New Triassic Terror: Rauisuchian Packs, New Evidence from the Middle Triassic of Brazil

I'm glad everyone enjoyed my April Fools joke; however, the Triassic is here to stay and especially Triassic vertebrate paleontology because the month of April is going to see the publication of numerous new papers on the subject. In fact, April 2011 has the possibility of being the best month ever for Triassic VP, especially archosaurs. We'll revisit this at the end of the month and see if this is true.

The first paper of April 2011 describes the recovery of nine articulated and associated rauisuchian skeletons from the Middle Triassic of Brazil. They have been assigned to a new Prestosuchus-like taxon named Decuriasuchus quartacolonia ("legion of ten crocodile from Quarta Colonia"). The material looks amazing and finds like this are extremely rare in the Triassic so I hope that a full description of the material will be forthcoming.

The phylogenetic analysis utilizes the dataset of Brusatte et al. (2010) and recovers a paraphyletic "rauisuchidae" with Decuriasuchus nesting with Prestosuchus and Batrachotomus; however, the paper and supplemental materials do discuss problems with the existing dataset and therefore these results should be considered very tentative.

One interesting point of the paper is the potential social implications of finding nine rauisuchians clustered together in a single spot. Franca et al., argue that the taphomomy of the site precludes that this is not a time averaged assemblage and instead it appears that these animals (all adults) congregated for some reason before dying. Whether the group was assembled and killed by a sudden catastrophy (e.g, flash flood) or accumulated and died because of another type of event (e.g., drought) is undeterminable with the current evidence, but in either case this would call for a certain degree of interaction between these predators. Modern crocodilians congregate in large number for various reasons, but mainly for feeding and in some cases have been reported to work together to herd fish. Thus, the circumstantial evidence from this new find may suggest that rauisuchians may also have worked together in some cases for hunting strategies. As stated by Franca et al in their concluding sentence, this would represent "the earliest evidence of possible group behavior among archosaurs". I personally would think that a hunting pack of large rauisuchians would be quite formidable, but then again this Brazilian group appears to have lacked invulnerability. Furthermore this would be extremely difficult to support with the evidence.

Note: The Revueltosaurus bonebed in Petrified Forest National Park is a monotypic assemblage of at least a dozen individuals (all of the same size and situated on a levee adjacent to the river channel).

Franca, M. A. G., Ferigolo, J., and M. C. Langer. 2011. Associated skeletons of a new middle Triassic “Rauisuchia” 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.