the amniotic egg is the single most important advancement in the terrestrialization of vertebrates.
i don't think terrestrialization is a word.
before The Amniotic Egg, amphibians [the most derived vertebrates at the time] were dorking around in water/on land/in water, always one foot out the door ...
then egg-laying reptiles hit, and SHAZAM! all of a sudden the marine environment was unnecessary for vertebrate survival. bird eggs are a lot like reptilian-dinosaur-lizard eggs; the reason for this is that birds are actually considered part of the reptilian clade.
according to Paleontology, fossilized eggs are hard to come by. the oldest egg found is thought to have been laid by a pelycosaur, aka a sailback:
[to note: mammals eventually evolve from the pelycosaur lineage]
though birds are on the whole terrestrial, the embryos still need a watery environment to develop in, and the egg provides that. other things an egg provides for a birdembryobabyfetus:
3. waste storage
4. free continental breakfast and wireless!
instead of trying to explain what happens inside of an egg as it develops, i will just show you this:
the inside is great and all, but my favorite part of the egg is the shell. hel-lo, it's comprised of one of my favorite chemical compounds, calcium carbonate (CaCO3)! Calcium carbonate is also featured in: bones, chalk, limestone, fossils and shells of marine organisms, cement, and tums. wow, how versatile!
i figure a fertilized egg needs two things in order for the birdembryobabyfetus to "come to term," as it were:
bird nests hold the fragile egg as the birdembryobabyfetus develops. in some cases, nests also serve as a home to young chicks. nests are really diverse: of the types, you got your cavity, your bark, your platform, your spherical, your pendant...the list goes on. within those, many different kinds of materials are used. cool, natural architectural triumphs!
my favorite is the nest that is made by a cave-dwelling swift [aerodramus] from its own saliva.
i guess it's a delicacy in china in a soup called bird's nest soup. ah, quelle originalit