Lower rotational inertia and larger leg muscles indicate more rapid turns in tyrannosaurids than in other large theropods.

Eric Snively,Lawrence M Witmer,John R Cotton,Heinrich Mallison,Anthony P Russell,Scott A Hartman,Donald M Henderson,Philip J Currie,Lara A Surring,Thomas R Holtz,Haley O’Brien,Michael E Burns

doi:10.7717/peerj.6432

Abstract

SynopsisTyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods.MethodsTo compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a(p) to stringently test the null hypothesis of equal agility.ResultsTyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration).ImplicationsThe capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.

Highlights

Tyrannosaurid theropods were ecologically unusual dinosaurs (Brusatte et al, 2010), and were as adults the only toothed terrestrial carnivores larger than 60 kg (Farlow & Holtz, 2002) across much of the northern continents in the late Cretaceous
The l-adjusted phylogenetically-informed generalized least squares (PGLS) regression line under-predicts agility, fitting non-tyrannosaur theropods more closely than tyrannosaurids (Figs. 4 and 5), indicating that theropods as a whole are more agile than predicted by phylogeny
Our phylogenetic regression analysis finds that agility and mass are strongly correlated among all theropods (R2 > 0.94; p < 0.001), and exhibit a high degree of phylogenetic signal (l > 0.88)

Summary

Introduction

Tyrannosaurid theropods were ecologically unusual dinosaurs (Brusatte et al, 2010), and were as adults the only toothed terrestrial carnivores larger than 60 kg (Farlow & Holtz, 2002) across much of the northern continents in the late Cretaceous. Large theropods would turn by applying torques (cross products of muscle forces and moment arms) to impart angular acceleration to their bodies. This angular acceleration can be calculated as musculoskeletal torque divided by the body’s mass moment of inertia (=rotational inertia). Terrestrial vertebrates such as cheetahs can induce a tight turn by lateroflexing and twisting one part of their axial skeleton, such as the tail, and rapidly counterbending with the remainder, which pivots and tilts the body (Wilson et al, 2013; Patel & Braae, 2014; Patel et al, 2016). Such turning balances magnitudes of velocity and lean angle, and centripetal and centrifugal limb-ground

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Conclusion