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.

Biology Department College of Science Taibah University Al Madinah Al Munawwarah Kingdom of Saudi Arabia

Centre for Craniofacial and Regenerative Biology Floor 27 Guy's Tower Guy's Hospital King's College London London Bridge London UK

Department of Anatomy Faculty of Medicine Pontificia Universidad Católica de Chile Santiago Chile

Department of Developmental Biology Institute of Experimental Medicine Czech Academy of Sciences Prague Czech Republic

Department of Experimental Biology Faculty of Science Masaryk University Brno Czech Republic

Department of Oral Health Sciences Life Sciences Institute University of British Columbia Vancouver BC Canada

Laboratory of Molecular Morphogenesis Institute of Animal Physiology and Genetics Czech Academy of Sciences Brno Czech Republic

Zoology Department Faculty of Science Fayoum University Fayoum Egypt

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