Why are terrestrial vertebrates so highly diverse? In times of anthropogenic change and mass extinctions, this key question to evolutionary research is more relevant than ever. We often attribute this phenomenon to modifications in accordance with different environments and lifestyles. However, it is not always easy to reconstruct from the fossil record how historic key events determined the destinies of entire lineages millions of years ago. Therefore, 160 years after Darwin's theory of evolution, our understanding of certain evolutionary mechanisms remains incomplete.
In order to better understand the complex interrelations between vertebrate lifestyle, form, and evolution, we looked into the skull roof structure of squamate reptiles – i.e., lizards and snakes. We wanted to understand to what extent their bone structure reflects certain lifestyles. Although many of these reptiles use their skulls as a digging tool, this question has never been systematically investigated. In our paper First Evidence of Convergent Lifestyle Signal in Reptiles Skull Roof Microanatomy, we therefore employed computer simulations to reconstruct the evolution of a specialised burrowing lifestyle over a period of 240 million years. Remarkably, we found that burrowing evolved independently in 54 squamate lineages. These reptiles are therefore particularly well suited as a model system for the study of convergent evolution.
In the second phase of our study, we compared the skull roof structure of lizards and snakes in accordance with their lifestyles. To this end, we made use of high-resolution computed tomography (micro-CT) for the 3D visualisation of bone tissue and employed a new, effective protocol for data analysis. Doing this, we achieved a large sample size, allowing us to draw conclusions about the entire squamate reptilian clade with over 11,000 species. We found that burrowing lizards and snakes have repeatedly evolved a particularly dense and compact skull roof in independent evolutionary processes. We further identified typical proportions of both the skull and between the skull roof bones as convergently evolved modifications associated with this lifestyle.
In our study, we present a novel case of convergent evolution: in different lineages, very similar structures have independently evolved in response to a common lifestyle. Such similarities reflect a certain function, in this case burrowing, and may therefore provide little information on the phylogenetics relations between the considered taxa. Nonetheless – or precisely therefore, the knowledge of these processes is of outstanding importance: by means of skull roof structure, we can now reconstruct the lifestyle of reptiles that became extinct millions of years ago. Our findings thus cast completely new light on the evolutionary history of certain lineages. This may particularly apply to snakes, for which both an aquatic and burrowing origin have been controversially discussed for decades. Our work now adds to a growing body of evidence for the latter. Having said this, our findings may also have important implications beyond squamate reptiles. A burrowing lifestyle has likely played a key role in the evolutionary history of turtles and certain amphibians. Recent studies even suggest that the mammalian stem lineage may not have survived the largest mass extinction in earth's history at the end of the Permian about 250 million years ago without adopting a burrowing lifestyle. It is therefore invaluable to understand such historical lifestyle transitions.
Should our work have sparked your interest in the fascinating field of squamate skull morphology and its implications for their ecology and evolution, you may be pleased to hear that we uploaded the µCT-scans employed in our study into the Data Repository of the Museum für Naturkunde Berlin. They will thus be available for future research projects worldwide.