Are Alligators Amniotes?

Alligators are considered to be amniotes. Amniotes are a group of tetrapods or four limbed animals that possess a backbone and spinal column and have a terrestrially adapted egg. These include synapsids (mammals along with their extinct kin) and sauropsides (reptiles and birds, as well as their fossil ancestors).

Amniotic embryos whether they are laid as eggs or carried by the female, are protected and aided by several extensive membranes. The first amniotes were referred to as “basal amniotes” and were creatures that resembled small lizards that had evolved from the amphibianreptiliomorphs about 340 million years ago, in the carboniferous geologic period. These amniotes eggs were also able to survive out of water which allowed them to branch out into drier environments. The eggs could also “breathe” and “cope” with waste, allowing the eggs and the amniotes to evolve into larger forms. The amniotes spread across the globe and became the dominate land vertebrates.

Early on in the evolutionary history of amniotes, basal amniotes evolved into 2 main lines of amniotes that came to be known as the synapids and the sauropsides, both of which persist into the modern era.

Amniotes are able to be characterized based in part by embryonic development that includes the formation of several extensive membranes known as the amnion, chorion, and allantois. Amniotes develop directly into a typical terrestrial form with limbs, as well as a thick stratified epithelium, rather than entering a feeding larval tadpole stage followed by metamorphosis as in amphibian.

In creatures that can be classified as amniotes, these amniotes unlike amphibians, transition from a two layered periderm to cornified epithelium that is triggered by thyroid hormone during embryonic development, rather than metamorphosis. This unique embryonic feature of amniotic creatures may reflect specializations of eggs to survive drier environments, as their massive size and yolk content of eggs has also suggested an evolution for direct development to a larger size.

Features of amniotes evolved for survival on land and include certain characteristics such as the following: a sturdy and leathery hard eggshell, an allantois that evolved to facilitate respiration while also providing a reservoir for disposal of wastes. Kidneys and large intestines that are well suited to water retention.

The first amniotes such as Casineria kiddi lived about 340 million years ago and evolved from amphibian reptoliomorphs and resembled small lizards. Their eggs were small and covered with a leathery membrane, however they were not in a hard shell like those of the birds and crocodiles that we know of today.

Classifications of amniotes have traditionally recognized the three following classes based on major traits as well as physiology:

• Class Reptilia (reptiles)

1. Subclass Anapsida (“proto-reptiles”, possibly including turtles).
2. Subclass Diapsida (majority of reptiles, progenitors of birds).
3. Subclass Synapsida (Mammal like reptiles, progenitors of mammals).

 

• Class Aves (birds)

1. Subclass Neornithes (all modern birds & several extinct subclasses).

 

• Class Mammals (mammals)

1. Subclass Monotremata (egg laying mammals).
2. Subclass Theria (marsupials and placental mammals).