The amniotes are a group of vertebrates, comprising the mammals, birds, and various other groups collectively referred to as reptiles. Most are adapted to a fully terrestrial existence, although some are secondarily aquatic. In contrast, amphibians are only partially terrestrial and pass through an aquatic stage. The name comes from the amniotic egg, in which the developing embryo is protected by a series of membranes and a hard shell which resists desiccation. Their kidneys and large intestines are also designed to retain water. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta. Most reptiles also have protective scales, which are modified in birds to form feathers.
There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number temporal fenestrae (openings) behind the eye. In anapsids there are none, in synapsids there is one, and in most diapsids there are two.
The skeletal remains of amniotes can be identified by their having at least two pairs of sacralribs and an astragalus bone in the ankle.
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