Skeletal Anatomy ofAcaenasuchus GeoffreyiLong and Murry, 1995 (Archosauria: Pseudosuchia) and its Implications for the Origin of the Aetosaurian Carapace

Autor: William G. Parker, Matthew E. Smith, Ben T. Kligman, Adam D. Marsh, Randall B. Irmis
Přispěvatelé: Geosciences
Rok vydání: 2020
Předmět:
Zdroj: Journal of Vertebrate Paleontology. 40:e1794885
ISSN: 1937-2809
0272-4634
DOI: 10.1080/02724634.2020.1794885
Popis: Acaenasuchus geoffreyi is a diminutive armored archosaur from the Upper Triassic Chinle Formation of northern Arizona, U.S.A., with uncertain evolutionary relationships and skeletal maturity. Known only from osteoderms, the taxon has been considered a valid taxon of aetosaur, juvenile specimens synonymous with the aetosaur Desmatosuchus spurensis, or a non-aetosaurian pseudosuchian archosaur. Here, we describe new fossils of Acaenasuchus geoffreyi that represent cranial, vertebral, and appendicular elements as well as previously unknown variations in the dorsal carapace and ventral shield. The skull bones are ornamented with the same anastomosing complex of ridges and grooves found on the paramedian and lateral osteoderms, and the appendicular skeleton resembles that of Revueltosaurus callenderi, Euscolosuchus olseni, aetosaurs, and other armored archosaurs such as erpetosuchids. Histology of osteoderms from the hypodigm of Acaenasuchus geoffreyi shows multiple growth lines, laminar tissue, and low vascularity, evidence that the individuals were close to skeletal maturity and not young juveniles. A revised phylogenetic analysis of early archosaurs recovers Acaenasuchus geoffreyi and Euscolosuchus olsenias sister taxa and members of a new clade that is the sister taxon of Aetosauria. This new phylogeny depicts a broader distribution of osteoderm character states previously thought to only occur in aetosaurs, demonstrating the danger of using only armor character states in aetosaur taxonomy and phylogeny. Acaenasuchus geoffreyi is also a good example of how new fossils can stabilize 'wild card' taxa in phylogenetic analyses and contributes to our understanding of the evolution of the aetosaur carapace. Petrified Forest Museum Association; Friends of Petrified Forest National Park We thank D. and J. Gillette (MNA), P. Holroyd (UCMP), and C. Levitt-Bussian (UMNH) for providing collections and archive access, and for facilitating loans from their institutions. M. Polcyn (SMU) graciously sent us locality information and photographs of SMU 75403. H.-D. Sues provided photographs of and helpful comments on Euscolosuchus olseni, and J. Strotman facilitated the molding, casting, and transfer of casts of USNM 448587 and USNM 448582. P. Holroyd (UCMP) graciously provided permission to histologically section two osteoderms, and A. Lee (Midwestern University) kindly assisted with sectioning. K. Ritterbush (University of Utah) generously provided access to her petrographic microscope imaging system, and the histological slides were imaged by N. Ong. R. Long's field notes are on file in the archives at PEFO and the UCMP and those of Charles Camp are available at the UCMP. Thanks to T. Olson for finding MDM specimens at PFV 211. We thank J. Kirkland and D. DeBlieux (Utah Geological Survey) for permission to include in this study material they collected. The collection and preparation of the specimens from PEFO was funded by the Petrified Forest Museum Association and Friends of Petrified Forest National Park. Silhouettes from phylopic.org in Figure 9 were created by S. Hartman, D. Bogdanov, and Smokeybjb and reused under the Creative Commons Attribution 3.0 Unported license (creativecommons.org/licenses/by/3.0). Thanks to V. Paes Neto and D. Gillette for constructive comments and discussion on aetosaur and glyptodont anatomy, and to A. Turner for help in constraining tree topology in TNT. Comments and suggestions made by M. B. von Baczko and I. Cerda improved the manuscript. This is Petrified Forest National Park Contribution No. 61. The conclusions presented here are those of the authors and do not represent the views of the United States Government.
Databáze: OpenAIRE
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