In addition to these membranes, the eggs of birds, reptiles, and a few mammals have shells. An amniote embryo was then enclosed in the amnion, which was in turn encased in an extra-embryonic coelom contained within the chorion.
Between the shell and the chorion was the albumin of the egg, which provided additional fluid and cushioning. This was a significant development that further distinguishes the amniotes from amphibians, which were and continue to be restricted to moist environments due their shell-less eggs. Although the shells of various reptilian amniotic species vary significantly, they all permit the retention of water and nutrients for the developing embryo.
The egg shells of bird avian reptiles are hardened with calcium carbonate, making them rigid, but fragile. The shells of most nonavian reptile eggs, such as turtles, are leathery and require a moist environment. Most mammals do not lay eggs except for monotremes such as the echindnas and platypuses. The amniotic egg is the key characteristic of amniotes.
Most mammals do not lay eggs except for monotremes. The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians.
However, the eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois Figure 1. Extra-embryonic membranes are membranes present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac.
The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta. Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal rib ventilation of the lungs. The first amniotes evolved from amphibian ancestors approximately million years ago during the Carboniferous period.
The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids.
Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds, and can be further divided into anapsids and diapsids. Many groups of early 'amphibians' radiate during this period, collectively called the 'stem tetrapods'.
However, the dry periods of the Permian and Mesozoic were not optimal for these animals limited to areas near water. Today, three groups of amphibians remain.
The Caecilians are unusual, rare, legless amphibians. Salamanders are the second group, and the giant Japanese salamander is the largest living amphibian - reaching six feet in length! The most diverse group is the Anura - mening "tailless" - the frogs and toads. Most amphibians have gills or lungs at some point in their life cycle, but many also or exclusively respire through their skin.
For this to happen, the gases must diffuse into a thin film of water, and then move across cell membranes by diffusion. So, most amphibians need to keep their skin moist to respire; and it also means that their skin, as an absorptive surface, is also prone to absorb toxins in the environment.
For these reasons, amphibians are particularly sensitive to pollutants and environmental contaminants. Over the last 50 years, there has been a dramatic decline in amphibian populations worldwide, probably due to exposure to human-produced pollution, climate change, and chytrid fungal infections. It is tough to say which of these variables is 'most' responsible for the decline. In all likelihood, each factor increases sensitivity to the other two.
In any case, these three factors are having a decimating effect on amphibian populations. The Amniota: Within the tetrapods, a new type of animal evolved in the Carboniferous; an animal that laid an egg surrounded by a series of membranes and a shell.
These layers served to protect the developing embryo and yolk sac from dessication, allowing these species to colonize land further from water and exploit food resources and habitats that amphibians could not reach.
This clade of animals is the amniotes, and their evolutionary innovation is the amniotic egg. The primitive condition within this group probably produced a leathery covering, much like turtles.
A more rigid, desiccation resistant egg like birds have is probably a derived trait, as is the retention of the egg and live bearing of young, as seen in some snakes, and most mammals. The amniotes radiated during the Permian Period, when the drying of the unified land mass of Pangaea gave them a decided advantage over the ancestral, amphibious tetrapods.
Three groups evolved at this time, distinguished by their skull morphology. Gymnophionia worm and snakelike amphibians. Home Wild Files N. Learn more about some of the amphibians that are found in New Hampshire. Anura frogs,toads Caudata salamanders, newts Gymnophionia worm and snakelike amphibians.
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