THE MAMMALIAN ERA
The first mammals were tiny, shrewlike creatures that appeared in the late Trias-
sic about 220 million years ago, at roughly the same time as the dinosaurs. The
two groups coexisted for about 150 million years thereafter. Mammals evolved
from bulky, cold-blooded creatures that were themselves descendants of the rep-
tiles. Mammals descended from the mammal-like reptiles, which were later dri-
ven into extinction by the dinosaurs about 160 million years ago.The mammals
then began to branch into new forms, following the breakup of Pangaea. The
multituberculates were probably the most interesting group of mammals that ever
lived. They evolved about the same time as the dinosaurs and became extinct a
little more than 30 million years ago, long after the dinosaurs disappeared.
One of the oldest mammalian skeletons ever uncovered is that of a 140-
million-year-old symmetrodont. It appears to be a missing link between egg-
laying and therian (live-birth) mammals. A bizarre, rat-sized animal dating
back at least 120 million years walked on mammalian front legs and splayed
reptilian hind legs.The animal was a close relative to the common ancestor of
all mammals alive today. Around 100 million years ago, mammals began to
branch into novel forms as new environments became available after the
breakup of the supercontinent Pangaea.
The early mammals (Fig. 173) developed over a period of more than 100
million years into the first therian mammals, the ancestors of all living marsu-
pials and placentals. During this time, mammals progressed and became more
adaptive in a terrestrial environment.Mammalian teeth evolved from simple
cones that were replaced repeatedly to more complex forms replaced only
once at maturity. However, the mammalian jaw and other parts of the skull
still shared many similarities with reptiles. One mysterious group known as
the triconodonts, ranging from 150 to 80 million years ago, were primitive
protomammals. They were possible ancestors of the monotremes, represented
today by the Australian platypus and echidna, which lay eggs and walk with a
reptilian sprawling gait.
The ancient mammals were forced into a nocturnal lifestyle, requiring
the evolution of highly acute senses along with an enlarged brain to process
the information. The archaic mammals developed a fourfold increase in rel-
ative brain size compared with the reptiles.Thereafter, they achieved no sub-
stantial increase in brain size for at least 100 million years, indicating an
adaptation to a lengthy, stable ecological niche during the Mesozoic.
Following the extinction of the dinosaurs 65 million years ago, mammals
became the recipients of daytime niches along with a preponderance of new
sensory signals for the brain to organize as they competed with each other in
a challenging environment. Another fourfold increase in relative brain size
occurred about 50 million years ago in response to adaptive radiation of mam-
TERTIARY MAMMALS