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.
Which is an egg that if you really look at it in the big sense of the thing, what this egg isdoing is allowing vertebrateanimals to produce larger newborn young than they would be able toproduce otherwise.
When we look at a fish egg, for example, or an amphibianegg, what we will see is thatthese eggs, which are -- I'm using these because these are representative of the ancestors fromwhich the amnioticegg evolved, these eggs have one structure which is called a extra embryonicmembrane that provides for nutrition.
So in an amnioticegg, the water is provided by one of the extra embryonicmembranes, which is called the "amnion." This is the membrane that gives the egg its name.
Considering the eggs in light of the possible sequence of events which occurred during their laying (Figure 6), egg 1 was laid and prior to the time eggs 2 and 3 were laid, approximately 12 to 15 cm of sediment, (half the diameter of egg 1), were deposited.
Eggs are positioned in the substrate as though their final positions were determined by the viscosity of wet sediments into which they were dropped.
Eggs are found with pieces of hatching windows lying inside the lower portion of the shells, usually with the concave surface upward, indicating breakage by causes other than the hatching of young and also indicating negligent treatment of the eggs when they were laid.
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