WHAT ARE AMNIOTES? - BIOLOGY - 2026

The amniotas are monophyletic group formed by animals whose embryos are surrounded by membranes (amnion, allantois, yolk sac and chorion) and are frequently covered with a leathery or calcareous shell.

Amniotic radiation is made up of two main lineages: the sauropsids and the synapsids. As seen in the fossil record, both groups diverged very early in evolutionary time - close to the Carboniferous, or probably earlier.

Reptiles are amniotes.

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The sauropsid lineage is made up of birds, the now-extinct dinosaurs, and modern reptiles. Synapsids, for their part, are a monophyletic grouping made up of therapsids and modern mammals.

The amniotic egg

Sea turtle hatching from egg. Author: Mayer Richard. Wikimedia Commons.

The amniotic egg achieves independent reproduction of the aquatic environment

Amphibians exhibit a series of characteristics - at a physiological and anatomical level - that allow them to develop a life outside of water. However, life on land occurs in a partial way as reproduction continues to tie amphibians to bodies of water.

The ancestor of the clade that contains non-avian reptiles, birds and mammals evolved an egg adapted to terrestrial conditions and allowed the total independence of aquatic ecosystems. In fact, the amniotic egg is so distinctive that it gives the clade its name.

Other characteristics also favored the independence of the water. Mainly the lack of gills and internal fertilization. Logically, the existence of a hard shell that surrounds the egg requires that fertilization be internal, because the sperm is not able to penetrate this structure.

For this reason, a copulatory organ appears in amniotes (except in tuataras and in most birds) that is responsible for transferring the sperm. The most popular organ among the members of the group is a penis, derived from the walls of the cloaca.

The four extra-embryonic membranes

Amniotic eggs have four extra-embryonic membranes: the amnion, allantois, chorion, and the yolk sac.

Amnion

The amnion is the first membrane that surrounds the embryo. It is responsible for providing an aqueous medium to the embryo for its growth, in addition to having buffer functions.

Allantois

The metabolic wastes produced by the newly forming organism are stored in the allantois. In this layer we find a significant vascularization.

Chorion

The chorion is responsible for surrounding the entire contents of the egg and, like the allantois, it is a highly vascular layer. For this reason, both the chorion and the allantois participate as a respiratory organ, mediating the exchange of carbon dioxide and oxygen between the embryo and the exterior.

Yolk sac

A characteristic shared with the egg of non-amniotic animals is the presence of a yolk sac. This functions as a storage of nutrients and its size is much larger in the eggs of amniotes.

Additional layer: mineralized or leathery rind

In most cases, the described structure is surrounded by an additional layer or shell, highly mineralized and in certain species flexible. However, this coverage is absent in many lizards, snakes, and the vast majority of mammals.

In birds, this mineralized cover is an important mechanical barrier. One of the characteristics of the shell is that it allows the passage of gases, but reduces the loss of water, that is, it is semi-permeable.

Evolution of the amniotic egg

One idea that may be attractive to many is to think that the amniotic egg is the “earth” egg. However, many amphibians are capable of laying their eggs in wet lands and many amniotes spawn in wet locations, such as turtles.

Clearly, the characteristics of the amniotic egg allow it to develop in much drier areas - compared to the optimal sites for amphibian eggs. Thus, the evolution of the amniotic egg was a key factor in the success of tetrapods on earth.

The greatest selective advantage that the amniotic egg gave to the group was to allow the growth of a much larger embryo and in much less time.

Furthermore, calcium deposits in the shell can be dissolved and subsequently absorbed by the developing organism. This material can be incorporated into the skeleton and promote its construction.

Characteristics derived from amniotes

In addition to the amniotic egg, this animal group is characterized by ventilating its lungs by means of aspiration. They do this by filling their lungs with air by expanding the rib cage using different muscle structures. If we compare it with amphibians, there is a change from positive to negative ventilation.

Also, compared to amphibian skin, amniote skin is much thicker and resistant to water loss. The skin tends to be more keratinized and much less permeable to water. There is a wide variety of structures composed of keratin, such as scales, hair, feathers, among others

Keratin gives the skin physical protection, and the lipids in the skin are responsible for limiting water loss.

Relationship between amniotes

The differentiation between both lineages of amniotes into sauropsids and synapsids is based on the fenestration of the skull, in the temporal region - the area before each eye. This region appears to be a reliable indicator of evolutionary lineages.

The temporal region of the amniotes can occur in two ways. The first criterion takes into account the number of openings or temporal fenestra and the second includes the position of the temporal arches. Here we will focus only on the first distinction (number of fenestras).

In non-amniotic organisms and in the most primitive amniotes, the temporal region is characterized by being entirely covered by bone. This condition is called annapsid.

A group that separated early from the anapsids formed the synapsids. This type of skull, with a single temporal opening, is found in the ancestors of mammals and in current mammals.

The second group that diverged from the anapsids were the diapsids, whose skull has two temporal openings. This anatomical pattern is found in pterosaurs and dinosaurs, birds and reptiles - with the marked exception of turtles, which are anapsids.

References

1. Divers, SJ, & Stahl, SJ (Eds.). (2018). Mader's Reptile and Amphibian Medicine and Surgery-E-Book. Elsevier Health Sciences.
2. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
3. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
4. Llosa, ZB (2003). General zoology. EUNED.
5. Vitt, LJ, & Caldwell, JP (2013). Herpetology: an introductory biology of amphibians and reptiles. Academic press.