Large neotheropods from the Upper Triassic of North America and the early evolution of large theropod body sizes

Christopher T Griffin

doi:10.1017/jpa.2019.13

Abstract

AbstractLarge body sizes among nonavian theropod dinosaurs is a major feature in the evolution of this clade, with theropods reaching greater sizes than any other terrestrial carnivores. However, the early evolution of large body sizes among theropods is obscured by an incomplete fossil record, with the largest Triassic theropods represented by only a few individuals of uncertain ontogenetic stage. Here I describe two neotheropod specimens from the Upper Triassic Bull Canyon Formation of New Mexico and place them in a broader comparative context of early theropod anatomy. These specimens possess morphologies indicative of ontogenetic immaturity (e.g., absence of femoral bone scars, lack of co-ossification between the astragalus and calcaneum), and phylogenetic analyses recover these specimens as early-diverging neotheropods in a polytomy with other early neotheropods at the base of the clade. Ancestral state reconstruction for body size suggests that the ancestral theropod condition was small (~240 mm femur length), but the ancestral neotheropod was larger (~300–340 mm femur length), with coelophysoids experiencing secondary body size reduction, although this is highly dependent on the phylogenetic position of a few key taxa. Theropods evolved large body sizes before the Triassic–Jurassic extinction, as hypothesized in most other ancestral state reconstructions of theropod body sizes, but remained rare relative to smaller theropods until the Jurassic.

Highlights

IntroductionThe evolution of large body sizes among theropod dinosaurs, a clade that includes some of the largest known terrestrial carnivores, is a major feature of the history of this group, influencing growth dynamics (Erickson et al, 2004; Carrano, 2006), biomechanics (Day et al, 2002; Preuschoft et al, 2011), ecology (Farlow and Pianka, 2002; Benson et al, 2014), and evolutionary trends (Carrano, 2006; Sereno et al, 2009; Brusatte et al, 2010a; Benson et al, 2014, 2018)
Footprint data have been used to suggest that there was a sharp increase in the maximum body size of theropods only at the beginning of the Jurassic (Olsen et al, 2002), more recently reconstructed ancestral states of theropod body sizes in the Triassic suggest that theropods larger than the typical coelophysoid size (∼200–230 mm femoral length, ∼10 kg) evolved prior to the Early Jurassic (Irmis, 2011; Benson et al, 2012, 2014, 2018; Turner and Nesbitt, 2013), most of these reconstructed body sizes are within the size range of the largest Triassic theropod trackmakers from Olsen and colleagues’ (2002) study
The Bull Canyon specimens (NMMNH P-4563, P-4569) are earlydiverging neotheropods and, judging from several morphological characters known to be ontogenetically variable in other earlydiverging neotheropods, represent skeletally immature individuals

Summary

Introduction

The evolution of large body sizes among theropod dinosaurs, a clade that includes some of the largest known terrestrial carnivores, is a major feature of the history of this group, influencing growth dynamics (Erickson et al, 2004; Carrano, 2006), biomechanics (Day et al, 2002; Preuschoft et al, 2011), ecology (Farlow and Pianka, 2002; Benson et al, 2014), and evolutionary trends (Carrano, 2006; Sereno et al, 2009; Brusatte et al, 2010a; Benson et al, 2014, 2018). Few body fossils of relatively large-bodied theropods are known from the Late Triassic, these specimens (e.g., PULR 076, HMN MB.R.2175, UCM 47721) represent skeletally immature individuals that would have reached even larger body sizes than those of their known skeletal remains (Griffin and Nesbitt, in press), further challenging the hypothesis that theropods experienced a dramatic increase in maximum body size at the onset of the Early Jurassic All this suggests that they were rare, large-bodied theropods were present in the Late Triassic and their replacement of nondinosaurian reptiles as the largest terrestrial carnivores was an extended process throughout the latter portion of the Late Triassic. Including the ‘Bull Canyon Neotheropod’ (NMMNH P-4569) and an isolated neotheropod fibula (NMMNH P-4563; Griffin and Nesbitt, in press), worldwide there are only six known individuals of definitive Triassic

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Conclusion