Encyclopedia of Evolution.pdf - Online Reading Center

would have become so great that the legs would not be able
to move (see allometry).
Sauropods were herbivorous dinosaurs, whose large size
and long necks allowed them to eat leaves even in the tallest
trees. Instead of having large teeth to grind the branches
and leaves, they had small sharp teeth that could tear leaves
and branches to be swallowed whole. Grinding occurred in
the stomach with the aid of gastroliths. If dinosaurs such as
Apatosaurus (formerly Brontosaurus) held their heads and
necks upright, they would have needed seven times the blood
pressure of a human, and the huge heart would have required
the energy equivalent to about half of the food intake of the
dinosaur. Some paleontologists say that huge, long-necked
dinosaurs did not raise their heads. Perhaps they lifted their
heads just long enough to eat. In contrast, the carnivorous
dinosaurs, even the famous Tyrannosaurus rex, were much
smaller (about five tons) because meat was much less available
than vegetation.
The huge sauropod dinosaurs were probably also stupid.
The huge, long-necked dinosaurs had small brains (and
small teeth) because the force of leverage that would have
been necessary to raise a large head at the end of a long neck
would have been too great. Large dinosaurs were also stupid
because a single, large brain in the head would have been too
far away from the tail to allow quick control over muscle
movement. The large dinosaurs had a brain, but also had a
secondary ganglion of nerves near the base of the tail, which
aided in control of posterior muscles.
Among the oldest known dinosaur fossils are Herrerasaurus
and Eoraptor, from the late Triassic period
dinosaurs
about 230 million years ago. Because these fossils already
had dinosaur specializations, the ancestral dinosaur must
have lived even earlier. These earliest dinosaurs were about
the size of a human and had narrow heads, long pointed
snouts, mouths that extended from ear to ear, many small
sharp teeth, and large eyes. Unlike nearly all other reptiles,
these early dinosaurs walked on their two hind legs, which
allowed their hands to evolve the ability to grasp prey. From
animals similar to Herrerasaurus, two lineages of reptiles
evolved: the bird-hipped dinosaurs (ornithischians) and the
lizard-hipped dinosaurs (saurischians), which evolved into
numerous forms during the ensuing Jurassic and Cretaceous
periods.
Ornithischians had a pelvis that reminded early paleontologists
of birds. Their teeth and jaws allowed the tearing
and grinding of plant materials, and the enlarged ribs and pelvis
allowed the digestion of large quantities of plant material.
Despite their name, the ornithischians were not the ancestors
of birds; they all met extinction at the end of the Cretaceous
period. As time went on, ornithischians diverged into different
lineages, all of which became much larger than their Herrerasaurus
ancestors. They included:
• Pachycephalosaurs or “thick-headed” dinosaurs had skulls
with thick domes that may have served as battering rams
during fights. Recent research has failed to find evidence of
tiny fractures that would have resulted from such conflicts.
• Ceratopsians, or horned dinosaurs, reverted to walking on
all fours. By the late Paleozoic, some were quite large and
had large horns, such as Triceratops.
Supersaurus was one of the largest land animals ever to live. At 00 tons, it was nearly as large as an animal could be and still be able to support its
weight on four legs. A single leg bone was larger than a human being.