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An egg tooth is a small protuberance on the beak or nose of vertebrates that are hatched from eggs, ie: birds and reptiles. Some lizards and snakes develop a true tooth that is shed after use; birds and other reptiles generally develop an analogous epidermal horn that is reabsorbed or falls off. Both varieties of egg tooth assist in the breaking or tearing of the eggshell, a process known as pipping.
Birds
Baby birds have a pipping muscle on the back of their necks. It is this muscle which gives them the strength to force the egg tooth through the inner membrane of the eggshell.
When a baby bird becomes too large to absorb oxygen through the pores of its eggshell, it uses its egg tooth to peck a hole in the air sac located at the flat end of the egg. This sac provides about a few hours' worth of air, during which the baby bird breaks through the eggshell to the outside. The egg tooth falls off several weeks after hatching.
Reptiles
Baby snakes generally hatch from eggs with tough, leathery shells. A baby snake's egg tooth tears a hole directly through the shell, and falls off the first time the snake sheds its skin. Lizards have similarly leathery eggshells.
A baby crocodile has an egg tooth on the end of its snout. It is a tough piece of skin which is totally reabsorbed less than two months after hatching. Crocodile eggs are similar to those of birds in that they have an inner membrane and an outer one. The egg tooth is used to tear open the inner membrane; the baby crocodile can then push its way through the outer shell. If conditions are particularly dry that year, the inner membrane may be too tough for the crocodile to break through, and without assistance it will simply die inside the egg. Generally, however, the mother crocodile is there to help free it.
Development of the Egg Tooth
A 121 million year old fossilized egg, complete with well-developed embryo, was discovered in northeastern China in 2004. The bird has no egg tooth, indicating that they are a more recent innovation. Prior to the development of the egg tooth, baby birds likely used sharp claws to break through their shells.
References
"Egg tooth"