Everything about The Amniotes totally explained
The
amniotes are a group of
tetrapod vertebrates that include the
Synapsida (
mammals and
mammal-like reptiles) and
Sauropsida (
reptiles and
dinosaurs, including
birds). Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In humans, these membranes include the
amniotic sac that surrounds the fetus. These embryonic membranes, and the lack of a larval stage, distinguish amniotes from tetrapod
amphibians.
The first amniotes, which resembled small lizards, evolved 340 million years ago. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with waste, allowing the eggs and the amniotes themselves to evolve into larger forms. The amniotes spread across the globe and became the dominant land vertebrates. The oldest known amniote is
Paleothyris.
Definition and traits
They are defined by
embryonic development that includes the formation of several extensive membranes, the
amnion,
chorion, and
allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in
amphibians. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis
(External Link
). The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments, or the massive size and yolk content of eggs evolved for direct development to a larger size.
Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell, and an
allantois evolved to facilitate respiration while providing a reservoir for disposal of wastes. Their
kidneys and
large intestines are also well-suited to water retention. Most mammals don't lay eggs, but corresponding structures may be found inside the
placenta.
The first amniotes, such as
Casineria kiddi, which lived about 340 million years ago, resembled small lizards. Their eggs were small and covered with a membrane, not a hard shell like most modern amniote eggs. Although some modern amphibians lay eggs on land, with or without significant protection, they all lack advanced traits like an amnion. This kind of egg only became possible with internal fertilization. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land, as species evolved to lay their eggs on land where they were safer than in the water. One can assume the ancestors of the amniotes laid their eggs in moist places, as such modest-sized animals wouldn't have too many difficulties in finding depressions under fallen logs or other suitable places in the ancient forests, and dry conditions were probably not the main reason why the soft shell emerged.
In fish and amphibians there's only one inner membrane, also called an embryonic membrane. In amniotes the inner anatomy of the egg has evolved further and new structures have developed to take care of the gas exchanges between the embryo and the atmosphere, as well as dealing with the waste problems. In order to grow a thicker and tougher shell new ways to supply the embryo with oxygen had to be developed as diffusion alone wasn't enough. After the egg had developed these structures, further sophistication allowed the amniotes to lay much bigger eggs in much drier habitats. Bigger eggs allowed for bigger offspring and bigger adults could produce bigger eggs, which meant that the amniotes developed the opportunity to grow bigger than their ancestors. Real growth wasn't possible however, until they stopped relying on small invertebrates as their main food source and started to eat plants or other vertebrates, or returned to the water. New habits and heavier bodies meant further evolution for the amniotes, both in behavior and anatomy.
There are three main lines of amniotes, which may be distinguished by the structure of the
skull and in particular the number of
temporal fenestrae (openings) behind the eye. In
anapsids (turtles) there are none, in
synapsids (mammals and their extinct relatives) there's one, and in most
diapsids (non-anapsid reptiles, dinosaurs, and birds) there are two.
The skeletal remains of amniotes can be identified by their having at least two pairs of
sacral ribs and an astragalus bone in the
ankle.
Taxonomy
(see sub-pages for in depth classification for each group).
Further Information
Get more info on 'Amniotes'.
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