Geology Reference
In-Depth Information
living gymnosperms show that they are closely related to
angiosperms, but unfortunately, the early fossil record of
angiosperms is sparse, so their precise ancestry remains
obscure.
Since they fi rst evolved, angiosperms have adapted to nearly
every terrestrial habitat—from mountains to deserts—and some
have even adapted to shallow coastal waters. Several factors
account for their phenomenal success, but chief among them
is their method of reproduction (Figure 22.23a). Particularly
important was the evolution of fl owers, which attract animal
pollinators, especially insects, and the evolution of enclosed
seeds.
Seedless vascular plants and gymnosperms are impor-
tant and still fl ourish in many environments; in fact, many
botanists regard ferns and conifers as emerging groups.
Nevertheless, a measure of the angiosperms' success is that
today, with 250,000 to 300,000 species, they account for
more than 90% of all land-plant species, and they occupy
some habitats in which other land plants do poorly or can-
not exist.
a
10
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10
b
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c
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The first reptiles evolved during the Mississippian Pe-
riod, and from this basic stock of so-called
stem reptiles
,
all other reptiles, as well as birds and mammals, evolved
(
◗
Figure 22.22
Primary Producers
a
A Miocene coccolith from the Gulf of Mexico (left); a Pliocene-
Miocene coccolith from the Gulf of Mexico (right).
b
Upper Miocene
diatoms from Java (left and right).
c
Eocene dinofl agellates from
Alabama (left) and the Gulf of Mexico (right).
Figure 22.24). Reptiles continued to diversify during the
Mesozoic Era, and, in fact, they were the most varied and
abundant vertebrate animals of that time. Several skeletal
modifi cations differentiate reptiles from amphibians, al-
though the earliest reptiles did not look much different
from their ancestors (see Chapter 21). All living reptiles,
lizards, snakes, crocodiles, and turtles lay amniotic eggs,
are cold-blooded, have a tough, scaly skin, and practice
internal fertilization. Also, with the exception of living
and fossil turtles, living reptiles, as well as dinosaurs, have
two openings on the side of the skull in the temporal re-
gion. So dinosaurs have traditionally been classified as
reptiles, but they also possessed several characteristics that
set them apart.
Dinosaurs had teeth set in individual sockets, a reduced
lower leg bone (fibula), a pelvis anchored to the verte-
bral column by three or more vertebrae, a ball-like head
on their upper leg bone (femur), and elongate bones in the
palate. Another significant feature is that dinosaurs had a
fully upright posture with their limbs directly beneath their
bodies, rather than the sprawling stance of other reptiles
(
◗
In general terms, we can think of the Mesozoic as a time
of increasing complexity among the marine invertebrate
fauna. At the beginning of the Triassic, diversity was low
and food chains were short. Near the end of the Cretaceous,
though, the marine invertebrate fauna was highly complex,
with interrelated food chains. This evolutionary history re-
fl ects changing geologic conditions infl uenced by plate tec-
tonic activity, as discussed in Chapter 2.
Just as during the Late Paleozoic, seedless vascular plants and
gymnosperms dominated Triassic and Jurassic land-plant
communities, and, in fact, representatives of both groups
are still common. Among the gymnosperms, the large seed
ferns became extinct by the end of the Triassic, but,
ginkgos
remained abundant and still exist in isolated regions, and
conifers
continued to diversify and are now widespread in
some terrestrial habitats, particularly at high elevations and
high latitudes. A new group of gymnosperms known as
cy-
cads
made its appearance during the Triassic. These palm-
like plants became widespread and now exist in tropical and
semitropical areas.
The long dominance of seedless plants and gymno-
sperms ended during the Early Cretaceous, perhaps the
Late Jurassic, when many were replaced by
angiosperms
,
or fl owering plants (
Figure 22.25). In fact, their upright posture, coupled with
modifi cations in their limbs, accounts for more effi cient lo-
comotion than in other reptiles; this may have been one fac-
tor in their incredible success.
Contrary to popular belief, there were no fl ying dino-
saurs or fully aquatic ones, although there were Mesozoic
reptiles that occupied these niches. Nor were all dinosaurs
large, although some certainly were. Note that dinosaurs
lived only during the Mesozoic Era, unless we consider their
evolutionary descendants, the birds.
◗
◗
Figure 22.23). Studies of fossils and
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