What is an Amniote?
When vertebrate animals first moved from the water to the land they had one major problem: How to reproduce without access to water. The earliest vertebrates to live on land somewhat resembled modern day amphibians. They had eggs that needed to stay wet to survive and began life as aquatic larva, much like modern tadpoles. These primitive animals also participated in external fertilization, where eggs and sperm are released into the water beside one another in order for fertilization to occur.
In order for vertebrate life to move away from the water and further inland, internal fertilization needed to evolve. Internal fertilization is what modern-day reptiles, mammals, and birds employ; sperm enters the female animal and the eggs are fertilized inside her body.
Early terrestrial vertebrates also needed eggs that wouldn’t desiccate outside of the water. This obstacle was solved by the “amniotic egg”. Amniotic eggs have a hard external shell that prevents water from escaping, as well as a series of moist membranes called the “amnion”. Together these protect the developing embryo and prevent it from drying out.
In modern times, fish and amphibians both lack amniotic eggs and their reproduction is tied to the presence of water. Reptiles, birds, and mammals, on the other hand, are amniotes with internal fertilization and eggs that can survive outside away from the water.
Yes, mammals are amniotes even though most of them don’t lay eggs! Although mammals usually gestate their offspring internally instead of in an egg, they still have an amnion and evolved from ancestors that laid amniotic eggs. Some mammals, like echidnas and platypuses, still lay eggs just like their ancient ancestors.
Zilles, Myriam (photographer). (2017). Hatching Chick Egg Shell Break [photography]. Retrieved from https://pixabay.com/photos/hatching-chicks-egg-shell-break-bill-2448541/