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fifteen-foot-long Plateosaurus, which possessed a grasping hand that
it inherited from the common ancestor of all saurischians. Its hind
limbs were much longer and stronger than its arms, and it is clear
from the proportions of these limbs that Plateosaurus, like the smaller
Saturnalia, walked only on its hind legs.
The main branch on the sauropodomorph limb of the dinosaur's
family tree contains all the enormous sauropods. In these animals, all
four limbs were strongly developed like the columns of a building,
indicating that they walked on four legs. The bones of their wrists and
ankles were greatly reduced in number, presumably an adaptation for
supporting their heavy weight. Fossilized trackways confirm their
quadrupedal mode of locomotion. The four-legged gait of sauropods
represents another example of the recurrent evolution of this type of
locomotion in large dinosaurs, probably to help support the weight of
their colossal bodies.
Within sauropods, there are several branches on the evolutionary
tree, although scientists are still debating how the different groups are
related to one another. These groups include diplodocids, dicraeo-
saurids, camarasaurids, brachiosaurids, and titanosaurs. Diplodocids
are exemplified by Diplodocus, the extremely long-necked, whip-
tailed sauropod of the late Jurassic in North America. Although one of
the longest dinosaurs ever discovered, it probably did not weigh as
much as some of its more robust sauropod relatives. Its teeth are elon-
gated, peglike or pencil-like structures that were probably used to strip
vegetation off branches.
The dicraeosaurids include Dicraeosaurus, a relatively small, short-
necked sauropod from the late Jurassic of Tanzania, and Amar-
gasaurus, a Patagonian form that lived in the early Cretaceous.
Dicraeosaurids evolved from a common ancestor with tall spines on
top of its vertebrae, especially in the hip region, and peglike teeth sim-
ilar to those of diplodocids. Most paleontologists agree that
dicraeosaurids are very closely related to diplodocids and believe that
their peglike teeth evolved in the common ancestor of these two
groups.
Brachiosaurids have longer front legs than hind legs, a posture
present in their common ancestor that made them some of the
tallest dinosaurs ever found. The immense Brachiosaurus was dis-
covered in the late Jurassic of Tanzania by a German expedition at
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