Amniotes

Scientific name: Amniota

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.

Amniotes (Amniota) are a group of tetrapods that includes birds, reptiles, and mammals. Amniotes evolved during the late Paleozoic era. The characteristic that sets amniotes apart from other tetrapods is that amniotes lay eggs that are well-adapted to survive in a terrestrial environment. The amniotic egg generally consists of four membranes: the amnion, the allantois, the chorion, and the 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 contents 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 (called the yolk) that the embryo consumes as it grows (in placental mammals and marsupials, the yolk sac only stores nutrients temporarily and contains no yolk).

The eggs of many amniotes (such as birds and most reptiles) are enclosed in a hard, mineralized shells. 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 amniotes. 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, and it was these muscles that enabled early amniotes and their descendants to more successfully 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

Species Diversity

Approximately 25,000 species

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 gamebirds, birds of prey, hummingbirds, perching birds, kingfishers, buttonquail, loons, owls, pigeons, parrots, albatrosses, waterfowl, penguins, woodpeckers and many others. Birds have many adaptations for flight such as lightweight, hollow bones, feathers, and wings.
  • Mammals (Mammalia) - There are about 5,400 species of mammals alive today. Members of this group include primates, bats, aardvarks, carnivores, seals and sea lions, cetaceans, insectivores, hyraxes, elephants, hoofed mammals, rodents, and many other groups. Mammals have several unique adaptations including mammary glands and hair.
  • Reptiles (Reptilia) - There are about 7,900 species of reptiles alive today. Members of this group include crocodiles, snakes, alligators, lizards, caimans, tortoises, worm lizards, turtles, and tuataras. Reptiles have scales that cover their skin and are cold-blooded animals.

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.