Eoraptor is a Sauropodomorph, and a New Basal Dinosaur, Eodromaeus murphi, from the Late Triassic of Argentina

The last decade has seen a renaissance in the study of basal dinosaurs and as a result of the resurgence in study we have seen a bunch of new dinosaur taxa coming from the Upper Triassic of North and South America (see excellent syntheses by Langer et al. 2010, and Brusatte et al., 2010).  These include the stem-sauropodomorphs Saturnalia, Panphagia, and most recently Chromigosaurus (Langer, 2003; Martinez and Alcober, 2009; Ezcurra, 2010).  Other new early dinosaurs include the herrerasaurid Sanjuansaurus, and the basal theropod Tawa (Nesbitt et al. 2010; Alcober and Martinez, 2010).  We have also seen updated description and discussion of older taxa such as Chindesaurus and Pisanosaurus as well as the discovery and reinvestigation of numerous dinosauromorphs (e.g., Dzik, 2003; Ferigolo and Langer, 2007; Irmis et al., 2007a, b; Nesbitt et al., 2007, 2009, 2010).  Furthermore, a plethora of phylogentic studies are attempting to work out the relationships of all of these taxa (e.g., Langer, 2004; Langer and Benton, 2006; Ezcurra, 2006, 2010; Irmis et al., 2007; Upchurch et al., 2007; Yates, 2007; Martinez and Alcobar, 2009; Nesbitt et al., 2009).

One point of contention in all of these analyses is the phylogenetic position of Eoraptor lunensis from the Upper Triassic Ischigualasto Formation.  Originally considered the basal most theropod (e.g., Sereno et al. 1993), recent analysis have supported this hypothesis (Ezcurra, 2006, 2010; Nesbitt et al., 2009) or cast it as a basal saurischian outside of Eusaurischia (e.g., Langer, 2004; Langer and Benton, 2004; Upchurch et al., 2007; Yates, 2007; Martinez and Alcober, 2009).

A new paper out today in Science by Martinez et al. describes a new basal theropod taxon, Eodromaeus murphi, from the Ischigualasto Formation.  This new taxon is known by much of the skeleton and represents the new theropod previously mentioned by Martinez et al. (2008). A phylogenetic analysis places Eodromaeus within Theropoda as the sister taxon to Neotheropoda (a position previously held by Tawa, which in this new analysis is a neotheropod) and the sister taxon to the herrerasaurid theropods. 

Very striking in this analysis is the recovery of Eoraptor lunensis not only as a Eusaurischian but as a stem-sauropodomorph in a polytomy with Panphagia and Saturnalia (thus it would be a saturnaliine according to Ezcurra 2010; Guaibasaurus and Chromogisaurus were not included in the analysis).  This positioning for Eoraptor had previously been alluded to by Martinez and Alcobar (2009), but this is the first time it has been supported by a phylogenetic analysis.

I'm sure this new analysis will cause quite a debate among basal dinosaur workers, especially regarding the ideas of taxon sampling and character inclusion/codings in phylogenetic analyses.  The analysis by Martinez et al. excludes lots of incomplete/poorly known taxa as well as taxa that these authors percieve as being too far out phylogentically to have consequence to the question of basal dinosaur relationships.  The character matrix is an updated version of Sereno (1999) and does not utilize many characters found in other studies such as Langer and Benton (2006), Ezcurra (2006), and Nesbitt et al. (2009). I don't really have a horse in this race given that I don't have a basal dinosaur analysis that I am personally working on, but as someone outside looking in, it is really difficult to compare across these various analyses given the strong differences in datasets being utilized.  Hopefully we will see a consensus at some point.

Nonetheless, if Eoraptor is indeed found to be a basal sauropodomorph, this is of great interest because it gets us a step closer to seeing what the common ancestor of theropods and sauropodomorphs may have looked like.

Some other neat tidbits and conclusions from the paper:

-Like Eoraptor, Eodromaeous possesses a small row of teeth on the palatal ramus of the pterygoid.

-A new radioisotopic date for the top of the Ischigualasto Formation constrains the majority of the formation between 231.4 Ma and 225.9Ma, thus the Ischigualasto Formation spans the Carnian/Norian boundary and does not overlap with dinosaur bearing strata in North America supporting the hypothesis of Irmis and Mundil, 2008, 2010. The previously reported date of ~217 Ma for the middle of the formation by Shipman (2004) and Currie et al (2009) has been considered unreliable because of a lack of stratigraphic control and the inability to reproduce the methodology.

-The Ischigualasto is divided into three biozones based on vertebrate fossil occurrences.  One of the boundaries is supposed to represent the Carnian/Norian boundary, although it seems to me that these boundaries are somewhat ambiguous as they all depend on negative evidence.

-Placing the vertebrate occurrences in stratigraphic order suggests to the authors that rhynchosaurs went extinct at the Carnian/Norian boundary.  Interestingly they also propose that dinosaurs went extinct locally and did not reappear in the area until deposition of the Los Colorados Formation later in the Norian.

Martinez, R. N., Sereno, P. C., Alcober, O. A., Colombi, C. E., Renne, P. R., Montañez, I. P., and B. S. Currie. 2011. A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea. Science 331:206-210 DOI: 10.1126/science.1198467.

Abstract - Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.

New Scientist article by Jeff Hecht here. Other links at Live Science and the BBC.