Compiling a List of Possible Victims - Walking On Eggs - page 37

prints called trackways, which show that when dinosaurs walked, the
left and right footprints form a nearly straight line with one another.
With this arrangement of the hind limbs, the femur exerted force in
a much different direction where it fit into the acetabulum. The
force generated by the dinosaur's weight was directed toward the
upper margin of the hip socket rather than toward the center, with
the bony ridge at the top of the acetabulum helping to counteract
that force. And because no force was directed toward the center of
the socket, a hole is found there. Dinosaurs were, thus, the first rep-
tiles in which a fully erect stance evolved.
Two major groups within dinosaurs represent the two large limbs
on the family tree of dinosaurs. One is called Ornithischia, which
contains most of the groups of herbivorous dinosaurs, including
armored dinosaurs, duckbills, horned dinosaurs, and dome-headed
dinosaurs. The 230-million-year-old Pisanosaurus is the oldest known
ornithischian dinosaur, and like all other early dinosaurs, Pisanosaurus
walked on its two hind legs. This suggests that the common ancestor
of dinosaurs must also have been bipedal. Even at this early date in
dinosaur evolution, the plant-eating Pisanosaurus already exhibits the
physical characteristics that are typically found in the group.
Omithischians evolved from a common ancestor in which one of its
hipbones, called the pubis, points toward the rear of the animal. In
the common ancestor of all dinosaurs, the pubis pointed forward. It
is not immediately obvious what purpose this change in direction
served. Perhaps it provided extra support for the gut, which had to be
large to process the enormous amount of vegetation that was required
to nourish the herbivorous omithischians, but no one knows for
sure.
Armored dinosaurs represent one of the most spectacular branches
on the ornithischian limb of the dinosaur family tree. Noteworthy
members include the tanklike ankylosaurs and the spectacularly orna-
mented stegosaurs. Together, these two lineages make up the group
called thyreophorans, which evolved from a common ancestor with a
mosaic of bony armor that essentially covered the entire body. Even
the earliest thyreophorans walked on all four legs, probably to better
support the weight of their heavy, bulky body. The armor, which
often included enormous spikes and plates, almost certainly played a
role in protecting these dinosaurs from the predatory dinosaurs that