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The early amniotes evolved into two separate lines: synapsids and sauropsids. Sauropsids are further divided into anapsids and diapsids. The main difference between synapsids, anapsids and diapsids are the structures of the skull and number of temporal fenestrae they possess. Temporal fenestrae refer to the post-orbital openings in the skull. Anapsids have no temporal fenestrae, synapsids have one and diapsids have two.

Anapsids include extinct organisms and testudines. Testudines include mainly the turtles. Recent molecular studies show that the ancestors of turtles possessed a diapsid skull, but modern turtles show an anapsid-like skull, this might be due to evolutionary changes that took place over the years which led to disappearance of the temporal fenestrae in the modern turtles.