Geology Reference
In-Depth Information
EVOLUTION OF THE
REPTILES—THE LAND
IS CONQUERED
Amphibians were limited in colonizing
the land because they had to return to wa-
ter to lay their gelatinous eggs. The evolu-
tion of the amniote egg (
Figure 21.27)
freed reptiles from this constraint. In such
an egg, the developing embryo is sur-
rounded by a liquid-fi lled sac called the
amnion and provided with both a yolk, or
food sac, and an allantois, or waste sac. In
this way, the emerging reptile is in essence
a miniature adult, bypassing the need for
a larval stage in the water. The evolution
of the amniote egg allowed vertebrates to
colonize all parts of the land because they
no longer had to return to the water as
part of their reproductive cycle.
Many of the differences between
amphibians and reptiles are physi-
ologic and are not preserved in the
fossil record. Nevertheless, amphib-
ians and reptiles differ sufficiently in
skull structure, jawbones, ear loca-
tion, and limb and vertebral construc-
tion to suggest that reptiles evolved
from labyrinthodont ancestors by the
Late Mississippian. This assessment
is based on the discovery of a well-
preserved fossil skeleton of the oldest
known reptile, Westlothiana , and other
fossil reptile skeletons from Late Mississippian-age rocks
in Scotland.
Other early reptile fossils occur in the Lower Pennsylvania
Joggins Formation in Nova Scotia, Canada. Here, remains of
Hylonomus are found in the sediments fi lling in tree trunks.
These earliest reptiles from Scotland and Canada were small
and agile and fed largely on grubs and insects. They are
loosely grouped together as protorothyrids , whose mem-
bers include the earliest reptiles (
Figure 21.24 Late Devonian Landscape in Eastern Greenland Shown is Ichthyostega ,
an amphibian that grew to a length of about 1 m. The fl ora of the time was diverse,
consisting of a variety of small and large seedless vascular plants.
As previously mentioned, the oldest known amphibian,
Ichthyostega , had skeletal features that allowed it to spend
its life on land. Because amphibians did not evolve until the
Late Devonian, they were a minor element of the Devonian
terrestrial ecosystem. Like other groups that moved into new
and previously unoccupied niches, amphibians underwent
rapid adaptive radiation and became abundant during the
Carboniferous and Early Permian.
The Late Paleozoic amphibians did not at all resemble
the familiar frogs, toads, newts, and salamanders that
make up the modern amphibian fauna. Rather, they dis-
played a broad spectrum of sizes, shapes, and modes of
life (
Figure 21.28). During
the Permian Period, reptiles diversifi ed and began displac-
ing many amphibians. The reptiles succeeded partly because
of their advanced method of reproduction and their more
advanced jaws and teeth, as well as their ability to move rap-
idly on land.
The pelycosaurs , or fi nback reptiles, evolved from the
protorothyrids during the Pennsylvanian and were the dom-
inant reptile group by the Early Permian. They evolved into
a diverse assemblage of herbivores, exemplifi ed by Edapho-
saurus , and carnivores such as Dimetrodon (
Figure 21.26). One group of amphibians were the
labyrinthodonts , so named for the labyrinthine wrinkling
and folding of the chewing surface of their teeth (Figure
21.23). Most labyrinthodonts were large animals, as much
as 2 m in length. These typically sluggish creatures lived in
swamps and streams, eating fish, vegetation, insects, and
other small amphibians (Figure 21.26).
Labyrinthodonts were abundant during the Car-
boniferous, when swampy conditions were widespread
(see Chapter 20), but they soon declined in abundance dur-
ing the Permian, perhaps in response to changing climatic
conditions. Only a few species survived into the Triassic.
Figure 21.29).
An interesting feature of the pelycosaurs is their sail. It was
formed by vertebral spines that, in life, were covered with
skin. The sail has been variously explained as a type of sexual
display, a means of protection, and a display to look more
ferocious. The current consensus seems to be that the sail
 
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