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PALEOZOIC ERA

PERMIAN PERIOD

299-252 million years ago

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therapsid

Therapsids modified their skulls.

A number of modifications of the skull evolved in therapsid reptiles. The snout became progressively shorter, the two vomer bones partially fused, the ectopterygoid and pterygoid attached to basisphenoid of the braincase, and the supratemporal bone was lost. After Biarmosuchus, the synapsid opening enlarged for larger jaw muscles and the squamosal became enlarged. Some therapsids still possessed the primitive characteristic of teeth on pterygoid. Therapsids were the first members of the mammalian lineage to evolve turbinals which affected the passage of air in the nasal cavity. The stapes was smaller, rested on the quadrate, and included a large stapedial foramen.