Amniotes are vertebrate organisms which have a fetal tissue known as the amnion. The amnion is a membrane derived from fetal tissue which surrounds and protects the fetus. The amnion can be found within the egg, as in lizards and birds, or the amnion can simply enclose the fetus within the uterus.
Amniotes include most of the vertebrates, excluding fish and amphibians. Fish and amphibians are anamniotes, meaning “without an amnion”. The eggs of these species are often laid in water, which protects them from being damaged or squished. Most amniotes, by contrast, are terrestrial and require the amnion to protect the developing fetus under the weight of gravity. The only exceptions to this are the whales, which live an entirely aquatic life. They developed an amnion before their ancestors returned to the sea. Some marine turtles also like in the sea, but return to land to lay their eggs, making the amnion necessary.
Characteristics of Amniotes
All amniotes have three membranes surrounding the fetus of one offspring. These membranes are the amnion, or protective layer, the top chorion layer, and the waste-absorbing allantois. These layers can be seen in the image of a chicken egg, below.
While amniotes share a number of other characteristics in general (being vertebrates, tetrapods, etc.), they all developed from a common ancestor which developed the amnion character. The amnion is seen within egg-laying species, such as birds and reptiles, as well as in mammals. While human eggs have lost the shell, in many ways they are identical to chicken eggs as they develop within the uterus.
Animals Considered Amniotes
There are two main divisions of amniotes, the sauropsid amniotes and the synapsid amniotes. The sauropsid amniotes include the reptiles and birds. Formally, this constitutes many different groups, but the sauropsid amniotes share many derived characteristics which separate them from the synapsids. The two groups evolved around the same time, from a common ancestor which was likely not terrestrial.
This means that both the synapsids and sauropsids had to adapt to the new terrestrial environment in a number of different ways. These differences are reflected in the heart, lungs, and kidneys mainly. In sauropsids, there will usually be found faveolar lungs, which differ from the lungs of the synapsids. Faveolar lungs have small chambers which open to a common space. The heart of sauropsids lacks a permanently divided ventricle. While some sauropsids (turtles and crocodiles) have developed hearts that are almost 4 chambered, they are not the synapsid heart with a physical separation between the ventricles.
The sauropsid amniotes also excrete waste differently than the synapsids. Sauropsids typically excrete uric acid (the white paste in bird poop). This substance precipitates out of the urine in the cloaca, where much of the water can be reabsorbed. This makes uric acid a more water-efficient way of excreting nitrogenous wastes, versus the synapsid method. Sauropsids include turtles, lizards, crocodiles, and birds. Typically, they lay eggs although there are some exceptions.
The synapsid amniotes do things slightly different, since they separated from the sauropsids millions of years ago. The synapsid strategy for expelling waste, for instance, is typically all urea. Urea can be concentrated in the synapsid kidney, and excreted with small bits of water. While this may not be as water-efficient as uric acid, it is much more efficient than excreting straight ammonia, which is what fish and amphibians typically do.
The heart of synapsids is 4-chambered, with a well-defined partition between ventricles. This improves the efficiency of oxygenating the blood, by insuring separate paths for blood going to and from the heart. Synapsids developed different lungs than the sauropsids. The synapsid lung is an aveolar lung. Instead of small pockets from a central chamber, the aveolar lung has many branches of trachea, each which ends at an aveolar sac.
There are only 3 extant groups of synapsids, all of which are mammals. While all of these amniotes still have amniotic sacs, they also have very different methods of reproducing. The monotremes, like the platypus, still lay eggs in nests. When the young hatch, they feed them milk from glands in their skin, like all other mammals. The marsupials represent a median between the monotremes and the placental mammals. They develop their young within a uterus, but the young are born at an extremely early age. They must climb along the mother into the marsupial pouch, where they can feed on milk for the rest of development.
The placental mammals represent the rest of the synapsid amniotes. These animals use a placenta, or oxygen and nutrient passing maternal membrane, to feed and nourish offspring within the womb. At birth, offspring of these animals are the largest of all amniotes comparatively. However, placental mammals also have fewer offspring compared to sauropsid amniotes.
Evolution of the Amniotes
Amniotes likely emerged as many of the first terrestrial animals were venturing onto land. The much different terrestrial environment is likely what drove the divisions between the two main groups of amniotes. This likely occurred in the Devonian period, around 400 million years ago. Since then, the two groups have evolved considerable differences in their anatomy and physiology, as discussed above. At the time, the new terrestrial environment provided a number of new niches for the animals to fill, which also diversified them significantly.
Below is an organism which could have been a common ancestor of modern amniotes. This is a parieasaur, a cow-sized organism from the Devonian period. This large reptile-looking organism likely had primitive lungs, heart, and kidneys. It also likely had an amnion, making it one of the first amniotes.