Stocker, M. R. 2010. A new taxon of phytosaur (Archosauria: Pseudosuchia) from the Late Triassic (Norian) Sonsela Member (Chinle Formation) in Arizona, and a critical re-evaluation of Leptosuchus Case 1922. Palaeontology 53:997-1022. doi: 10.1111/j.1475-4983.2010.00983.x
Abstract - Leptosuchus Case, 1922 (Reptilia: Phytosauria) from the Late Triassic of the American West is represented by many specimens. Here, I present complete morphological descriptions of the skull material of a new taxon from the Sonsela Member (Chinle Formation) of Petrified Forest National Park, Arizona, with the first rigorous phylogenetic analysis focused on the interrelationships of Leptosuchus. The new taxon is recovered as the sister taxon to Pseudopalatinae. It possesses one unambiguous synapomorphy (the ‘septomaxillae’ form part of the lateral borders of the nares) and shares the presence of a subsidiary opisthotic process with Pseudopalatinae. The new taxon does not fall within the restricted clade Leptosuchus. In my analysis, the previously proposed, but undemonstrated, sister taxon relationship between Angistorhinus and Rutiodon is not supported, Paleorhinus is recovered as paraphyletic, and a subset of taxa traditionally included within Leptosuchus are found to be more closely related to Pseudopalatinae, rendering Leptosuchus paraphyletic. ‘Leptosuchus’ adamanensis emerges as sister taxon to Smilosuchus gregorii and is here referred to as Smilosuchus adamanensis nov. comb., and ‘Machaeroprosopus’ lithodendrorum is also transferred to Smilosuchus lithodendrorum nov. comb. Documentation of the variation present within Phytosauria, and specifically within Leptosuchus sensu lato, demonstrates higher diversity within Phytosauria than previously appreciated and places the character states previously proposed for Pseudopalatinae into a broader context of shared characters.
One specific subject that even the heartiest of Triassic workers will often go out of their way to avoid is the issue of phytosaur taxonomy, and for good reason, it is a mess. I previously supplied a flowchart I created in 2001 that demonstrates how much of a mess I’m talking about. Recent revisions (e.g., Hungerbühler 2002) have dealt almost exclusively with the pseudopalatine phytosaurs and the rest of the group has been relatively neglected since Long and Murry’s (1995) treatment. The genus Leptosuchus (still assigned to Rutiodon by some) has been poorly understood, and most new specimens assigned to this genus have been lumped into existing species without clear discussion of the characters used to support these assignments (I’m am guilty of this myself).
In the Spring of 2003 Daniel Woody and I discovered a large phytosaur skull in the Devil’s Playground portion of Petrified Forest National Park during a geological reconnaissance. That summer a crew consisting of Randall Irmis, Michelle Stocker, Jeff Shuman, and I collected the skull (now PEFO 31218) and our group initiated preparation. A couple of months later Jeff, Randy, and I discovered a second skull very close by; however, this skull appeared to only consist of a set of badly eroded lower jaws and was not collected. Interestingly we made the discovery while we were relocating and rephotographing a photo taken by Edwin Colbert of a phytosaur skull excavation conducted by the AMNH in 1946. Amazingly both skull sites are in this historic photo.
In 2006 Michelle and I reinvestigated the lower jaws and discovered that much of the skull was indeed present. This skull (PEFO 34239) was collected and Michelle began the preparation. About this time Michelle was looking for a good project for her Master’s thesis at the University of Iowa and despite knowing the potential pitfalls in the morass of phytosaur taxonomy, I suggested that maybe she should describe the two PEFO skulls (she had helped excavate and prep both of them) and determine their taxonomic affinities. Fast forwarding to the present it would seem that this was indeed a very good project and Michelle produced a very fine thesis (Stocker 2008), a portion of which is presented in this paper. She has since become quite the expert in the phytosaurs of the American Southwest and I look forward to more of her work on this subject (it is badly needed).
Some important points from this paper and details on the specimens:
1) Erects a new phytosaur taxon, Pravusuchus hortus, from the Sonsela Member of the Chinle Formation of Petrified Forest National Park. Existing specimens of Pravusuchus were originally assigned to the genus Leptosuchus, but detailed analysis shows that the taxon is distinct, possessing skull characters found in both ‘leptosuchines’ and pseudopalatines.
2) The holotype (AMNH FR 30646) was originally collected from the Petrified Forest in 1946 by Edwin Colbert of the American Museum of Natural History. The referred specimens are from the same geographical area and stratigraphic horizon and were collected by park staff (including Michelle Stocker) in 2003 and 2006.
3) Stratigraphically the specimens were found between the highest specimens of “Leptosuchus” and the lowest specimens of Pseudopalatus (Parker and Martz in press).
4) The genus Leptosuchus (sensu Long and Murry 1995) is paraphyletic and specimens referred to the genus are now only found in the Dockum Group. Arizona specimens of “Leptosuchus” are reassigned to Pravusuchus and Smilosuchus.
5) Stocker considers there to be three valid species of Smilosuchus; S. gregorii (the type species), S. adamanensis, and S. lithodendrorum. These last two are new combinations and this study resurrects the species S. lithodendrorum, which was considered a junior sysnonym of Leptosuchus crosbiensis by Long and Murry (1995).
This paper contains the most inclusive phylogenetic analysis of the Phytosauria since Ballew (1989). Some results:
1) The genus Paleorhinus is paraphyletic. Although this idea has been floating around for awhile (e.g., Fara and Hungerbuehler 2000) this is the first time it has been supported with a phylogenetic analysis.
2) The genera Anghistorhinus and Rutiodon are distinct. This synonymy was first put forth in an abstract by Hungerbuehler and Sues in 2001, but is not recovered in this study.
3) The name Rutiodon is restricted to specimens from the eastern United States. The name Rutiodon is often used for phytosaur specimens from the western United States stemming back to work by Gregory (1962) and Ballew (1989); however, this paper clearly shows that these referrals are erroneous and Rutiodon should be restricted to material from the eastern U.S.
This analysis should form the basis of future studies on the lower part of the phytosaur tree for a long time to come.
Ballew, K. L. 1989. A phylogenetic analysis of Phytosauria from the Late Triassic of the western United States. 309–339. In LUCAS, S. G. and HUNT, A. P. (eds). Dawn of the age of dinosaurs in the American Southwest. New Mexico Museum of Natural History, Albuquerque, 414 pp.
Fara, E., and Hungerbühler, A. 2000. Paleorhinus magnoculus from the Upper Triassic of Morocco: a juvenile primitive phytosaur (Archosauria). Comptes Rendus de l’Académie des Sciences, Paris, Sciences de la Terre et des planétes, 331, 831–836.
Gregory, J. T. 1962. The genera of the phytosaurs. American Journal of Science, 260, 652–690.
Hungerbühler, A. 2002. The Late Triassic phytosaur Mystriosuchus westphali, with a revision of the genus. Palaeontology, 45, 377–418.
Hungerbühler, A., and Sues, H.-D. 2001. Status and phylogenetic relationships of the Late Triassic phytosaur Rutiodon carolinensis. Journal of Vertebrate Paleontology, 21(3-Suppl.), 64A.
Long, R. A. and Murry, P. A. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. Bulletin of the New Mexico Museum of Natural History and Science, 4, 1–254.
Stocker, M. R. 2008. Relationships of the phytosaur Leptosuchus Case 1922 with descriptions of new material from Petrified Forest National Park, Arizona. Unpublished MS thesis, University of Iowa, Iowa City, 220 pp.
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