Early Mammals 131distinct tooth types, the structure of which varies by spe-
cies. Knowledge of the way the teeth fit together indicates
the arrangement of muscles needed to operate the jaws.
Reconstruction of the jaw muscles, in turn, indicates
how the skull must have been shaped to provide a place
for these muscles to attach. The shape of the jaws and de-
tails of the teeth also suggest the type of food that these
animals consumed. Thus a mere jawbone fragment with
a few teeth contains a great deal of information about the
animal from which it came.
An interesting observation about the evolution of the
mammals is that the diverse forms with which we are fa-
miliar today, including the primates, are the products of
an adaptive radiation: the rapid increase in number of re-
lated species following a change in their environment. This
did not begin until after mammals had been present on
the earth for over 100 million years. With the mass extinc-
tion of many reptiles at the end of the Cretaceous, how-
ever, a number of existing ecological niches, or functional
positions in their habitats, became available to mammals.
A species’ niche incorporates factors such as diet, activity,
terrain, vegetation, predators, prey, and climate.
The story of mammalian evolution starts as early as 230
to 280 mya (Figure 6.4). From deposits of this period, which
geologists call the Permian, we have the remains of reptiles
with features pointing in a distinctly mammalian direction.
These mammal-like reptiles were slimmer than most other
reptiles and were flesh eaters. Graded fossils demonstrate
trends toward a mammalian pattern such as a reduction in
the number of bones, the shifting of limbs underneath the
body, the development of a separation between the mouth
and nasal cavity, differentiation of the teeth, and so forth.
Eventually these creatures became extinct, but not be-
fore some of them developed into true mammals by the
Triassic period. During the Jurassic period that followed,
dinosaurs and other large reptiles dominated the earth,
and mammals remained tiny, inconspicuous creatures
occupying a nocturnal niche.
By chance, mammals were preadapted—possessing
the biological equipment to take advantage of the new op-
portunities available to them through the mass extinction
of the dinosaurs and other reptiles 65 million years ago. As
homeotherms, mammals possess the ability to maintain a
constant body temperature, a trait that appears to have pro-
moted the adaptive radiation of the mammals. Mammals
can be active at a wide range of environmental tempera-
tures, whereas reptiles, as isotherms that take their body
temperature from the surrounding environment, become
progressively sluggish as the surrounding temperature
drops. Cold global temperatures 65 mya appear to be re-
sponsible for the mass extinction of dinosaurs and some
other reptiles, while mammals, as homeotherms, were
preadapted for this climate change.
Figure 6.4 This timeline highlights some major milestones
in the course of mammalian primate evolution that ultimately
led to humans and their ancestors. The Paleocene, Eocene,
Oligocene, and Miocene epochs are subsets of the Tertiary
period. The Quaternary period begins with the Pleistocene and
continues today.adaptive radiation Rapid diversification of an evolving
population as it adapts to a variety of available niches.
preadapted Possessing characteristics that, by chance, are
advantageous in future environmental conditions.
homeotherm An animal that maintains a relatively constant
body temperature despite environmental fluctuations.
isotherm An animal whose body temperature rises or
falls according to the temperature of the surrounding
environment.