Primitive amniotes and reptiles

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Amphibians lay eggs in water and their young pass the first part of their life in an aquatic environment, like the contemporary tadpole that transforms into a land-living adult frog. The amniotes developed the ability to live without the water-living part of this cycle by laying leathery-shelled eggs lined by a anmiotic membrane that kept water in but allowed oxygen to pass in and carbon dioxide out. Realistic Dinosaur Costume

 

Many of these Carboniferous amphibians became highly adapted to living and feeding underwater in the shallow pools and streams of the tropical swamps. Animatronic Dinosaur
Their limbs became weaker and their bodies flatter. One problem, though, was the articulation of the jaw. In primitive forms the articulation is at the back of the skull, behind the point of attachment of the vertebral column. This means that the mouth could only be opened by dropping the jaw downwards – the stiff neck being unable to lift the skull itself. This is no problem in open water, but on land and in shallow water there would not have been room for the mouth to open properly. Eventually the articulation changed, so that the jaw joint moved directly beneath the condyle – the joint between the back bone and the skull and this enabled the skull to tilt upwards to open the mouth while the jaw was still on the ground.
Below: If the backbone and the jaw are in the same plane, the mouth opens easily.