Cranial biomechanics underpins high sauropod diversity in resource-poor environments.

David J Button,Emily J Rayfield,Paul M Barrett

doi:10.1098/rspb.2014.2114

Abstract

High megaherbivore species richness is documented in both fossil and contemporary ecosystems despite their high individual energy requirements. An extreme example of this is the Late Jurassic Morrison Formation, which was dominated by sauropod dinosaurs, the largest known terrestrial vertebrates. High sauropod diversity within the resource-limited Morrison is paradoxical, but might be explicable through sophisticated resource partitioning. This hypothesis was tested through finite-element analysis of the crania of the Morrison taxa Camarasaurus and Diplodocus. Results demonstrate divergent specialization, with Camarasaurus capable of exerting and accommodating greater bite forces than Diplodocus, permitting consumption of harder food items. Analysis of craniodental biomechanical characters taken from 35 sauropod taxa demonstrates a functional dichotomy in terms of bite force, cranial robustness and occlusal relationships yielding two polyphyletic functional ‘grades’. Morrison taxa are widely distributed within and between these two morphotypes, reflecting distinctive foraging specializations that formed a biomechanical basis for niche partitioning between them. This partitioning, coupled with benefits associated with large body size, would have enabled the high sauropod diversities present in the Morrison Formation. Further, this provides insight into the mechanisms responsible for supporting the high diversities of large megaherbivores observed in other Mesozoic and Cenozoic communities, particularly those occurring in resource-limited environments.

Highlights

Large herbivores are primarily limited by their high gross energy requirements, and so by plant productivity [1,2]
An extreme example of this paradox is presented by the fauna of the Late Jurassic Morrison Formation of North America
Biomechanical modelling demonstrates that Camarasaurus was capable of exerting much greater bite forces than Diplodocus through its more mechanically efficient skull, greater overall adductor muscle mass and a greater relative contribution of the external adductor muscle group

Summary

Introduction

Large herbivores are primarily limited by their high gross energy requirements, and so by plant productivity [1,2]. The derived diplodocoid and titanosaur lineages show the independent derivation of a ‘narrow-crowned’ morphology of a reduced dentition of peglike teeth occupying a gracile skull with a narrow, inclined adductor chamber End members of this spectrum are exemplified by Camarasaurus and Diplodocus, respectively, two of the most abundant and regularly co-occurring sauropod taxa in the Morrison Formation [11]. Posterior bite force was taken as the sum of reaction forces from two point constraints between the left and right posterior-most teeth To compare both Camarasaurus and Diplodocus two groups of analyses were performed. Analyses replicating ‘branch-stripping’ behaviour were performed, including loading from the jaw musculature and plant-stripping forces following Young et al [36], and including the loading consequences of the craniocervical musculature For these additional analyses see electronic supplementary material, §§8 and 11. The time-calibrated supertree was mapped onto the first two PC axes of the biomechanical morphospace within R to yield the biomechanical phylomorphospace

Results

Discussion

Findings

Patagosaurus Turiasaurus