The archosaur and synapsid lineages of ancient reptiles diverged from ancient amniotes about 280 million years ago.

Archosaur lineage

The dinosaurs belonged to archosaur lineage. This lineage gave rise to crocodilians. It also gave rise to two groups of fliers.

1. Pterosaurs (pteros, wing + saurus, lizard): Their size is ranged from sparrow size to animals with wing spans of 13 m. They developed an elongation of the fourth finger. It supports their membranous wings. Their sternum was adapted for the attachment of flight muscles. Their bones were hollow to lighten the skeleton for attachment of flight muscles.  These adaptations are paralleled to the adaptations in the birds.

2. Birds: Birds are the descendants of the second lineage of flying archosaurs.

Synapsid lineage



Synapsid lineage gave rise to the mammals. The legs of synapsids were long. They held their bodies above the ground. Teeth and jaws were adapted for chewing and tearing. Additional bones were incorporated into the middle ear. These mammal-like characteristics developed between the Carboniferous and Triassic periods. It flnally gave rise to mammals.

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