RESUMO
BACKGROUND: The egg tooth is a vital structure allowing hatchlings to escape from the egg. In squamates (snakes and lizards), the egg tooth is a real tooth that develops within the oral cavity at the top of the upper jaw. Most squamates have a single large midline egg tooth at hatching, but a few families, such as Gekkonidae, have two egg teeth. In snakes the egg tooth is significantly larger than the rest of the dentition and is one of the first teeth to develop. RESULTS: We follow the development of the egg tooth in four snake species and show that the single egg tooth is formed by two tooth germs. These two tooth germs are united at the midline and grow together to produce a single tooth. In culture, this merging can be perturbed to give rise to separate smaller teeth, confirming the potential of the developing egg tooth to form two teeth. CONCLUSIONS: Our data agrees with previous hypotheses that during evolution one potential mechanism to generate a large tooth is through congrescence of multiple tooth germs and suggests that the ancestors of snakes could have had two egg teeth.