Along with amphibians and reptiles, mammals belong to the group of animals called __, named for having four limbs. Mammals have a number of unique anatomical features that fossilize readily, allowing scientists to trace their orignin.
For example, the lower jaw is composed of one bone (the __) in mammals but several bones n other tetrapos. In addition, the lower and upper jaw hinge between a different set of bones in mammals than in other __. Mammals also have a unique set of three bones that transit sound in th e middle ear (__,__,__), whereas other tetrapods have only one such bone (the __). Finally, the teeth of mammals are differentiated into __ (for tearing), __ (for piercing), and the mulit-pointed __ and __ (for grinding)> In contrast, the teeth of other tetrapods usually consist of a row of undifferentiated, single-pointed teeth.
- hammer, stirrup, anvil
- the stirrup