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By the end of this section, you will be able to:
  • Describe the main characteristics of amniotes
  • Explain the difference between anapsids, synapsids, and diapsids, and give an example of each
  • Identify the characteristics of reptiles
  • Discuss the evolution of reptiles

The amniotes —reptiles, birds, and mammals—are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments. This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their shell-less eggs. Although the shells of various amniotic species vary significantly, they all allow retention of water. The shells of bird eggs are composed of calcium carbonate and are hard, but fragile. The shells of reptile eggs are leathery and require a moist environment. Most mammals do not lay eggs (except for monotremes). Instead, the embryo grows within the mother’s body; however, even with this internal gestation, amniotic membranes are still present.

Characteristics of amniotes

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 ( [link] ). 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 chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. 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.

Art connection

The illustration shows an egg with the shell, embryo, yolk, yolk sac, and the extra-embryonic membranes
The key features of an amniotic egg are shown.

Which of the following statements about the parts of an egg are false?

  1. The allantois stores nitrogenous waste and facilitates respiration.
  2. The chorion facilitates gas exchange.
  3. The yolk provides food for the growing embryo.
  4. The amniotic cavity is filled with albumen.

Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal (rib) ventilation of the lungs.

Evolution of amniotes

The first amniotes evolved from amphibian ancestors approximately 340 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. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye ( [link] ). Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.

Questions & Answers

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The scientific study of life.
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the virus that causes mumps in humans is composed of a protein outer Shell containing a core of DNA
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Basic science and applied science question about cancer
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what are the importance of ATPs
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How can biology be studied from a microscopic approach to a global approach
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The large central opening in the poriferan body is called
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You go for a long walk on a hot day. Give an example of a way in which homeostasis keeps your body healthy.
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You sweat.
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sweating is your bodies way of keeping you from overheating.
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Biology is the study of Life
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is the branch of science which deals with the of living things.
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Gyamfi
Describe the steps and results of reintroducing wolves to Yellowstone National Park.
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tag and release wolves into Yellowstone. wolves eventually reproduce and the pack grows. as wolves hunt they cull the sick and weak prey. the carcass that is left provides food for other species (scavengers and insect.. etc). this heals the circle of life and contributes to the biodiversity...
Will
before you know it species that are critical to the eco system return. having apex predators is crucial to an ecosystem... it helps run the deer and elk , etc around.
Will
example: there was a species of shrub/ plant that grows along river banks that moose love to eat.. the moose have no predator so they decimate that food source which also helps prevent erosion. when the wolves were reintroduced this changed. oddly enough this plant species started to repopulate in
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the areas where wolf feces sat and decayed
Will
which of the following statements about the parts of an egg are false?
Israel Reply
Monotremes include...?
Israel
medicinal plants including microbs
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,medicinal plants including microbes
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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