The simian shelf is a bony thickening on the front of the ape mandible

Mandible

The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

. Its function is to reinforce the jaw, though it also has the effect of considerably reducing the movement of the tongue by restricting the area available for muscles.

Humans are the only apes to have protruding chins, though some fossils of early humans show evidence of a simian shelf.