Eoraptor Confirmed as Basal Phytodinosaur

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

Figure 1. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image.

Abstract – We (Sereno et al. 2013) describe the basal sauropodomorph Eoraptor lunensis, based on the nearly complete holotypic skeleton and referred specimens, all of which were discovered in the Cancha de Bochas Member of the Ischigualasto Formation in northwestern Argentina. The lightly built skull has a slightly enlarged external naris and a spacious antorbital fossa with a prominent, everted dorsal margin and internal wall lacking any pneumatic extensions into surrounding bones. The tall quadrate is lapped along its anterior margin by the long, slender ventral process of the squamosal, and the lower jaw has a mid-mandibular joint between a tongue-shaped splenial process and a trough in the angular. All but the posterior-most maxillary and dentary crowns have a basal constriction, and the marginal denticles are larger and oriented more vertically than in typical theropod serrations. Rows of rudimentary palatal teeth are present on the pterygoid. Vertebral centra are hollow, although not demonstrably pneumatized,and all long bones have hollow shafts. The radius and ulna are more robust, the manus proportionately shorter, and the manual unguals less recurved than in the contemporaneous basal theropod Eodromaeus murphi. An outstanding feature of the manus of Eoraptor is the twisted shaft of the first phalanx of the pollex, which deflects medially the tip of the ungual as in basal sauropodomorphs. The long bones of the hind limb have more robust shafts than those of Eodromaeus, although in both genera the tibia remains slightly longer than the femur.

From the text – Eoraptor lunensis was placed by Sereno et al. (1993) and Sereno (1999) as the basal member of Theropoda on the basis of phylogenetic analyses that identified synapomorphies uniting Eoraptor with Herrerasaurus and other theropods.

An opposing camp emerged with the view that Eoraptor was a more basal saurischian, outside both Theropoda and Sauropodomorpha (Langer, 2004; Mart´ınez and Alcober, 2009; Brusatte et al., 2010; Langer et al., 2010).

We now regard Eoraptor as a basal sauropodomorph (Mart´ınez et al., 2011), and there are important events that led us to this new understanding. It was not until excellent remains of this dinosaur were discovered in 1996 and prepared several years later that its distinction from Eoraptor was revealed (Mart´ınez et al., 2011).

Secondly, two key discoveries came to light while working on the holotypic skeleton of Eoraptor for this monograph. We discovered that, prior to its final fossilization, slight disarticulation of digit I in the well-preserved right manus of Eoraptor (Fig. 69) had obscured a remarkable derived feature known only among large bodied basal sauropodomorph dinosaurs (Sereno, 2007b)—the medial rotation in the shaft of proximal phalanx of manual digit I that directs the tip of the ungual inward (Fig. 73D). 

We also realized that the lower jaws of Eoraptor seemed slightly short relative to the upper jaws (Figs. 16, 17) and that the anterior end of the dentaries also had vascular openings (Fig. 23) similar to those of many larger-bodied basal sauropodomorphs thought to have a small keratinous lower bill (Sereno, 2007b; Mart´ınez, 2009). By preparing between the premaxillary teeth, we were able to verify evidence from the computed tomography (CT) data that the first dentary tooth in Eoraptor, as in Panphagia (Mart´ınez and Alcober, 2009), is inset a short distance from the anterior end of the dentary.

Thirdly, the discovery of Panphagia in Ischigualasto (Martínez and Alcober, 2009) and Saturnalia in southeastern Brazil (Langer et al., 1999, 2007; Langer, 2003) highlighted postcranial features in the girdles and hind limb shared with later sauropodomorphs.

The striking similarities between Eoraptor and Panphagia and Saturnalia became apparent. 

More recently, the discovery in southeastern Brazil of wellpreserved cranial remains of Pampadromaeus (Cabreira et al., 2011) has extended the striking similarities between Eoraptor and Brazilian genera to include the skull.

We reconsider the relationships of Eoraptor and other basal dinosaurs elsewhere (Sereno and Martínez, in review). Evidence is mounting that Eoraptor and several other taxa from the Ischigualasto and Santa Maria formations (Panphagia, SaturnaliaPampadromaeus) are basal sauropodomorphs.

Based only on Sereno et al. 1993 data and whatever was online at the time
Now that several traits in Eoraptor are now published, the large reptile tree (and its limited number of characters, will be updated soon) also nested Eoraptor with Pampadromaeus and these two with Panphagia in a clade basal to the Phytodinosauria (= Sacisaurus and the poposaurs + Sauropodomorpha + Ornithischia).

This order is confirmed by Martínez et al. (2013) which found, “The analysis positions Panphagia as the basal-most sauropodomorph, followed by Eoraptor, Pampadromaeus, and a clade that includes Chromogisaurus and Saturnalia.”

So, another confirmation for a much maligned study. Nice.

References
Martínez RN, Apaldetti C and Abelin D 2013. 
Basal sauropodomorphs from the Ischigualasto Formation. Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12:51-69.
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, Martínez RN and  Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Journal of Vertebrate Paleontology Memoir 12:83-179.

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