Geology Reference
In-Depth Information
thesis in hopes of one day having a greater understanding of how the various systems of planet earth
interact.
Mammals are descendants of
cynodont therapsids,
a group of Mesozoic reptiles that have
mammalian features, such as fur and specialized teeth. The transition from cynodonts to
true mammals is illustrated in fossils showing subtle changes in the bone structure of
the inner ear, jaw, and teeth. These creatures occupied some habitats during the Meso-
zoic but began to flourish only after the mass extinction of dinosaurs and other reptiles
at the end of the Cretaceous period (about 65 million years ago).
Three different types of mammals exist:
Monotreme mammals:
Those that lay eggs instead of giving birth to live young.
Modern examples include the duck-billed platypus and spiny anteater that live in
Australia.
Marsupials:
Mammals whose young are carried in a pouch until fully developed.
Modern examples of marsupials include kangaroos and opossums.
Placental mammals:
By far the most successful mammal group. Most
modern mammals are placental mammals, which evolved during the early Cenozo-
ic. Placental mammals have an organ called the
placenta
that provides nourish-
ment in the womb so that the live offspring are fairly developed by the time they
are born.
Regulating body temperature
A huge advantage that mammals have over reptiles and other cold-blooded or
exothermic
animals is the ability to control and regulate their own body temperat-
ure. This ability allows them to live in environments with fluctuating or extreme
temperatures because their bodies will burn or conserve energy as needed to
keep a steady temperature.
