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PALEOZOIC ERA

PERMIAN PERIOD

299-252 million years ago

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therapsid

Advanced therapsids possessed a mix of primitive and advanced features. Aspects of the hip and leg had become more mammal-like.

In the therapsid hip, the acetabulum was open more ventrally. In the most primitive therapsids, such as Biarmasuchus, the ilium was not as expanded as in later therapsids (such as Titanophoneus in the following image). A dual gate was possible in that the hip accommodated both an erect and a sprawling posture. The legs were longer and more slender. In the foot, some phalanges were shorter and the foot was more symmetrical, indicating that it was more forward-facing. The 5th distal element in the tarsus fused with 4th. Therapsids possessed a plantigrade foot with a more powerful astragalo-calcaneal joint. An extension of the calcaneus allowed it to act as a lever and the ankle accommodated a dual gate.