The Herrerasaurian Radiation

Image from Ischigualasto Park (http://www.ischigualasto.gob.ar/)

The Triassic beds of Argentina and Brazil play a key role in the understanding of the origin and early diversification of Dinosauria. The first recorded dinosaurs include some predatory forms, such as the herrerasaurids from the lower Ischigualasto Formation in northwestern Argentina, and the Santa Maria formation in southern Brazil. In 1911, Guillermo Bodenbender briefly refers to the fossils of Ischigualasto, but intensive paleontological study of the Ischigualasto and Chañares Formations began only in the late 1950s. In Brazil, the fossil record of Triassic dinosaurs has greatly expanded since the discovery of Staurikosaurus in 1970.

Herrerasauridae is a basal clade of predatory, obligatorily bipedal dinosaurs recorded from the Upper Triassic of Argentina and Brazil. There are putative records of herrerasaurids from the mid-late Norian strata of Europe, North America, and from the Maleri Formation of India. The Herrerasaurid family includes Staurikosaurus, Gnathovorax, Herrerasaurus, Sanjuansaurus and possibly Frenguellisaurus. Staurikosaurus and Gnathovorax were recovered from the Hyperodapedon Assemblage Zone of the Santa Maria Supersequence in southern Brazil, whilst Herrerasaurus and Sanjuansaurus were collected from the lower levels of the Ischigualasto Formation, and Frenguellisaurus (considered by many authors as a junior synonym of Herrerasaurus) was collected from the upper levels of this unit. 

Skull of Herrerasaurus ischigualastensis (Sereno, 2013)

Herrerasaurus ischigualastensis is one of the best known Triassic dinosaurs and the largest dinosaur of the Ischigualasto Formation. It was described by Osvaldo Reig in 1963. Herrerasaurus was fully bipedal, with strong hind limbs, short thighs and long feet. The skull has a rectangular profile and a transversely narrow snout (Sereno and Novas, 1992). The presence of two sacral vertebrae and lack of brevis fossa made Herrerasaurus, and other herrerasaurids, a controversial group.

Due to this combination of features, including some traits that are nearly exclusive of theropod dinosaurs (e.g., serrated dentition, grasping hands, pubis with distal pubic boot, distal caudal prezygapophyses elongated), with some remarkable plesiomorphic traits (e.g., primitive-looking pelvic girdle and tarsus), the phylogenetic relationships of herrerasaurs are problematic. Some authors suggested that Herrerasauridae may constitute the sister group to Dinosauria, whereas others proposed theropod affinities for the group. Other proposal indicates that herrerasaurids may constitute a non-Eusaurischia branch of Saurischia.

 

The holotype of Daemonosaurus chauliodus (CM 76821). From Sues et al., 2011.

The putative North American herrerasaurs Tawa, Chindesaurus, Caseosaurus and Daemonosaurus, have been recorded from Norian to Rhaetian beds in different fossil sites in southwestern USA. Chindesaurus, from the Petrified Forest National Park in Arizona, shows several features of herrerasaurian affinities, while Tawa shares with Herrerasaurus, Sanjuansaurus and Gnathovorax several derived features that are considered diagnostic for Herrerasauria. These include: dorsoventrally deep jugal; cervical vertebrae with pronounced ventral keel; atrophied metacarpals IV and V; pubic shaft fan-shaped distally, resulting from the posterior flexion of the lateral margin of pubis; and anteroposteriorly expanded pubic boot. Among the features that Daemonosaurus shares with Herrerasauridae are: a dorsoventrally deep premaxilla, jugal dorsoventrally tall, and fang-shaped maxillary teeth.

The presence of herrerasaurs outside South America during the Late Triassic suggests the group was globally dispersed, contrasting with their apparent South America endemism. Furthermore, the recognition of Daemonosaurus as a herrerasaurian provides evidence that this clade survived into the Rhaetian and the group was likely one of the victims of the end-Triassic mass extinction.

References:

Novas, F.E., Agnolin, F.L., Ezcurra, Martí.D., Müller, R.T., Martinelli, Agustì., Langer, M., Review of the fossil record of early dinosaurs from South America, and its phylogenetic implications, Journal of South American Earth Sciences (2021), doi: https://doi.org/10.1016/j.jsames.2021.103341.

Pacheco C, Müller RT, Langer M, Pretto FA, Kerber L, Dias da Silva S. 2019. Gnathovorax cabreirai: a new early dinosaur and the origin and initial radiation of predatory dinosaurs. PeerJ 7:e7963 https://doi.org/10.7717/peerj.7963

Sues, Hans-Dieter, Nesbitt, Sterling J., Berman, David S., and Henrici, Amy C. 2011. “A late-surviving basal theropod dinosaur from the latest Triassic of North America.” Proceedings of the Royal Society B: Biological Sciences 278 (1723):3459– 3464. https://doi.org/10.1098/rspb.2011.0410

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