Eucoelophysis baldwini Sullivan and Lucas, 1999

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 47-48

Eucoelophysis baldwini Sullivan and Lucas, 1999

AGE: Mid to late Norian ( Litwin et al., 1991; Lucas, 1998a; Heckert et al., 2005; Irmis et al., 2007b).

OCCURRENCE: Petrified Forest Member, Chinle Formation, New Mexico ( Sullivan and Lucas, 1999).

HOLOTYPE: NMMNH P-22298, incomplete postcranial material consisting of two dorsal and four incomplete caudal vertebrae, nearly complete right pubis, partial right ischium, ilium fragment, fragmentary femora, proximal half of the left tibia, incomplete right metatarsals II and IV, complete metatarsal III, phalanges, unidentified bone fragments, and possibly an incomplete left scapulocoracoid.

REFERRED MATERIAL: GR 195, proximal portion of the femur ( Irmis et al., 2007a).

REMARKS: The incomplete specimen of Eucoelophysis was found in a multitaxic assemblage in the Petrified Forest Member, Chinle Formation, New Mexico. Originally described as a coelophysoid theropod dinosaur by Sullivan and Lucas (1999), Eucoelophysis is now considered to be a nondinosaurian dinosauriform (Nesbitt et al., 2005; Ezcurra, 2006; Irmis et al., 2007a; Nesbitt et al., 2007; Brusatte et al., 2008). In explicit phylogenetic analyses, Ezcurra (2006) found Eucoelophysis as the sister taxon to Dinosauria , whereas Irmis et al. (2007a) found Eucoelophysis in a clade with Silesaurus as the sister taxon to Dinosauria .

The studies of Ezcurra (2006) and Nesbitt et al. (2007) agreed for the most part. However, because the hind limbs were the only elements of Eucoelophysis that were definitely associated (within a multitaxic quarry), Nesbitt et al. (2007) considered the hind limbs and metatarsals the only definite material pertaining to the holotype of Eucoelophysis . Nesbitt et al. (2007) hypothesized that the pubis does not go to the hind limbs, whereas Ezcurra (2006) scored the pubis as part of Eucoelophysis in his data matrix. If the pubis character scores of Eucoelophysis are removed, Eucoelophysis , Silesaurus , and Dinosauria form a polytomy in Ezcurra’s (2006) matrix.

The following autapomorphies were listed by Ezcurra (2006): (1) noninvasive pleurocoels in the dorsal vertebrae; (2) strongly marked U-shaped ischio-acetabular groove in pubis ( Sullivan and Lucas, 1999); (3) absence of femoral trochanteric shelf of femur; (4) cnemial crest distinctively offset from the tibial shaft, cranially straight, and without lateral notch; and (5) femoral fourth trochanter reduced. The first character does not occur in any of the vertebrae of Eucoelophysis . The second character is present in the pubis, but this element cannot be unambiguously shown to belong to Eucoelophysis . Staurikosaurus (MCZ 1669) , some basal theropods (e.g., Dilophosaurus, UCMP 37302), Sacisaurus (MCN PV10019), Lagerpeton (PVL 4619), basal ornithischians (e.g., Scutellosaurus ), and basal sauropodomorphs (save Saturnalia ) all lack a trochanteric shelf. The proximal end of the tibia, including the cnemial crest, is eroded the absence of a posterior notch cannot be assessed. Furthermore, the shaft of the tibia is incomplete; therefore, it cannot be assumed that it is straight. The fourth trochanter of Eucoelophysis is reduced relative to other archosaurs. Nesbitt et al. (2007) cited an appressed surface of the tibia as an apomorphy of Eucoelophysis .

KEY REFERENCES: Sullivan and Lucas, 1999; Nesbitt et al., 2005; Ezcurra, 2006; Nesbitt et al., 2007.