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

Among amniotes, lizards are the only lineage that can lose and regrow a portion of the body, typically the tail. In modern lizards, this self-controlled tail amputation occurs at weak points known as fracture planes, found in the caudal vertebrae. However, little is known about the morphological and developmental variations of these fracture planes in fossil lizards. Utilizing high-resolution X-ray CT, we examined the fracture planes in two Early Cretaceous fossil lizards Yabeinosaurus and Dalinghosaurus . The fracture plane first appears at the sixth caudal vertebra in Yabeinosaurus and at the ninth caudal in Dalinghosaurus . In Yabeinosaurus , the fracture plane is located within the transverse processes, resembling the condition seen in modern scincids. In Dalinghosaurus , the fracture plane is found posterior to the transverse processes, similar to the condition found in extant phrynosomatid iguanians. Our findings suggest morphological disparity in the fracture planes of extant lizards first evolved in stem squamates such as Yabeinosaurus . Furthermore, the posterior caudal vertebrae of Dalinghosaurus demonstrate fused neural spines and divided vertebral centra – a condition known in extant adult lizards to restrict the ability of tail shedding. This fusion in Dalinghosaurus implies that this species could adapt escape strategies at different developmental stages. Most early squamates were small, and the increase in the use of CT and synchrotron imaging studies on damaged and compressed specimens enhances our understanding of not only the anatomical details but also the behavior and growth of fossil squamates.