THEROPODA

Marsh, Adam D. & Parker, William G., 2020, New dinosauromorph specimens from Petrified Forest National Park and a global biostratigraphic review of Triassic dinosauromorph body fossils, PaleoBios 37, pp. 1-56 : 15-17

THEROPODA MARSH, 1881 SENSU GAUTHIER, 1986

FIG. 7A–O

Referred specimens and localities — PEFO 38710 ( Fig. 7A–L), presacral and sacral vertebrae, PFV 302: Rabbit Foot Hills, PFM; PEFO 44470 ( Fig. 7M–O), distal end of right pubis, PFV 453: Puerco Ridge N, PFM.

Description and rationale for assignment —The presacral and sacral vertebrae (PEFO 38710) belong to a single individual and were collected as float; the fossils are covered in carbonate mineralization that obscures some details. The anterior cervical vertebrae ( Fig. 7A–D) are relatively more anteroposteriorly elongate than the more posterior vertebrae. The prezygapophysis of the anterior cervical vertebra is triangular in lateral outline and projects anteriorly past the centrum face. In proximal view the centroprezygapophyseal fossa is present dorsolateral to the neural canal. The diapophysis and parapophysis frame the anterior margin of an anterior pneumatic fossa (Nesbitt 2011:188-1; Fig. 7B); such a fossa is apomorphic for theropod dinosaurs, e.g., Chindesaurus bryansmalli (PEFO 10395, Marsh et al. 2019a: fig. 2a); Coelophysis bauri (AMNH FR 2701, Nesbitt 2011: fig. 28a), and Dilophosaurus wetherilli (UCMP 37302, Marsh and Rowe 2020: fig. 7.1). The posterior pneumatic fossa (Nesbitt 2011:129-1) found in coelophysids, e.g., Coelophysis bauri (AMNH FR 2701, Nesbitt 2011: fig. 28a), ‘ Syntarsus ’ kayentakatae (MNA V2623, Tykoski 1998: fig. 16c–d), and Lucianovenator bonoi Martínez and Apaldetti (2017, PVSJ 906: fig: 4) is not present in PEFO 38710. The neural spine of the more posterior cervical vertebra ( Fig. 7E, F) is complete and preserves a transverse dorsal expansion, which is also present in several pseudosuchian archosaurs (see discussion in Nesbitt 2011: p. 113), Eoraptor lunensis (PVSJ 512, Sereno et al. 2012: fig. 47b), Herrerasaurus ischigualastensis (PVL 2566, Novas 1993: fig. 1c), Ch. bryansmalli (PEFO 10395, Marsh et al. 2019a: fig. 2s, t). This vertebra also has a hyposphene ventral to the postzygapophyses, but it is broken at its base. A hypantrum is clearly present on the anterior trunk vertebra ( Fig. 7G). The presence of hyposphene-hypantrum articulations in the posterior cervical vertebrae and trunk vertebrae (Nesbitt 2011:195-1) is apomorphic for saurischian dinosaurs, e.g., Saturnalia tupiniquim (MCP PV 3845), Tawa hallae (GR 241), and Di. wetherilli (UCMP 37302), but they are also found in some aetosaurs, ctenosaurischid poposauroids, ‘rauisuchian’ loricatans, Teleocrater rhadinus Nesbitt et al., 2017 , and some silesaurids (Stefanic and Nesbitt 2019). The first sacral rib ( Fig. 7K–L) is located on the anterior half of the first sacral vertebra, and it is C-shaped in lateral view (Langer and Benton 2006, Nesbitt 2011:209-1). This condition is found in saurischian dinosaurs such as Sa. tupiniquim (MCP PV 3844, Langer 2003: fig. 1b) and Ch. bryansmalli (PEFO 10395, Marsh et al., 2019a: fig. 2w–x). We assign this partial vertebral column to the Theropoda owing to the presence of an anterior pneumatic fossa on the centrum.

PEFO 44470 is an isolated distal end of a right pubis that preserves the base of the pubic apron in medial view and a distinct posterior ‘boot’. The distal end of the shaft and the posterior boot form a right angle anteroventrally ( Fig. 7M), and the posterodorsal surface between them

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is gently concave. A pubic boot is present in theropod dinosaurs (Nesbitt 2011:283-1) such as H. ischigualastensis (PVL 2566, Novas 1993: fig. 5a), Staurikosaurus pricei (MCZ 1669, Colbert 1970: fig. 8b), Gnathovorax cabreirai Pacheco et al. (2019, CAPPA /UFSM 0009: fig. 7a), and T. hallae (GR 155, Nesbitt et al. 2009a: fig. 2h), but is also found in some ‘rauisuchian’ (Nesbitt 2011: fig. 35a, c) and poposauroid suchians (Nesbitt 2011: fig. 35e). The ventral margin of the pubic boot of PEFO 44470 and theropods (e.g., T. hallae ) is fairly straight, but it is greatly concave in pseudosuchian archosaurs such that the pubic boot curves posterodorsally, e.g., Poposaurus gracilis Mehl (1915, YPM 57100) (Schachner et al. 2019: fig. 28a), and Effigia okeeffeae Nesbitt and Norell (2006, AMNH FR 30587) (Nesbitt 2007: fig. 42). We assign PEFO 44470 to the Theropoda owing to the presence of a posteriorly-straight and relatively short pubic boot.However, if herrerasaurids and T. hallae are in fact not theropods but rather earlier-diverging saurischians (Nesbitt and Sues 2020; Müller and Garcia 2020), then PEFO 44470 would also belong to an earlier-diverging dinosaur group.