The cranial mechanics of herbivory in kangaroos and relatives: an integrated geometric morphometric and finite element approach

Abstract

<p>The structure and material properties of different foods are widely considered to be reflected in the mechanics of the vertebrate skull. In this thesis, the relationship between feeding ecology and cranial morphology is examined in extant kangaroos and relatives. Geometric Morphometrics (3D shape analysis) and Finite Element Analysis (a form of computational biomechanics) were employed in order to identify the influences that the physical properties of vegetation, and behaviours involving food procurement, have on the shape and structure of the cranium. </p> <p>The thesis focusses on the Macropodiformes, a sub-order of herbivorous diprotodont marsupials that includes three extant families: kangaroos and wallabies (Macropodidae), bettongs, potoroos, and rat-kangaroos (Potoroidae), and the musky rat-kangaroo (Hypsiprymnodontidae). This clade is represented by a diverse array of extant species from which to draw known ecology and diet across a broad range of body sizes. An initial interspecific study on sixteen macropodiform species found that feeding ecology is clearly associated with the relative lengths of the muzzle and zygomatic arch. Species that bite harder with the anterior dentition on more resistant foods possess a shorter muzzle and longer zygomatic arch.</p> <p>By applying these methods to three closely related pademelon (genus: <i> Thylogale</i>) taxa, biomechanical performance during anterior biting was also found to reflect the mechanical properties of their respective diets at the subspecies scale. A diet dominated by more resistant vegetation was again represented by a shorter, more robust cranium, while a longer, more gracile cranium was observed in species that either bite into softer foods or employ cervical musculature to obtain more vegetation via tension (plucking), thus not requiring harder bites.</p> <p>Wombats and koalas were then incorporated into a sample of other medium to large macropodiform species to test the viability of these previous findings at a broader scale. The results were then used to suggest the potential feeding ecology of an extinct species: the giant short-faced kangaroo, <i>Simosthenurus occidentalis</i>. Diet was clearly associated with palatodental arrangement across these species. However, muzzle length was not determined by diet, as in the previous chapters, but rather was associated with the hardest bites required of a species within its particular niche. A combination of cranial features and dental arrangements indicate that <i>S. occidentalis</i> was likely a browser, capable of consuming tough, bulky vegetation. </p> <p>The findings of this thesis suggest that craniofacial morphology is intimately linked with dietary ecology among these species and may be used to infer the potential feeding behaviours and diet of rare or remote marsupial herbivore species for conservation and management strategies and also of extinct species for inferring their paleoecology. The relationships identified here could be applicable to other herbivorous taxa and used in tandem with microwear and stable isotope analyses to draw more robust conclusions, regarding feeding ecology and behaviour. </p>