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Amniotes: The Innovators of the Four-Membraned Egg

Scientific name: Amniota

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These Nile crocodile hatchlings are one of about 25,000 species of amniotes alive today.

These Nile crocodile hatchlings are one of about 25,000 species of amniotes alive today.

Photo © Heinrich van den Berg / Getty Images.
Birds are one of the three basic groups of amniotes alive today.

Birds are one of the three basic groups of amniotes alive today.

Photo © Sijanto / Getty Images.
The Japanese macaque is a placental mammal and as such reproduces with an amniotic egg that lacks a mineralized shell.

The Japanese macaque is a placental mammal and as such reproduces with an amniotic egg that lacks a mineralized shell.

Photo © Wizdata / Shutterstock.

Amniotes (Amniota) are a group of tetrapods that includes birds, reptiles, and mammals. There are about 25,000 species of amniotes alive today. Amniotes appeared and diversified during the late Paleozoic era. Amnoites lay eggs that are well-adapted to survive in a terrestrial environment.

The amniotic egg generally consists of four membranes, the amnion, allantois, chorion, and yolk sac. The amnion encloses the embryo in a fluid that serves as a cushion and provides an aqueous environment in which it can grow. The allantois is a sac that holds metabolic wastes. The chorion encloses the entire conents of the egg and together with the allantois helps the embryo breath by providing oxygen and disposing of carbon dioxide. The yolk sac, in some amniotes, holds a nutrient-rich fluid (the yolk) that the embryo consumes as it grows (in placental mammals and marsupials, the yolk sac only stores nutrients temporarily and contains now yolk). The eggs of many amniotes (such as birds and most reptiles) is enclosed in a mineralized shell. In many lizards, this shell is flexible. The shell provides physical protection for the embryo and its resources and limits water loss. In amniotes that produce shell-less eggs (such as all mammals and some reptiles), the embryo develops within the female's reproductive tract.

Amniotes are often described and grouped by the number of openings (fenestrae) that are present in the temporal region of their skull. The three groups that have been identified on this basis include the anapsids, diapsids, and synapsids. Anapsids have no openings in the temporal region of their skull. The anapsid skull is characteristic of the earliest amnoites. Diapsids have two pairs of openings in the temporal region of their skull. Diapsids include birds and all modern reptiles. Turtles are also considered diapsids (although they have no temporal openings) because it is thought that their ancestors were diapsids. Synapsids, which include mammals, have a single pair of temporal openings in their skull.

The temporal openings characteristic of amniotes are thought to have developed in conjunction with stronger jaw muscles that enabled early amniotes and their descendents to better capture prey on land.

Key Characteristics

  • amniotic egg
  • thick, waterproof skin
  • strong jaws
  • more advanced respiratory system
  • high-pressure cardiovascular system
  • excretion processes that reduce water loss
  • large brain, modified sensory organs
  • larvae do not have gills
  • undergo internal fertilization

Classification

Amniotes are classified within the following taxonomic hierarchy:

Animals > Chordates > Vertebrates > Tetrapods > Amniotes

Amniotes are divided into the following taxonomic groups:

  • Birds (Aves) - There are about 10,000 species of birds alive today. Members of this group include albatrosses, gamebirds, herons, hummingbirds, kingfishers, loons, owls, parrots, penguins, perching birds, pigeons, waterfowl, woodpeckers and many others. Birds have wings, feathers, hollow bones and numerous other adaptations for an aerial lifestyle.
  • Mammals (Mammalia) - There are about 5,400 species of mammals alive today. Members of this group include bats, carnivores, cetaceans, seals and sea lions, hyraxes, insectivores, elephants, hoofed mammals, primates, rodents, and many other groups.
  • Reptiles (Reptilia) - There are about 7,900 species of reptiles alive today. Members of this group include snakes, worm lizards, lizards, alligators, caimans, crocodiles, tortoises, turtles, and tuataras.

References

Hickman C, Roberts L, Keen S. Animal Diversity. 6th ed. New York: McGraw Hill; 2012. 479 p.

Hickman C, Roberts L, Keen S, Larson A, l'Anson H, Eisenhour D. Integrated Principles of Zoology 14th ed. Boston MA: McGraw-Hill; 2006. 910 p.

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