Geoscience Reference
In-Depth Information
were feathered; even the colossal
Tyrannosaurus
must have had a
feathered body at some early stage of its life. However, scientists do
not believe that adult tyrannosaurs were feathered because the com-
bination of this insulating covering and their large size might have
posed a disadvantage in regulating the animal's body temperature.
One branch within coelurosaurs leads to the "bird-mimic"
dinosaurs called ornithomimids. Ornithomimids have a skeleton
that looks superficially like that of an ostrich, which explains the
derivation of their name. That these dinosaurs had long legs with
shortened toes, which would have reduced the amount of friction
with the ground, suggests that ornithomimids were swift runners.
Most ornithomimids lack teeth in their jaws, which has led to some
questions about what these animals ate. Recent discoveries of stones
called gastroliths inside the stomach area of some ornithomimid
skeletons suggest that these dinosaurs may have eaten plants because
similar stones are commonly found in the gizzard of herbivorous
birds and other plant-eating dinosaurs. Yet gastroliths are also known
from other theropods that clearly had a carnivorous diet.
The other branch within coelurosaurs leads to birds and small,
meat-eating dinosaurs, such as
Velociraptor.
These evolved from a
common ancestor that possessed a crescent-shaped bone in their
wrist called the semilunate, because it is shaped like a crescent moon,
and hips in which the pubic bone pointed toward the rear of the ani-
mal. The shape of the semilunate bone allows the wings to be folded
back against the body in the distinctive position that we see in birds
today. The presence of this bone in
Velociraptor
and its relatives
indicates that these dinosaurs were also able to fold their arms back
against their body in the same way that birds do. These dinosaurs form
the group called maniraptors, named for their extremely enlarged and
elongated hands, which were certainly formidable predatory weapons.
As odd as it may seem, birds are maniraptors and, therefore,
dinosaurs. They belong on the same branch of the dinosaur family tree
that contains
Velociraptor.
Earlier, when we introduced the funda-
mentals of cladistic methodology and the principle of parsimony,
we downplayed the role of physical differences and emphasized the
role of similarities in reconstructing the evolutionary relationships of
organisms. The same concepts must be applied to reconstruct the evo-
lutionary relationships between birds and other dinosaurs.
