Tail shedding in two Cretaceous lizards: autotomic structures revealed by X-ray CT scanning

Tail shedding is a widespread anti-predator strategy in modern lizards, facilitated by specialized autotomic structures in the caudal vertebrae. These structures include a fracture plane and, in some taxa, a pseudospine that creates predetermined breaking points along the tail. In caudal vertebrae, the position of the fracture plane relative to the transverse processes varies among extant squamates and has been used as a phylogenetic character. However, little is known about the three-dimensional morphology of autotomic structures in extinct lizards, particularly in early taxa from the Jurassic and Cretaceous Lagerstätten that are preserved in compressed slabs with limited exposure of the fracture plane. Using high-resolution X-ray computed tomography (CT), we examined the caudal vertebrae of two Early Cretaceous lizards from the Jehol Lagerstätten of China: Yabeinosaurus sp. and Dalinghosaurus longidigitus . Our CT reconstructions reveal that both taxa possess fracture planes, but with distinct morphologies. In Yabeinosaurus , the fracture plane traverses directly through the transverse processes, partitioning them into anterior and posterior segments, a condition present in certain species of extant Scincinae, Xantusiidae, Anguidae, and Shinisauridae. In Dalinghosaurus , the fracture plane lies posterior to the transverse processes, resembling the condition in extant gekkotans, lacertids and some iguanians. Additionally, Dalinghosaurus exhibits a fan-shaped pseudospine encompassing the fracture plane – a structure previously unrecognized in this genus – and shows partial fusion of the fracture plane, implying that autotomic capability may have varied through ontogeny. These findings demonstrate that fracture plane morphology was already disparate among Early Cretaceous squamates. High-resolution CT imaging of compressed fossils provides a powerful tool for resolving the three-dimensional anatomy of autotomic structures, enabling more robust morphological comparisons and providing novel characters for future phylogenetic analyses.