An investigation of tetrapod skull architecture using advanced computer modelling techniques

The skulls of animals must balance the conflicting demands of strength and stability with flexibility, so that they can open their mouths as wide as possible and apply a maximum bite force without damaging the enclosed brain and sense organs. Ancestors of reptiles, birds and mammals had a solid skull except for eye and ear openings, but they soon began to develop openings (fenestrae) in the side of the skull behind the eyes; now different skulls have different patterns of fenestrae, but it is still not understood why. Furthermore the overall geometries of skulls are different. There is variation in skull depth, in the size of the brain and/or sense organs, in the complexity of the jaw muscles, and in the length of the neck. All of these features, individually or in combination, have a major effect on skull function (biomechanics) and may underlie the radical differences in skull architecture of living animals. The aim of this research is to understand the relationship between biomechanical forces and skull shape in living animals, and in particular to determine the biomechanical significance of skull fenestrations. To do this work, we need to combine the expertise of mechanical engineers, digital imagers, bone biologists, and morphologists, and use advanced computer modelling techniques to perform sophisticated biomechanical analyses. In this project, information from museum specimens of living animal groups (obtained by advanced computer imaging - High Resolution Computed X-ray Tomography) will be combined to develop accurate models of a range of skulls. These can be modified to change basic parameters (e.g. eye size, brain size, patterns of fenestration), and then loaded in ways that simulate changing complexities of the jaw and neck muscles, changes in size of the brain and sense organs, and/or increasing bite force. As a result, we can, for the first time, test a series of theories to explain skull shape. Not only will the study advance our knowledge of the development of a key group of organisms but it will also deepen our understanding of the complex relationship between biomechanical forces, soft tissue structures and skeletal shape. Understanding this complex relationship is important, not only to general biology but also to medicine (e.g. bone repair and remodelling, over-use injuries, osteoporosis).

For land tetrapods, it is widely accepted that key anatomical features of the skull (e.g. diagnostic holes, fenestrae, emarginations) are genetically regulated and serve specific purposes (are adaptive). However, some of these features may be secondary phenomena reflecting optimisation of skull structure to the combined effects of stress (from jaw and neck muscles, from biting) and the changing proportions of the enclosed brain and sense organs. This impacts on our understanding of the extent to which skull features are there because they serve a particular function or are secondary mechanically or genetically mediated optimisations of form to function. This is a key biological issue. It requires an intimate understanding of cranial and bone biomechanics, structure, comparative function, and developmental biology, a range of expertise that can only be offered by a cross-disciplinary approach. We propose to undertake the first comprehensive study of skull form and function using a hybrid MDA/FEA approach to test a series of hypotheses relating to the biomechanical significance of fenestration and/or emargination. Multibody dynamics analysis (MDA) will be used to calculate the external forces and internal musculature arising during normal skull loading. Finite element analysis (FEA) and a unique adaptive FEA approach developed at Hull (BMU-SIM) will then be used to model the skulls and test their response to varying patterns of stress/strain that result from enlargement of the brain and/or sense organs, increased complexity of jaw muscles, and the presence of a mobile neck. We have access to skull data obtained by High Resolution X-ray CT and to a supercomputer powerful enough for highly detailed static and adaptive skull remodelling studies. This pioneering research will also be the first to model in detail effects of cranial sutures on skull biomechanics and function.