Geoscience Reference
In-Depth Information
lated that the large air passages contained within the crests served as
resonating chambers to amplify vocal calls made by the duckbills,
while other paleontologists have suggested that the crests were used
to identify other members of the same species and potential mates.
Again, it is difficult to be certain since all the duckbills are extinct, and
perhaps the crests were involved in both these activities.
The horned dinosaurs and dome-headed dinosaurs form a group
within cerapods called the Marginocephalia. The name comes from
the shelf of bone rimming the back of the skull, which was inherited
from the common ancestor of the group. Evolution took this bony
shelf and expanded it in two different directions in the horned
dinosaurs and dome-headed dinosaurs. In horned dinosaurs, also
known as ceratopsians, the bony shelf at the back of the skull
expanded backward to form a shieldlike structure called a frill.
Although the frill is relatively small in more primitive members of
the group, such as the parrotlike
Psittacosaurus
and its distant rela-
tive
Protoceratops,
it became a large and elaborate structure in many
later and larger members of the group, such as
Triceratops
and
Styra-
cosaurus.
In dome-headed dinosaurs, also known as pachy-
cephalosaurs, the bony shelf along the back of the skull expanded
forward to form a thick, bony helmet on top of the skull. In some
later members of the group, such as
Pachycephalosaurus,
this dome
is about six inches thick. Furthermore, while horned dinosaurs
developed a four-legged gait in the more advanced members of the
group, such as
Triceratops,
the dome-headed dinosaurs retained the
two-legged stance and gait of the earliest dinosaurs.
The second major limb at the base of the family tree contains the
saurischians, who evolved from a common ancestor that had a hand
capable of grasping. The thumb was slightly offset from the rest of
the fingers. The bony structure of the saurischian's grasping hand dif-
fers slightly from the structure of our hand, but the basic result was
the same. Within saurischians, we find two large branches on the evo-
lutionary tree. One contains all the giant, long-necked, four-legged
herbivores called sauropods. The other contains all the two-legged,
carnivorous theropods, including their descendants, birds.
Theropods evolved from a common ancestor that had only three
fully developed toes on the hind feet, with the central toe being the
longest. The common ancestor of theropods gave rise to many
