Origin and early evolution of vertebrate burrowing behaviour

The ability to live underground is widespread among continental vertebrates, so understanding the origin and early evolution of fossorial vertebrates and the architecture and function of the burrows they excavate is an important component of the history of Life on Earth. However, this topic has not been addressed in a synoptic manner; available information is scattered, hampering our understanding of the overall picture. Here, after a short overview of convergent morphological and behavioural adaptations seen in modern fossorial taxa and the diversity of extant vertebrate burrows, we review the fossil record of inferred vertebrate burrows and fossorial vertebrates from the Devonian to the Triassic. Results highlight a probable Devonian earliest occurrence of fossoriality in continental vertebrates (Dipnoi) and a Carboniferous earliest occurrence of fossoriality in tetrapods (Recumbirostra). During the Devonian-Carboniferous, burrows were probably used primarily for aestivation or temporary shelter and evidence of fossoriality is restricted so far to European and North American localities. During the Permian, fossoriality became geographically widespread and developed in new, distantlyrelated vertebrate lineages, such as diapsids and synapsids. Additionally, there was a size increase and all the main structural features of sub-vertical and sub-horizontal burrows were acquired, as well as the probable use of burrows as permanent shelters or for breeding. We show that the radiation of fossorial forms and increase in abundance and complexity of burrows are contemporaneous with climatic crises such as the Cisuralian aridification, culminating in the Artinskian Warming Event (AWE), and the end-Guadalupian and end-Permian extinction events. After the end-Permian mass extinction, vertebrate fossoriality became a common and widespread feature of continental environments and in more distal floodplain areas, probably as a consequence of changing fluvial regimes. During the Triassic, fossoriality is recorded in even more groups, such as Procolophonidae and Temnospondyli. The first definite complex burrows used as permanent shelters appeared as early as the Early Triassic, and evidence of shared burrow use by different clades appears, suggesting an emerging role of burrowers as ecosystem engineers.