Monday, January 07, 2008
Although new species are still being discovered at a rapid rate, the present count stands at 4,629 mammal species. Together, mammals represent one of the five major groups of vertebrates—animals that possess vertebral columns—a group that also includes fish, amphibians, reptiles, and birds.
Several of the features that distinguish mammals from other vertebrates only become evident through studying the mammal's basic skeletal structure. For example, unlike other vertebrates, mammals have three middle-ear bones, or ossicles.
Another skeletal difference can be seen in the mammalian dentary, the tooth-bearing part of the lower jaw, which changed considerably as mammals evolved. In keeping with a generalized evolutionary trend toward a reduction in the number of bones in the skull, most of the lower jaw bones became smaller and eventually disappeared. Some, however, became the ossicles of the middle ear, and one, the dentary, grew larger. From taking up only about half of the jaw, the dentary of today's mammals has expanded until it forms a new jaw joint with the skull.
The teeth of mammals have also undergone significant changes in the course of evolution. Some 290 million years ago, the precursors of modern mammals had teeth that were all the same size and shape. Gradually, as hunting and eating habits diversified, teeth became more specialized, designed to process food before digestion can occur. The mammalian stomach is usually made of a single saclike organ, although among some mammals, such as ruminant artiodactyls and cetaceans, it is subdivided into chambers.
Several of the features that distinguish mammals from other vertebrates only become evident through studying the mammal's basic skeletal structure. For example, unlike other vertebrates, mammals have three middle-ear bones, or ossicles.
Another skeletal difference can be seen in the mammalian dentary, the tooth-bearing part of the lower jaw, which changed considerably as mammals evolved. In keeping with a generalized evolutionary trend toward a reduction in the number of bones in the skull, most of the lower jaw bones became smaller and eventually disappeared. Some, however, became the ossicles of the middle ear, and one, the dentary, grew larger. From taking up only about half of the jaw, the dentary of today's mammals has expanded until it forms a new jaw joint with the skull.
The teeth of mammals have also undergone significant changes in the course of evolution. Some 290 million years ago, the precursors of modern mammals had teeth that were all the same size and shape. Gradually, as hunting and eating habits diversified, teeth became more specialized, designed to process food before digestion can occur. The mammalian stomach is usually made of a single saclike organ, although among some mammals, such as ruminant artiodactyls and cetaceans, it is subdivided into chambers.
# posted by saffron @ 7:00 AM 
