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

This article focuses on the embryonic development and morphology of the egg tooth in the sand lizard (Lacerta agilis), a member of the squamate clade Unidentata. The egg tooth, a true tooth used to break the eggshell during hatching, develops through all classical tooth stages and is a large, midline premaxillary structure with pleurodont implantation, attached by periodontal ligament-like tissue, acellular cementum-like tissue, and alveolar bone. Contrary to recent suggestions that the enlarged egg tooth in snakes results from merging two tooth primordia, this study found no evidence of such merging in Lacerta agilis; instead, the egg tooth's size is attributed to delayed development of adjacent regular premaxillary teeth, allowing resource allocation to the egg tooth. The findings contribute to understanding squamate phylogeny by supporting that two egg teeth, as seen in gekkotans and dibamids, represent an ancestral trait, while Unidentata species possess a single egg tooth.