Amniotes vs Anamniotes: Main Characteristics of Amniotes and Anamniotes

What are Amniotes?

Amniotes are a subclass of reptiles that includes mammals, birds, and turtles. Amniotes are distinguished from other reptiles by their ability to lay eggs on land that are protected by an amniotic membrane.

This membrane allows for the embryos to develop without being in contact with water, which is necessary for the eggs of most other reptiles.

Amniotes include some of the most successful animals on Earth and have been around for over 300 million years.

What distinguishes Amniotes?

Amniotes are a tetrapod vertebrate clade that includes sauropsids (which includes reptiles and birds) and synapsids (including mammals). They are identified by the presence of a membrane (amnion) that protects the embryo and the absence of a larval stage.

Why reptiles are called first amniotes

The first amniotes termed “basal amniotes,” looked like little lizards and developed from amphibian reptiliomorphs around 312 million years ago, during the Carboniferous geologic epoch.

Amniotes colonized the land on Earth and eventually became the dominating terrestrial vertebrates.

They quickly separated into synapsids and sauropsids, both of which survive to the present day. Protoclepsydrops is the earliest known fossil synapsid, dating from around 312 million years ago, whereas Paleothyris, a member of the order Captorhinida, dates from the Middle Pennsylvanian era (c. 306–312 million years ago).

The earliest reptiles lay leathery amniotic eggs on land. The first amniotes were reptiles. Mammalian amniotes also have an amnion and a vestigial yolk sac.

Are Amphibians Amniotes?

Amphibians are not amniotes. Reptiles, birds, and mammals are classified as amniotes. They are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes.

The anamniotes are an informal category that includes fish and amphibians, which are classified as “lower vertebrates” since they deposit their eggs in water. They differ from amniotes, or “higher vertebrates” (reptiles, birds, and mammals), which deposit their eggs on land or keep the fertilized egg within the mother.

This was an important breakthrough since amphibians were constrained to damp habitats due to their shellless eggs.

What are Anamniotes?

The anamniotes are an undefined category that includes fish and amphibians, which are classified as “lower vertebrates” since they deposit their eggs in water.

They differ from amniotes, or “higher vertebrates” (reptiles, birds, and mammals), which deposit their eggs on land or keep the fertilized egg within the mother.

The term “amnion” refers to an embryonic membrane generated during amniotes’ fetal development that functions to carry oxygen into the egg and expel carbon dioxide.

As their name implies, anamniotes lack an amnion throughout fetal development and are therefore capable of exchanging oxygen, carbon dioxide, and waste metabolites with the surrounding water.

The water aids in the dispersion of waste products (especially ammonia), allowing the embryo to develop normally without being poisoned by its own waste products.

All anamniote classes undergo a stage of growth that resembles that of a fish, showing their close physiological link.

Amniotes vs Anamniotes

There are two main types of vertebrates: amniotes and anamniotes. Amniotes are a group of vertebrates that have an extra membrane, the amnion, surrounding their embryos.

This allows amniotes to lay eggs on land, as opposed to in water like their anamniotic ancestors.

Amniotes include reptiles, birds, and mammals. Anamniotes are a group of vertebrates that lack an amnion. They include amphibians and fish.