Evolutionary morphology of the rabbit skull

Abstract

The skull of leporids (rabbits and hares) is highly transformed, typified by pronounced arching of the dorsal skull and ventral flexion of the facial region (i.e., facial tilt). Previous studies show that locomotor behavior influences aspects of cranial shape in leporids, and here we use an extensive 3D geometric morphometrics dataset to further explore what influences leporid cranial diversity. Facial tilt angle, a trait that strongly correlates with locomotor mode, significantly predicts the cranial shape variation captured by the primary axis of cranial shape space, and describes a small proportion (13.2%) of overall cranial shape variation in the clade. However, locomotor mode does not correlate with overall cranial shape variation in the clade, because there are two district morphologies of generalist species, and saltators and cursorial species have similar morphologies. Cranial shape changes due to phyletic size change (evolutionary allometry) also describes a small proportion (12.5%) of cranial shape variation in the clade, but this is largely driven by the smallest living leporid, the pygmy rabbit (Brachylagus idahoensis). By integrating phylogenetic history with our geometric morphometric data, we show that the leporid cranium exhibits weak phylogenetic signal and substantial homoplasy. Though these results make it difficult to reconstruct what the ‘ancestral’ leporid skull looked like, the fossil records suggest that dorsal arching and facial tilt could have occurred before the origin of the crown group. Lastly, our study highlights the diversity of cranial variation in crown leporids, and highlights a need for additional phylogenetic work that includes stem (fossil) leporids and includes morphological data that captures the transformed morphology of rabbits and hares.

Lagomorpha as a Model Morphological System

“…Considerable homoplasy in the morphology of leporid species was identified by Corbet (1983) after examining 21 morphological characteristics for 22 leporid species. More recent morphometric studies of lagomorphs have also found a high degree of homoplasy, low phylogenetic signal, and adaptive divergence in skull shape (Ge et al, 2015;Kraatz and Sherratt, 2016;Feijó et al, 2020). These studies highlight the difficulties in reconstructing a robust phylogeny for lagomorphs, particularly at the intergeneric level, by using morphological data.…”

Evolution and expansion of the RUNX2 QA repeat corresponds with the emergence of vertebrate complexity

“…For example, the previously mentioned Anoles represent a natural case study by which RUNX2 repeat evolution may influence adaptive convergence of cranial ecomorphs 107 and studies in birds will elucidate whether RUNX2 QA repeat variation promotes adaptive changes in beak shape diversity 107 . In addition, leporid rabbits and hares (order Lagomorpha) are a relatively recent (~20 mya) evolutionary radiation that display substantial craniofacial length and angle variation in response to different ecologies 116 , 117 . However, how RUNX2 QA repeat variation corresponds with the natural craniofacial variation in this group remains unknown.…”

The Intertwined Evolution and Development of Sutures and Cranial Morphology

“…In geckos, cranial kinesis, which is supported by the cranial sutures, offers the capability for a larger bite force ( Herrel et al, 2000 ). It is possible this functional role of sutures observed in cranial kinesis of geckos is mimicked in mammals which also exhibit cranial kinesis, such as rabbits ( Kraatz and Sherratt, 2016 ). Not only are sutures functionally important during feeding, but they also act as shock absorbers to absorb strains from other external inputs, such as fighting behaviours and locomotion ( Curtis et al, 2013 ).…”