 Another transitional feature of Dimetrodon is a ridge in the back of the jaw called the reflected lamina . The reflected lamina is found on the articular bone , which connects to the quadrate bone of the skull to form the jaw joint . In later mammal ancestors , the articular and quadrate separated from the jaw joint while the articular developed into the malleus bone of the middle ear . The reflected lamina became part of a ring called the tympanic annulus that supports the ear drum in all living mammals . 
