Poposaurus and the future of 'Crurotarsi'

Like many rauisuchians, Poposaurus has been plagued with a lack of fossil material. However, a recent find in the Upper Triassic Chinle Formation of Utah has helped shed some light into this early archosaur. Although a skull still remains to be found, paleontologists have managed to uncover  an almost complete post-cranial skeleton. What is also interesting is the phylogenetic analysis associated with the new Poposaurus material. The most startling result is that Phytosauria falls out of the crocodile side of the archosaur tree and becomes the sister taxon of Archosauria. Depending on your definition of Crurotarsi, this could mean that dinosaurs are crurotarsans. However, under the Sereno 2005 definition, Crurotarsi is safe. For now ;-). Rauisuchia may also be in trouble based on the new phylogeny, which preserves Poposauroidea, but not Rauisuchidae. A more detailed phylogeny by Sterling Nesbitt will be coming out soon in the Bulletin of the American Museum of Natural History.

Gauthier, J. A., Nesbitt, S. J., Schachner, E. R., Bever, G. S., and W. G. Joyce. 2011. "The bipedal stem-crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion." Bulletin of the Peabody Museum of Natural History 52: 107-126.  

We introduce a spectacular new specimen of a Late Triassic stem crocodilian identified as Poposaurus gracilis. It is part of a poorly known group, Poposauroidea, that, because of its striking similarities with contemporaneous stem avians (“dinosaurs”), has long puzzled archosaur paleontologists. Observed vertebrate locomotor behaviors, together with exceptional preservation of distinctive anatomical clues in this fossil, enable us to examine locomotor evolution in light of new advances in phylogenetic relationships among Triassic archosaurs. Because this stem crocodilian is unambiguously an archosaur, a diapsid, a tetrapod and a choanate sarcopterygian, we can safely infer major components of its locomotor behavior. These inferences, together with form-function constraints, suggest that P. gracilis was a fleet-footed, obligately erect-postured,
striding biped. That behavior seems to have been superimposed on the ancestral archosaur’s innovative locomotor repertoire, which includes the capacity to “high walk.” These novelties persist in a recognizable form in archosaurs for at least 245 million years and are widely distributed across Earth’s surface in diverse ecological settings. They thus qualify as evolutionary innovations even regardless of significant differences in diversification rates among extant diapsid reptiles.

Crocs in JVP: Goniopholis and Metriorhynchus

de Andrade, M. B. and J. J. Hornung. 2011. 'A New Look into the Periorbital Morphology of Goniopholis (Mesoeucrocodylia: Neosuchia) and Related Forms' Journal of Vertebrate Paleontology 31(2):352-368 doi: 10.1080/02724634.2011.550353

Abstract
The periorbital morphology of goniopholidids is discussed, exploring the diversity of patterns and the relevance of the data for phylogenetic studies. Revision of material is focused on Goniopholis spp. and aff. Goniopholis spp., from England, Germany, and Belgium, providing a comparative description of their interorbital morphology. Traditional interpretation of the interorbital elements in species of Goniopholis (G. simus, G. baryglyphaeus), where the frontal is interpreted as excluded from the orbit by a prefrontal-postorbital contact in the skull roof, is contested and clarified through the analysis of new specimens, including a morphometric analysis. In Goniopholis, failure to identify the palpebral and its subtle contact with the prefrontal has lead to misinterpretation of elements and structures near the orbit, and the differential preservation/loss of palpebrals explains variability of the orbit in shape and orientation. In all European goniophilidids the frontal reaches the primary orbital border and there is no prefrontal-postorbital contact on the dorsal surface of the skull. Extensive contact of the palpebral with the primary orbital border creates a secondary (functional) orbital border, from which the frontal is excluded in most taxa. The condition is not exclusive of European goniopholidids and is paralleled by protosuchids, peirosaurids, and baurusuchids. At least four main morphological patterns are recognized, revealing a high diversity of European goniopholidids.

Fernandez, M. B., A. P. Carabajal, Z. Gasparini, and G. C. Diaz. 2011. 'A Metriorhynchid Crocodyliform Braincase from Northern Chile' Journal of Vertebrate Paleontology 31(2):369-377 doi: 10.1080/02724634.2011.550361

Abstract
A three-dimensionally preserved metriorhynchid braincase from the Oxfordian of northern Chile is described. The specimen is referred to the metriorhynchid Metriorhynchus cf. M. westermanni. The excellent preservation provides clear sutures and a detailed description, and X-ray computed tomographic (CT) scanning provides internal anatomical details. The general pattern of the orbitotemporal region is consistent with that of the basal thalattosuchian Pelagosaurus typus as described recently. The specimen from northern Chile shares with other metriorhynchids (e.g., Cricosaurus araucanensis, Metriorhynchus westermanni, M. casamiquelai, and Dakosaurus andiniensis) a dorsally exposed laterosphenoid, a laterosphenoid-prootic suture forming a blunt crest separating the supratemporal fenestra into two fossae for muscular attachment, and the quadrate incompletely sutured to the braincase. Thus, these features characterize not only basal but derived Thalattosuchia, as suggested by previous authors. The main difference in the orbitotemporal region is that in the specimen described herein, and in the other metriorhynchids examined, the trigeminal fossa is developed mainly caudal to the trigeminal foramen, whereas in P. typus the fossa is developed rostral and caudal to the trigeminal foramen. CT scanning indicates the presence of enlarged dorsal dural venous sinuses overlying the brain, as it has been described recently in Steneosaurus pictaviensis, and a well-developed sinus within the quadrate. The large foramen ventrolateral to the occipital condyle, which characterizes metriorhynchids, is confirmed as the entry for the internal carotid artery.

Great Rauisuchian Find: Decuriasuchus quatracolonia

Franca, M. A. G. and J. Ferigolo. 2011. 'Associated skeletons of a new middle Triassic "Rauisuchian" from Brazil' Naturwissenschaften (online first) DOI: 10.1007/s00114-011-0782-3 

Abstract
For more than 30 million years, in early Mesozoic Pangea, “rauisuchian” archosaurs were the apex predators in most terrestrial ecosystems, but their biology and evolutionary history remain poorly understood. We describe a new “rauisuchian” based on ten individuals found in a single locality from the Middle Triassic (Ladinian) Santa Maria Formation of southern Brazil. Nine articulated and associated skeletons were discovered, three of which have nearly complete skulls. Along with sedimentological and taphonomic data, this suggests that those highly successful predators exhibited some kind of intraspecific interaction. Other monotaxic assemblages of Triassic archosaurs are Late Triassic (Norian-Rhaetian) in age, approximately 10 million years younger than the material described here. Indeed, the studied assemblage may represent the earliest evidence of gregariousness among archosaurs, adding to our knowledge on the origin of a behavior pattern typical of extant taxa.

Updates

First of all, I'd like to apologize for the long hiatus. I've been back at the University of Maryland this semester, working and taking classes (and studying for the general and biology GRE), and time has gotten away from me. I'll be getting back to posts on crurotarsans soon.

The first thing that may interest you all is that this summer, I'll be coming at you live from Petrified Forest National Park. I'll be working as an intern there for approximately 10 weeks, doing "hardcore paleontology". I will be working with Bill Parker, a vertebrate paleontologist for the park, whom many of you may know from his blog Chinleana, which focusses on issues of the Late Triassic. Petrified Forest is known for it's petrified wood, but the Late Triassic Chinle Formation that dominates the park is also known for it's diversity of archosaurs, both crurotarsan and dinosaur.

Image by Jeff Marz.