While digging the garden have found what appears to be part of a skull, how do i find out what it is from?

on first look it looked human, it has a horeshoe shaped piece indented in the back and there are remnants of teeth, these teeth go right back to the rear so looking at human ones on line it doesnt appear to be human, have looked on line and it doesnt appear to be normal domesticated animals or farmyard, any clue how to find out what it is from?