Geoscience Reference
In-Depth Information
branches near the base of this limb on the dinosaur family tree. For the
most part, these branches contain relatively primitive, meat-eating
dinosaurs. Some of these are among the oldest dinosaurs that we know
about, such as
Herrerasaurus
and
Eoraptor,
which were collected
from rocks in northwest Argentina that are about 230 million years old.
Another branch near the base of the limb represents the abelisaurs,
including the Argentine
Carnotaurus.
This fearsome predator lived
later in the Mesozoic era, about 75-80 million years ago, although the
lineage leading to this imposing branch of dinosaurs must extend back
millions of years before this. As mentioned earlier,
Carnotaurus
was
unusual in that it had short arms and sported two prominent bony
horns above its eyes. We will delve into more details regarding
abelisaurs later in our story.
The other larger branch of theropods contains a group called
tetanurans. These dinosaurs evolved from a common ancestor that
possessed collarbones that were fused together like the wishbone of
birds, as well as a hand that had no more than three fingers. Branches
near the base of the tetanuran limb of the tree include the giant
meat-eaters
Allosaurus
and
Giganotosaurus. Giganotosaurus,
one of
the largest of all the known carnivorous dinosaurs, had been found
in the same province of Argentina where our expedition conducted
its exploration, and our colleague Rodolfo had been instrumental in
collecting and describing the massive skeleton.
Another branch within tetanurans contains smaller meat-eaters,
such as
Velociraptor
and
Compsognathus
of
Jurassic Park
fame, as well
as birds and the infamous
'Yyrannosaurus.
This group contains all
dinosaurs called coelurosaurs, who evolved from a common ancestor
that had an extra hole in the snout for reducing the weight of its skull
and an elongated ilium, the main bone that forms the top part of the
hip. The first coelurosaur also had elongated arms, although this
feature was later modified in many of its descendants, including
Tyrannosaurus
and the kiwi. For those descendants that retained
long arms, these appendages may well have been an advantage in cap-
turing prey.
Spectacular new discoveries in China have shown that primitive
coelurosaurs were covered with downy structures that are interpreted
to be the evolutionary precursors of the vaned feathers found in
birds. These discoveries suggest that most, if not all, coelurosaurs
