The “Megatambichnus” riddle: Investigating possible producers of large vertebrate burrows from the Cisuralian Bromacker locality

Fossoriality is common among land vertebrates and offers many ecological advantages. Evidence for burrowing lifestyles dates back to the Paleozoic era, but large tetrapod burrows are rare, and their producers remain largely unknown. One of the oldest known large vertebrate burrows (up to ~70 cm wide) comes from the Cisuralian Bromacker locality in Thuringia, central Germany. Recent excavations at the Bromacker site have uncovered new material of this trace fossil, previously informally named “Megatambichnus”, including ramps, horizontal tunnels, and terminal chambers. These structures display distinct scratch traces, often in sets of five. Such features allow for a confident classification of the trace fossils as tetrapod burrows. Here, we take advantage of the unique simultaneous occurrence of trace fossils and exceptional preserved skeletons at the Bromacker locality to correlate potential producers with the “Megatambichnus” burrows. For that, we introduce a new method comparing burrow dimensions with the body sizes and proportions of fossil taxa from the same locality. In addition, we investigate the potential fossorial adaptations of the taxa that fit the burrow dimensions, based on osteological features observed in modern fossorial tetrapods. Our results show that diadectids, such as Orobates pabsti and Diadectes absitus, and caseids, such as Martensius bromackerensis, match the burrow dimensions and show skeletal traits consistent with a fossorial lifestyle, making them reliable tracemaker candidates. Further morphometric analyses of the trace fossils and skeletons, combined with biomechanical and histological approaches, will be conducted to more precisely identify the producer.