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[dinosaur] Burnetiamorphs (Therapsida) boneheads + dinocephalian extinction from track evidence




Ben Creisler
bcreisler@gmail.com


Recent early synapsid papers:

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Zoe T. Kulik &Â Christian A. Sidor (2019)
The original boneheads: histologic analysis of the pachyostotic skull roof in Permian burnetiamorphs (Therapsida: Biarmosuchia).
Journal of Anatomy (advance online publication)
doi: https://doi.org/10.1111/joa.12987
https://onlinelibrary.wiley.com/doi/10.1111/joa.12987


Thickened, pachyostotic skulls are best known in pachycephalosaur dinosaurs, but evolved convergently in Permian burnetiamorphs as well as in some other stemâmammal groups and Triassic archosauromorphs. Until now, only pachycephalosaur domes have been histologically sampled to reveal patterns of bone tissue microstructure and growth. Using computed tomography and osteohistology, we serially thinâsectioned one of the smallest burnetiamorph skull caps ever recovered (estimated skull length = 10 cm), as well as an individual nearly twice as large, and here report the first cranial histological data from this clade. We recognize four highly vascularized histological zones visible in coronal thinâsections, only one of which shares morphological similarities with the tripartite zonation previously reported in pachycephalosaur domes. Zone A forms the endocranial region of the skull cap and records disorganized primary osteons in a fibrolamellar complex. Zone B preserves a border of compact, avascular layers of parallelâfibered bone surrounding an interior of partially remodeled vascular canals. Interestingly, the outline of Zone B resembles the shape of an incipient skull roof. Zone C forms the thickest portion of the skull cap and is composed of fastâgrowing woven bone with minimal osteonal development. The superficial Zone D has a matrix of predominantly woven bone with narrower primary vascular canals than in deeper regions of the skull caps. Unlike in pachycephalosaurs, where primary vascular porosity is thought to decrease through ontogeny, both burnetiamorph skull caps preserve a thick Zone C of highly vascularized tissue. Additionally, the remnants of sutures are visible as radial struts that taper superficially, leaving no trace on the surface of the skull. Even in the smallest individual, the sutures are closed ectocranially, which is unusual, given that some large, presumably adult pachycephalosaur domes preserve open sutural gaps. Although pachycephalosaur and burnetiamorph skull domes are superficially similar, histological analysis reveals differences in their vascularity and construction that imply multiple evolutionary pathways to form an elaborate pachyostotic dome.


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Lorenzo Marchetti, Sebastian Voigt & Spencer G. Lucas (2019)
An anatomy-consistent study of the Lopingian eolian tracks of Germany and Scotland reveals the first evidence of the end-Guadalupian mass extinction at low paleolatitudes of Pangea.
Gondwana Research 73: 32-53
doi: https://doi.org/10.1016/j.gr.2019.03.013Â
https://www.sciencedirect.com/science/article/pii/S1342937X19300991


Highlights

Chelichnus is a taphotaxon due to the locomotion of tetrapods on inclined planes.
The eolian tracks from Scotland and Germany belong to the Paradoxichnium biochron.
These revised records extend the dinocephalian extinction globally.

Abstract

The poor preservation and apparent monospecifity of Permian tetrapod footprints from eolian paleoenvironments have thus far hampered their reliable interpretation. This study clarifies how this is due to distinct and repeated ichnotaphonomic effects on trackway pattern and footprint morphology on originally inclined planes. Once these effects are excluded, the anatomy-consistent ichnotaxobases useful for ichnotaxonomy can be recognized. Several nomina dubia are identified, among these the ichnogenus Chelichnus, here considered a taphotaxon. The eolian ichnoassociations from the Lopingian of Scotland and Germany include six different ichnotaxa: cf. Capitosauroides isp. (?eutheriodont therapsid), Dicynodontipus geinitzi (cynodont therapsid), Dolomitipes isp. (dicynodont therapsid), Pachypes loxodactylus n. comb. (pareiasaurian parareptile), Procolophonichnium isp. (small parareptile) and Rhynchosauroides isp. (non-archosauriform neodiapsid). This is completely different from the interpretations of the last 20âyears, which postulated that these paleoenvironments comprised monospecific associations of synapsid tracks. These ichnoassociations are instead moderately diverse, similar to low-latitude marginal marine to floodplain ichnoassociations and belong to the Lopingian Paradoxichnium footprint biochron. The Cornberg Formation of Germany, being constrained between the Illawarra reversal and the mid-Wuchiapingian Kupferschiefer at the Rotliegend/Zechstein transition, constitutes the earliest evidence of Lopingian tetrapod faunas at low paleolatitudes and the first evidence of low-paleolatitude faunal turnover related to the end-Guadalupian mass extinction from both the skeleton and the track record. This suggests a global extension of the dinocephalian extinction event, which occurred at high-mid (South Africa and Russia) and low (Western Europe) paleolatitudes of Pangea about 259â260âMa and was probably triggered by the eruption of the Emeishan Large Igneous Province of SW China, which considerably changed global environmental conditions in both marine and continental settings.


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