A new theropod dinosaur from a peculiar Late Triassic assemblage of southern Brazil

Highlights

• A new theropod dinosaur from the Late Triassic of Brazil is described.

• It is one of the oldest known theropods worldwide.

• This sheds light on some of the earliest theropod features.

• The new theropod comes from a peculiar fauna dominated by traversodontid cynodonts.

• The new theropod represents the first carnivorous dinosaur from this assemblage.

Abstract

The early evolution of theropod dinosaurs is poorly understood. Whereas sauropodomorphs are well-known from the oldest dinosaur bearing outcrops, the record of theropods is fragmentary and ambiguous. The Triassic deposits from Brazil yielded some of the oldest dinosaurs worldwide. These dinosaurs came from two distinct Assemblage Zones (AZ): the Hyperodapedon AZ (Carnian) and the Riograndia AZ (early Norian). Here, a specimen previously assigned to cf. Dinosauromorpha is reassessed. CAPPA/UFSM 0157 comes from an enigmatic assemblage with predominance of the traversodotind cynodont Siriusgnathus. This assemblage has been tentatively assigned to the Riograndia AZ. However, the absence of index fossils still hampers a reliable assignation. The specimen, which comprises a proximal portion of a left femur, belongs to a new theropod taxon erected here as Erythrovenator jacuiensis gen. et sp. nov. The new dinosaur differs from all other known Triassic dinosaurs based on the absence of a raised dorsolateral trochanter of the femur. Erythrovenator jacuiensis gen. et sp. nov. is regarded as a theropod on the basis of the pyramidal shape of the anterior trochanter in anterior view. The results of a phylogenetic analysis corroborate this assignation. Therefore, the new dinosaur represents one of the oldest theropod dinosaurs worldwide, shedding lights on some of the earliest theropod features. Finally, the new specimen also represents the first carnivorous dinosaur from the assemblage dominated by the traversodontid cynodont Siriusgnathus, increasing our knowledge of the faunal content of this enigmatic assemblage.

Introduction

The Upper Triassic beds from Brazil provide evidences of the origin and initial radiation of dinosaurs (Colbert 1970; Langer et al., 1999; Bonaparte et al., 1999; Cabreira et al., 2016; Marsola et al., 2018a; Pacheco et al., 2019). Distinct Assemblage Zones (AZs) mark different evolutionary stages of the group (Schultz et al., 2020). For instance, the Hyperodapedon AZ encompasses some of the oldest dinosaurs worldwide (Langer et al., 1999; Cabreira et al. 2011, 2016; Pacheco et al., 2019), whereas the Riograndia AZ yielded evidences from the moment when dinosaurs become bigger, more abundant, and geographically widespread (Bonaparte et al., 1999; Leal et al., 2004; Müller et al., 2018). This framework relies on radioisotopic data from typical localities of both AZs (Langer et al., 2018). However, some fossiliferous localities lack index fossils. This is the situation of the peculiar Niemeyer Site (Pavanatto et al., 2018). This site yielded several specimens of the traversodontid cynodont Siriusgnathus niemeyerorum (Pavanatto et al., 2018). In addition, were also reported probainognathian cynodonts and archosauriform remains (Pavanatto et al., 2018). This fossiliferous content was tentatively assigned to the Riograndia AZ based on the presence of Siriusgnathus niemeyerorum (Miron et al., 2020; see Marsola et al. 2018b). Among the archosauriform remains exhumed from the Niemeyer site, one particular specimen draws attention. CAPPA/UFSM 0157 comprises a proximal portion of a left femur of a dinosauromorph. This specimen was previously described by Pavanatto et al. (2018) and was regarded as having dinosauromorph affinities. Despite its fragmentary condition, CAPPA/UFSM 0157 is particularly interesting because it combines a set of unusual traits among Late Triassic dinosauromorphs. However, the phylogenetic affinities of this specimens have not been hypothesized with computational analysis. Furthermore, the unique anatomy of CAPPA/UFSM 0157 suggests that it belongs to an undescribed taxon. Hence, here the specimen is ascribed to a new taxon and its phylogenetic affinities analysed. Moreover, the former description by Pavanatto et al. (2018) is expanded based on new observations and comparisons.

AMNH, American Museum of Natural History, New York, New York, USA; CAPPA/UFSM, Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia/Universidade Federal de Santa Maria, São João do Polêsine, Rio Grande do Sul, Brazil; MCN, Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Porto Alegre, Brazil; MCP, Museu de Ciências e Tecnologia Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil; MCZ, Museum of Comparative Zoology, Harvard, Cambridge, USA; MMACR, Museu Municipal Aristides Carlos Rodrigues, Candelária, Rio Grande do Sul, Brazil; MNA, Museum of Northern Arizona, Flagstaff, Arizona, USA; NHMUK, Natural History Museum, London, UK; NMT, National Museum of Tanzania, Dar es Salaam, Tanzania; PVL, Paleontología de Vertebrados, Instituto Miguel Lillo, Tucumán, Argentina; SAM-PK, Iziko South African Museum, Cape Town, South Africa; UCMP, University of California Museum of Paleontology, Berkeley, California, USA; ULBRA, Universidade Luterana do Brasil, Coleção de Paleovertebrados, Canoas, Brazil; ZPAL, Institute of Paleobiology of the Polish Academy of Sciences in Warsaw, Poland.