Form, function and development of the amniote skull: a case study using lepidosaurs

Each side of our skull behind our eyes bears a large opening that houses jaw-closing muscles. While the skull of our species and other mammals has a single opening, or temporal fenestra, the ancestors of all amniotes (mammals and reptiles, including birds) more than 300 million years ago had two. Transformations in the number and form of the temporal fenestrae during evolution is traditionally depicted in textbooks as giving rise to the major lineages of living amniotes, and their diversification in a broad array of environments. For instance, while mammals have evolved only one fenestra, snakes have lost their temporal fenestrae, and crocodiles retain two. The evolutionary history of temporal fenestrae is however more complex than previously thought, as similar fenestration has evolved multiple times in different groups, and species that share the same fenestration often exhibit a great variability in terms of size, shape, and arrangement of the associated bony scaffold.

Because the temporal fenestrae house the jaw-closing muscles, their presence or absence and changes in their morphology during evolution have been traditionally linked to the volume of the jaw-closing musculature, feeding forces, and diet. These evolutionary changes were interpreted as resulting from changes in the way the skull ossifies during embryonic development, which would alter the position of the developing muscles. Yet, we now know that muscles and other soft tissues form before the skeletal elements in the developing embryos, and we therefore think that the formation of the fenestrae in the cranium might be a consequence of the way muscles develop. In addition, it is still unclear how variation in the fenestration and in its associated musculature influences the functioning and mechanics of the overall skull during feeding, and thus how development, morphology and function of the skull interact to influence the evolution of the skull in amniotes. In this project we propose to address these questions by focusing on lepidosaurs (tuatara, lizards, and snakes), one of the most specious group of living amniotes with a remarkable ecological and anatomical diversity, that encompasses the major types of fenestration found in living amniotes.

Addressing these ideas and questions has been hindered by a lack of knowledge on how the skull bones and muscles develop in non-model animals, by the lack of a comparative dataset on muscle properties across living animals and the inability to apply computational approaches to deduce function on large sample sizes and therefore make inferences on function and evolution. Here we bring together a team of researchers who are leaders in developing and validating methods for imaging, reconstructing, and simulating the musculoskeletal anatomy, its development and function. The team will provide large amounts of developmental data (150 developmental stages) on species that represent the variation in skull construction across lepidosaurs, as well as data on musculoskeletal anatomy and function on adult animals (150 species) that encompass all major families. For the first time we will bring together an understanding of how the skull develops, and the relative timing of bone and muscle formation; how this generates variation in skull shape, muscle properties and the relationship between skull shape, muscles, and function (diet and ecology).
Our project will produce methodological advances that can be applied more broadly to evolutionary transitions and radiations, and to address long standing questions linking form and function. Palaeontologists, anatomists, biomechanists, evolutionary and developmental biologists and engineers will benefit from this work, which will establish new international collaborations. Its visual aspect and focus on lizards and snakes will appeal to the public, offering engagement opportunities and generating media interest.

The phenotypic variation of complex biological systems such as the head are influenced by different demands and constraints. These include development, evolutionary history, and essential functions such as feeding. Ideas on cranial fenestrae evolution and the skull form have predominantly focussed on skeletal structures, and their response to biomechanical demands associated with feeding. However, soft tissues such as muscles develop first, and dictate to some extent the formation of the skeleton. This close developmental integration of hard and soft tissues is understood by craniofacial clinicians but has received little attention in broader comparative, evolutionary studies. The relationship between the morphology of the temporal region, the rest of the cranium and the musculature is also little understood at broad comparative scale, as well as how craniofacial complexity translates to functional complexity. We will determine the relationship between the morphology of the temporal region observed in adults and the way muscles and bones are integrated during development, and quantify its significance for the functioning and mechanics of the feeding system. We will use a combination of cutting-edge imaging techniques to visualise the development of head tissues in situ, geometric morphometrics, experimental and biomechanical modelling approaches (e.g. finite element analysis) to measure musculoskeletal function and mechanics, underpinned by phylogenetic comparative methods and statistical analysis. The results of this study will represent a major advance in our understanding of how phenotypic diversity translates and equates to functional and ecological diversity, and will uncover how supposed constraints imposed by development and phylogenetic heritage manifest into phenotypic variation.