Craniocervical myology and functional morphology of the small-headed therizinosaurian theropods Falcarius utahensis and Nothronychus mckinleyi.

David K. Smith,Ulrich Joger

doi:10.1371/journal.pone.0117281

Abstract

Therizinosaurs represent a highly unusual clade of herbivorous theropods from the Cretaceous of North America and Asia. Following descriptions of the basicrania of the North American therizinosaurs Falcarius utahenisis and Nothronychus mckinleyi, the craniocervical musculature in both taxa is reconstructed using Tyrannosaurus, Allosaurus, and some extant birds as models. These muscles are subdivided into functional groups as dorsiflexors, lateroflexors, and ventroflexors. Lateroflexors and dorsiflexors in Nothronychus, but not Falcarius, are reduced, from the plesiomorphic theropod condition, but are still well developed. Attachments in both genera are favorable for an increase in ventroflexion in feeding, convergent with Allosaurus fragilis. Falcarius and Nothronychus are both characterized by a flat occipital condyle, followed by centra with shallow articular facets suggesting neck function very similar to that of an ostrich Struthio camelus. Neck movement was a combined result of minimal movement between the individual cervical vertebrae.

Highlights

Reconstructions of craniocervical musculature are regarded as being increasingly important in understanding feeding behavior in extinct vertebrates
Cervicocephalic muscles insert on the basicranium, which has recently been described for the therizinosaurs Falcarius and Nothronychus [11, 12]
Hypothesized therizinosaur function is deduced from the presented craniocervical muscle reconstructions (Fig. 3) and compared to other theropods

Summary

Introduction

Reconstructions of craniocervical musculature are regarded as being increasingly important in understanding feeding behavior in extinct vertebrates. Jaw muscles and function in such selection of theropods have been reconstructed [5,6,7,8]. Cervicocephalic muscles insert on the basicranium, which has recently been described for the therizinosaurs Falcarius and Nothronychus [11, 12]. A reconstruction of the major craniocervical muscles for Falcarius and Nothronychus is reported, using supplemental information for Erlikosaurus andrewsi [13, 8] along with nervous and vascular reconstructions The size of the epaxial muscles is taken to be a function of the height of the neural arch plus the spine, as evident in lariform birds and proposed for larger theropods [2]. Since the therizinosaurian braincase closely resembles the avian condition, avian terminology is retained, where possible, but homologies with crocodilian musculature are noted

Methods and Materials

Findings

Conclusions