the extreme environmental swings over the last 3 million years, when glaciers
spanned much of the Northern Hemisphere.
The number of mammalian genera rose to a high of 130 about 55 mil-
lion years ago.Thereafter, the number of genera waxed and waned. It dropped
to 60 and rose back up to 120 presumably in response to climate change and
migration.These fluctuations lasted millions of years, yet diversity always con-
verged on an equilibrium of about 90 genera. Greater speciation resulting in
stiffer competition caused higher extinction rates, which maintained mam-
malian genera at a constant number.
Mammals are warm-blooded, which gives them a tremendous advantage.
A stable body temperature finely tuned to operate within a narrow thermal
range provides a high rate of metabolism independent of the outside tempera-
ture. Therefore, the work output of heart, lung, and leg muscles increases enor-
mously, allowing mammals to out perform and out endure reptiles. MammalsFigure 175
Paleogeography of the
upper Tertiary in North
America when inland seas
withdrew from the
continents.
Historical Geology
NORTH
AMERICAPacific
Ocean Atlantic
OceanAppalachian MountainsRockyMountainsWesternCordelia