Breaking through the eggshell: embryonic development of the premaxillary dentition in Lacerta agilis (Squamata: Unidentata) with special emphasis on the egg tooth

Abstract The egg tooth of squamates is a true tooth that allows them to break, tear, or cut the eggshell during hatching. In this clade there are some uncertainties concerning the egg tooth implantation geometry, the number of germs, and their fates during embryonic development. Here, we used X-ray microtomography and light microscopy, focusing on the egg tooth and remaining premaxillary teeth of the sand lizard (Lacerta agilis, Squamata: Unidentata). The developing egg tooth of this species passes through all the classic stages of tooth development. We did not find any evidence that the large size of the egg tooth is related to the merging of two egg tooth germs, which has recently been suggested to occur in snakes. Instead, this feature can be attributed to the delayed formation of the neighbouring regular premaxillary teeth. This might provide more resources to the developing egg tooth. At the last developmental stage, the egg tooth is a large, midline structure, bent forward as in most oviparous Unidentata. It is characterized by pleurodont implantation, and its base is attached to the pleura and a peculiar ridge of the alveolar bone. The attachment tissue contains periodontal ligament-like tissue, acellular cementum-like tissue, and alveolar bone.