The exceptional vertebrate burrow record of the early Permian Bromacker locality

Burrows are crucial for the survival of vertebrates, providing shelter from unfavourable climate and predators, but also forming a safe space for feeding, breeding, hibernation, or aestivation. Nevertheless, the origin and early evolution of fossoriality is scarcely investigated. The early Permian Bromacker locality (Tambach Formation, Germany) is worldwide known for its exceptional vertebrate skeletal and footprint record. Nevertheless, vertebrate burrows were only briefly reported so far. New excavations in the frame of the BMBF-funded Bromacker project (2020-2025) yielded an unexpectedly large amount of potential vertebrate burrows of different size and morphology. Large-sized (about 30-40 cm wide), low-dipping burrows ending in oval chambers with dense concentrations of broad scratch traces at the bottom and at the sides, were found in high concentrations in fine-grained and cross-stratified sandstone layers. Some of the burrow infills contain tetrapod remains, including Diadectes, Orobates, and Martensius. Somewhat smaller (about 12 cm wide), long, low-dipping burrows ending in pentagonal chambers, characterise the fluvial sandstones at the bottom and at the top of the succession. Furthermore, small (about 5 cm wide), low-dipping burrows displaying branching, thin scratch traces and large quadrangular chambers, were found in the cross-stratified sandstone at the top of the succession. A further type of small (about 3-5 cm wide), simply built burrows, vertical or slightly dipping, characterise the laminated mudstone facies. This exceptional abundance and diversity of potential vertebrate burrows makes the Bromacker locality a key area for studying the early evolution of fossoriality, likely linked to survival strategies during early Permian global warming.