Evolution of Unique Skull Morphology and Function in Lagomorphs: A Combined Effect of Locomotion and Mastication?

The leporid lagomorphs, rabbits and hares, exhibit unique cranial traits that distinguish them from all other mammals, including their closest relatives, the Ochotonidae (pikas). Key features include fenestration of the rostrum and posterior cranial bones, a pronounced ventral flexion of the facial region, and an intracranial joint that suggests some form of cranial movement (kinesis). Despite both evolutionary and contemporary conservatism in diet, size and much of their morphology, leporid lagomorphs display considerable specialisa- tion in locomotor form, particularly in cursorial (running) and saltatorial (hopping) modes. These unique morphological traits are thought to enhance locomotor efficiency by reducing weight, improving visual acuity, mitigating shock and protecting the sensory organs during the deceleration phase of high-speed running. Despite the functional importance of the leporid intracranial joint, research on the evolu- tionary history, comparative anatomy and biomechanical function of the feature is lacking. Broadly, the project’s aims are three-fold: 1. To analyse the intracranial joint in the context of leporid evolutionary history, utilising phylogenetic comparative methods. 2. To quan- tify the complexity of the intracranial joint using two independent metrics and explore its relationship with overall cranial shape, locomotor mode, mass, burrowing habit and facial tilt angle. 3. To explore intracranial joint function via a series of finite element models, analysing the forces associated with mastication and locomotion. In summary, the work found that it is likely that the last common ancestor of living i leporids had some facial tilting and an intracranial joint, but the last common ancestor of all lagomorphs included in the dataset did not. We also found that it was likely that the last common ancestor of living leporids had fenestration that occupies the dorsal, but not the anteroventral, surface of the maxillary corpus. This suggests that these traits likely developed alongside the onset of specialised saltatorial and cursorial locomotor form. The work also found that there were significant correlations between complexity and locomotor mode as well as with facial tilt angle, indicating that cursorial species have less complex sutures and highlighting a potential link between the complexity of suture interdigitation and facial tilting. The assessment of biomechanical performance via finite element analysis found that the fastest species (Lepus) have lower strain energy in the intracranial joint and proportionally fewer compressive strain-dominated elements relative to tensile strain- dominated elements. Furthermore, variation in stress in the posterior portion of the cranium varies between species, with the fastest-running species exhibiting markedly lower stress during deceleration loading. Our results support previous studies suggesting that locomotion is likely a strong driver of cranial morphology in leporid lagomorphs. This work contributes to filling critical knowl- edge gaps regarding the unique traits in leporid crania by identifying new leporid-specific characters that can be used in phylogenetic studies, quantifying complexity in the intracra- nial joint and developing a new methodology to simulate the effect of inertial deceleration loads on the cranium. Future work should seek to further investigate the tissues present in the joint, better understand the anatomy and role of the neck musculature and elucidate the developmental trajectory of facial tilting.