Seals use their teeth to spear fish and hold onto them. The teeth of the seal are all very similar as are the teeth of the alligator. Reptiles have undifferentiated teeth whereas mammals have differentiated teeth. However, the seal is one example of a reversion to relatively undifferentiated teeth in mammals. Look closely at the jaws of the sheep and alligator.
All reptiles have more than one bone in their jaws.
The joint in reptile is at the very back of the skull. In the mammal it is moved forward more on the skull. As mentioned above reptiles have undifferentiated teeth whereas mammals have differentiated teeth. This transition took place from about 300 million years ago to about 200 million years ago, even before the appearance of most of the dinosaurs. Today's reptiles and mammals differ in a large number of ways, as indicated in the table below.