Massospondylidae Huene, 1914 sensu Yates, 2003b

Massospondylus carinatus Owen, 1854

....

Conclusion: 
The use of CT scanning and 3D visualization graphics allows for a better understanding of the internal and external morphological structures of the braincase as well as information about the soft tissues such as the vestibular canals. M. carinatus can be tentatively diagnosed cranially by basipterygoid processes that are separated by an angle smaller than 60° and a jugal process of the ectopterygoid that is strongly curved. A revision of cranial characters provides a basis for more comparative work on the braincase of sauropodomorphs in general. Results also show a well-supported Massospondylidae clade. Further phylogenetic analyses using individual specimens of known ontogenetic stages as operational taxonomic units would provide better resolution for the Massospondylidae clade as well as a better understanding of which sets of character states set M. carinatus aside from other taxa.


Kimberley E.J. Chapelle and Jonah N. Choiniere. 2018. A Revised Cranial Description of Massospondylus carinatus Owen (Dinosauria: Sauropodomorpha) based on Computed Tomographic Scans and A Review of Cranial Characters for Basal Sauropodomorpha.   PeerJ. 6:e4224.  DOI: 10.7717/peerj.4224
CT-scan study makes it possible to 3-D print the skull of the dinosaur species Massospondylus http://phy.so/434970306 via @physorg_com



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