Origin of Amniotes

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the most successful group of terrestrial organisms.

What are Amniotes?

amniotes are a group of tetrapods

The egg is equipped with an amnios which is an adaptation to lay eggs on land rather than in water.

Amniotic embryos, are either laid as eggs or carried and the amniotic sac surrounds the foetus.

The embryonic membranes, and lack of a larval stages, distinguish amniotes from non-amniote

Features of Amniotes

Amniotic egg – distinctive extraembryonic membrane.

Amniotic egg is characteristic of:

- Turtles

- Lepidosaurs

- Crocodilians

- Aves

- Monotremes

Embryonic membranes that contribute to the placenta of therian mammals - eutherians and metatherians.

How did amniotes arise?

Ancestors of modern amphibians were dependent upon water or very damp areas in which to lay their eggs.

Fertilised eggs covered by a moist, gelatinous mass, had to be kept wet or damp to avoid desiccation.

Modern amphibians, egg masses are laid in water, in very damp burrows or encased in a frothy mass to maintain the necessary humidity.

This limited the ability of amphibians to travel away from water bodies.

In the late Devonian period, a new form of egg evolved - the amniotic egg.

These eggs featured a tough outer layer that prevented the eggs from drying out.

This meant that the amniotes were not restricted to wet regions.

Instead, they were able to radiate out, settling into new habitats.

Amniotes arose in Carboniferous.

Initial radiation from late Carboniferous - early Permian.

Subsequent radiations throughout Mesozoic

Also Cenozoic.