Nesting Eoraptor in the Large Reptile Tree

Eoraptor lunensis (Fig. 1) is an early (Carnian, Triassic, 1 meter long, PVSJ 512) bipedal dinosaur introduced by Sereno et al. (1993). Only the skull has been published, and before being fully prepped. The whole skeleton (Fig. 4), sans the tail tip, is known. I understand a monograph should be out this year. Digimorph published the skull in several views. The post-cranial data I’m working with is from Greg Paul (Fig. 4), published online here.

Figure 1. Eoraptor skull from Sereno et al. 1993.

Figure 1. Eoraptor skull modified from Sereno et al. 1993.

I should have added Eoraptor earlier considering the importance of this taxon.

Figure 2. Eoraptor skull, traced from Digimorph image.

Figure 2. Eoraptor skull, traced from Digimorph image. Here the premaxillary teeth appear to be longer.

The Wiki/Eoraptor entry reports, Eoraptor was one of the world’s earliest dinosaurs. It was a two-legged saurischian, close to the ancestry of theropods and sauropodomorphs” and lacked specialized predatory dinosaur traits. Sereno (195) considered Eoraptor the earliest recorded theropod closest to “the hypothetical dinosaurian condition that any other dinosaurian subgroup.

Figure 3. Archosaur/Dinosaur family tree. Here's where Eoraptor nests in the large reptile tree.

Figure 3. Dinosaur family tree. Here’s where Eoraptor nests in the large reptile tree.

Indeed it is.
The large reptile tree (Fig. 3) nested Eoraptor with Panphagia and Pampadromaeus, a clade basal to the Phytodinosauria, derived from basal Theropoda like Herrerasaurus. According to Sereno et al. (1993), Eoraptor has distinct premaxillary and anterior maxillary leaf-shaped teeth. Currie (1997) also found Eoraptor closer to the hypothetical ancestor of both saurischian and ornithischian dinosaurs. More recently, Sues et al. (2011), in their study of Daemonosaurus, considered Eoraptor a basal theropod.

Eoraptor is a key taxon in the family tree of basal Dinosauria. While close to theropods, the snout is shorter and rounder, the teeth are “leaf-shaped,” and the orbit is larger. These traits continue to evolve in Phytodinosauria.

 

Figure 2. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image.

Figure 2. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image.

Interesting PostScript
When working with Sacisaurus, I thought it strange that the postorbital had an odd convex bump just posterior to the eye. Now I see that it is evidently derived from Eoraptor, which likewise shared that postorbital bump.

Figure 4. Dinosaur relations as recovered from the large reptile tree. Here short-faced plant-eaters, like Massospondylus and Heterodontosaurus, are derived from meat-eaters, like Herrerasaurus via Daemonosaurus.

Figure 4. Dinosaur relations as recovered from the large reptile tree. Here short-faced plant-eaters, like Massospondylus and Heterodontosaurus, are derived from meat-eaters, like Herrerasaurus via Daemonosaurus and Pampadromaeus.

A Key Taxon
The figure above represents the phylogenetic nesting of Eoraptor as a sister to Pampadromaeus, which is descended from a sister to Herrerasaurus and basal to the Phytodinosauria: Ornithischia,  Saurompodomorpha and Poposauridae. Seems to make sense on the face of it. And this is why Eoraptor has been hard to classify into traditional categories.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Currie PJ 1997. Theropoda. In Encyclopedia of Dinosaurs (P.J. Currie, and K. Padian, Eds.) pp 731-736. Academic Press, San Diego, California.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993. Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC 1995. Theropoda: early evolution and major patterns of diversification. Journal of Vertebrate Paleontology 15(3, suppl.):52A-53A
Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society Bpublished online.

Wiki/Eoraptor

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