Aspects of gorgonopsian paleobiology and evolution: insights from the basicranium, occiput, osseous labyrinth, vasculature, and neuroanatomy
- Published
- Accepted
- Subject Areas
- Developmental Biology, Evolutionary Studies, Neuroscience, Paleontology, Anatomy and Physiology
- Keywords
- braincase, mammals, modularity, vestibular organ, synchrotron, gorgonopsian, homology, brain, vasculature, therapsid
- Copyright
- © 2017 Araujo et al.
- Licence
- This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
- Cite this article
- 2017. Aspects of gorgonopsian paleobiology and evolution: insights from the basicranium, occiput, osseous labyrinth, vasculature, and neuroanatomy. PeerJ Preprints 5:e2313v2 https://doi.org/10.7287/peerj.preprints.2313v2
Abstract
Synapsida, the clade including therapsids and thus also mammals, is one of the two major branches of amniotes. Organismal design, with modularity as a concept, offers insights into the evolution of therapsids, a group that experienced profound anatomical transformations throughout the past 270Ma, eventually leading to the evolution of the mammalian bauplan. However, the anatomy of some therapsid groups remains obscure. Gorgonopsian braincase anatomy is poorly known and many anatomical aspects of the brain, cranial nerves, vasculature, and osseous labyrinth, remain unclear. We analyzed two gorgonopsian specimens, GPIT/RE/7124 and GPIT/RE/7119, using propagation phase contrast synchrotron micro-computed tomography. The lack of fusion between many basicranial and occipital bones in the immature specimen GPIT/RE/7124 allowed us to reconstruct its anatomy and ontogenetic sequence in comparison with the mature GPIT/RE/7119. We examined the braincase and rendered various skull cavities. Notably, there is a separate ossification between what was previously referred to as the “parasphenoid” and the basioccipital. We reinterpreted this element as a posterior ossification of the basisphenoid: the basipostsphenoid. Moreover, the “parasphenoid” is a co-ossification of the dermal parasphenoid and the endochondral basipresphenoid. Our detailed examination of the osseous labyrinth reveals a unique discoid, rather than toroidal, morphology of the horizontal semicircular canal that probably results from architectural constraints of the opisthotic and supraoccipital ossifications. In addition, the orientation of the horizontal semicircular canal suggests that gorgonopsians had an anteriorly tilted alert head posture. The morphology of the brain endocast is in accordance with the more reptilian endocast shape of other non-mammaliaform neotherapsids.
Author Comment
We succinctly list below the revisions we have done in the manuscript since the first submission:
1) We segmented an entirely new gorgonopsian specimen GPIT/RE/7119 showing the basicranium morphology in an ontogenetically more mature specimen. We used this specimen to further validate our observations as well as to give an idea of the variation expected within gorgonopsians.
2) We added many more comparative taxa to support our observations on the evolution of the basicranium in synapsids, including two synchrotron scans of cynodonts (one of them was entirely segmented and we provided a supplementary video), a synchrotron scan of a therocephalian, etc.
3) We re-rendered all the images with VgStudio MAX resulting in much better images plates and figures.
4) We added three new sections in the discussion: “Comparative anatomy of the occiput”, “Comparative anatomy of the basicranium”, and “The evolution of the synapsid basicranial axis: parabasisphenoid, prootic, basioccipital”, and three new figures to better explain and illustrate/demonstrate our results.
5) The occiput of GPIT/RE/7124 was completely re-segmented conforming now to a more typical, expected, gorgonopsian morphology. Namely, the tabulars, the interparietal, and the supraoccipital.
6) The synchrotron tomographic slices of GPIT/RE/7124 were re-reconstructed using a new, more up-to-date and powerful algorithm.
7) The reference list has been substantially expanded with 38 new references.
Supplemental Information
Basicranium of Galesaurus BP1-5973
Longitudinal rotation of the Galesaurus basicranium BP1-5973