Functional reorganisation of the cranial skeleton during the cynodont–mammaliaform transition

Abstract

Skeletal simplification occurred in multiple vertebrate clades over the last 500 million years, including the evolution from premammalian cynodonts to mammals. This transition is characterised by the loss and reduction of cranial bones, the emergence of a novel jaw joint, and the rearrangement of the jaw musculature. These modifications have long been hypothesised to increase skull strength and efficiency during feeding. Here, we combine digital reconstruction and biomechanical modelling to show that there is no evidence for an increase in cranial strength and biomechanical performance. Our analyses demonstrate the selective functional reorganisation of the cranial skeleton, leading to reduced stresses in the braincase and the skull roof but increased stresses in the zygomatic region through this transition. This cranial functional reorganisation, reduction in mechanical advantage, and overall miniaturisation in body size are linked with a dietary specialisation to insectivory, permitting the subsequent morphological and ecological diversification of the mammalian lineage.

Weighing in on miniaturization: New body mass estimates for Triassic eucynodonts and analyses of body size evolution during the cynodont‐mammal transition

“…If we mean just the emergence of extremely small sizes, independently of the evolutionary processes that lead to these phenotypes, then the answer would be "yes" regardless of the results presented here. Admittedly, this definition is useful in some contexts, like biomechanical studies that focus on the consequences of these small sizes (e.g., Lautenschlager et al, 2018Lautenschlager et al, , 2023; however, we think our approach is more appropriate for macroevolutionary studies. Here we followed Hanken and Wake (1993) and Gould and MacFadden (2004)'s phylogenetic oriented definitions, as such, our results indicate that no miniaturization took place in the probainognathian lineage during the Triassic.…”

“…However, Bonaparte (2012)'s interpretation is problematic, because it is based on a rather unorthodox phylogenetic hypothesis, where clades like Eucynodontia, Probainognathia, and Cynognathia are not monophyletic at all. Nonetheless, after Rowe (1993), it is well accepted that phenomena of miniaturization played an important role within the probainognathian lineage and in the origin of mammals (e.g., Kemp, 2005;Lautenschlager et al, 2018Lautenschlager et al, , 2023Rowe, 2017;Rowe & Shepherd, 2015). Unfortunately, without clear cut definitions on what a "miniaturized" cynodont is, most studies just assume that this process happened and focus on the mechanical and anatomical consequences of smaller sizes, without even defining what the authors consider as "miniaturized" (viz-a-viz small taxa), much less testing the hypothesis using modern phylogenetic hypotheses and methods.…”