Geology Reference
In-Depth Information
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to live young. Marsupial mammals are born in an imma-
ture, almost embryonic condition, and then undergo further
development in their mother's pouch. In placental mam-
mals, on the other hand, the amnion of the amniote egg (see
Figure 21.27) has fused with the walls of the uterus, forming
a
placenta.
Nutrients and oxygen carried from the mother
to the embryo through the placenta permit the young to
develop much more fully before birth.
A measure of the success of placental mammals is that
more than 90% of all mammals, fossil and living, are placental.
In contrast, judging from the fossil record, monotremes have
never been very common, and the only living ones are platy-
puses and spiny anteaters of the Australian region. Marsupi-
als have been more successful, at least in terms of number of
species and geographic distribution, but even they have been
largely restricted to South America and the Australian region.
Although mammals fi rst appeared during the Triassic,
major diversifi cation began during the Paleocene and con-
tinued throughout the Cenozoic. Several groups of Paleo-
cene mammals are considered
archaic,
meaning that they
were holdovers from the Mesozoic Era or they did not give
rise to any of today's mammals (
What Would You Do
You are a science teacher who through remarkably good
fortune receives numerous unlabeled mammal and plant fos-
sils from a generous benefactor. All of the fossils come from
Oligocene- and Miocene-age rocks. You're not too concerned
with identifying genera and species, but you do want to show
your students various mammal adaptations for diet and
speed. What features of the skulls, teeth, and bones would
allow you to infer which animals were herbivores (grazers ver-
sus browsers) and carnivores, and which ones were speedy
runners? Also, could you use the fossil leaves to make any
inference about ancient climates?
Several varieties of large flightless birds existed dur-
ing the Pleistocene (discussed later) and today, but some
remarkable predatory birds lived during the Paleogene
and Neogene. For instance,
Diatryma,
a heavily built,
flightless bird that stood about 2 m high, lived during the
Paleocene and Eocene in North America and Europe
(
Figure 23.19). About 25 species of fl ightless birds up to
3 m tall are known from South America, where they were the
dominant predators until they were replaced by big cats and
dogs that migrated from North America.
◗
Figure 23.20). Also among
these mammals were the first rodents, rabbits, primates,
carnivores, and hoofed mammals. However, even these had
not yet become clearly differentiated from their ancestors,
and the differences between herbivores and carnivores were
slight. Most were small; large mammals were not present un-
til the Late Paleocene, and the fi rst giant terrestrial mammals
did not appear until the Eocene (
◗
Among living mammals,
monotremes
, such as the platy-
pus, lay eggs, whereas
marsupials
and
placentals
give birth
◗
Figure 23.21).
◗
Figure 23.19
Restoration of
Diatryma
Diatryma
was a fl ightless, predatory bird that stood more
than 2 m tall. It lived during the Paleocene and Eocene in North America and Europe.
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