Encyclopedia of Evolution.pdf - Online Reading Center

mammals, evolution of The Class Mammalia consists of
vertebrates that have hair and produce milk. Linnaeus (see
Linnaeus, Carolus) classified mammals into one group, and
even invented the name, based on the Latin mammae, referring
to breasts. Mammals also have four-chambered hearts
and teeth clearly differentiated into incisors, canines, and
molars. There are or have been in the past about 5,000 genera
of mammals. The surviving 1,135 genera contain about 4,700
species. This is far less than the three-quarter million species
of beetles.
Mammals evolved from synapsids, one of the lineages
of reptiles. One synapsid reptile of the late Paleozoic era
was Dimetrodon, which had a huge sail of bony extensions
along its back. Although Dimetrodon itself is unlikely to
have been the ancestor of mammals, it may have shared a
characteristic that may have been widespread among synapsids:
warm blood. Modern mammals generate body heat
internally (are endothermic) and usually maintain a constant
body temperature (are homeothermic). Dimetrodon
may have used its sail to warm its blood in the morning (by
absorbing sunshine) and cool it off in midday (by radiating
heat through the skin surface). This may have helped
Dimetrodon to be, while not homeothermic, at least partly
endothermic.
Synapsids included the pelycosaurs and the therapsids
(from the Greek for “nurse”). Compared to earlier reptiles,
therapsids had fewer skull bones, teeth that were more differentiated,
and multiple sets of replacement teeth rather
than just juvenile “milk teeth” replaced by adult teeth. One
group of therapsids was the cynodonts (from the Greek for
“dog teeth”), with teeth even more differentiated (doglike)
than those of earlier therapsids. Small pits in the facial bones
of cynodonts may have been openings for extensive blood
vessels, and this has led some scientists to conclude that cynodonts
were fully warm-blooded. Cynodonts such as Morganucodon
and Sinocodon, which lived during the Jurassic
period, represent nearly perfect “missing links” (actually not
missing) between earlier synapsid reptiles and modern mammals.
Morganucodon is considered one of the earliest mammals,
even though it is intermediate between synapsid reptiles
and modern mammals. The very fact that so many “missing
links” have been discovered means that the line between reptiles
and mammals is somewhat arbitrary. Like modern mammals,
Morganucodon had juvenile milk teeth replaced by
adult teeth; Sinocodon retained the ancestral reptilian pattern
of multiple replacement sets of teeth.
The intermediate status of the cynodonts is illustrated in
three ways:
• Lower jaw. In early synapsid reptiles, as in modern reptiles,
the lower jaw consists of several bones, one of which
is the dentary bone; but in mammals the lower jaw consists
entirely of the dentary bone. In cynodonts, the dentary
bone was intermediate in size (see figure at right).
• Ear bones. In early synapsid reptiles, as in modern reptiles,
the inner ear has a single bone, the stapes, which conducts
vibrations to the cochlea. The mammalian inner ear consists
of three bones: the incus, which corresponds to the
reptilian quadrate bone; the malleus, which corresponds to
mammals, evolution of
Intermediate forms in the evolution of mammals have been found. The
mammal-like reptile Dimetrodon had cheek teeth but no molars; its jaw
contained large dentary and angular bones, and the jaw joint was between
the articular bone of the jaw and the quadrate bone of the skull. It had only
one bone, the stapes, inside the ear. In the early mammal Morganucodon,
the cheek teeth had become premolars and molars; its jaw consisted
mostly of the dentary bone, the angular bone being small; the dentary bone
connected directly with the skull. The quadrate and articular bones had
become the incus and malleus bones inside the ear of Morganucodon.
Thrinaxodon was intermediate in all of these respects between Dimetrodon
and Morganucodon. The triangle indicates the location of the jaw joint.
(Adapted from Benton)