Abstract

The echidna incubates the egg for ten days in the pouch after it has developed in the uterus. The fetuses acquire an egg tooth and caruncle during this time to aid in hatching. Micro-CT, histology, and immunofluorescence were used to evaluate the development of the egg tooth and caruncle by utilizing rare and unprecedented access to limited echidna pre- and post-hatching tissues. The echidna egg tooth developed via evagination, like the first teeth of some reptiles and fish, in contrast to therian tooth germs, which develop via placode invagination. Instead of forming a tooth root with a ligamentous attachment like in other mammals, the egg tooth ankylosed to the premaxilla. The caruncle formed as a separate mineralization from the adjacent nasal capsule. This suggests that the monotreme egg tooth has been preserved from a common ancestor of mammals and reptiles because it shares many similarities with typical reptilian teeth.