They are distinguished from other amniotes by having a single opening -LRB- temporal fenestra -RRB- in their skull behind each eye , which developed in the ancestral synapsid about NUMBER@1 million years ago -LRB- mya -RRB- during the late Carboniferous Period .

They developed one opening in their skull behind each eye , about NUMBER@1 million years ago during the late Carboniferous Period .
