bipedalism is an unusual locomotor mode for vertebrates, having
appeared elsewhere only in apes, some
rodents, and kangaroos. Thus dinosaurs show an unusual evolutionary
trend: many dinosaur species reverted to a quadrupedal posture.
This was especially true of larger herbivorous dinosaur lineages.
Early sauropodomorphs, ceratopsians, ornithopods, and thyreophorans
were all small and bipedal, but the later members of each group
were larger and partly or fully quadrupedal. In addition, many
different theropods develop the long, slender limbs that characterize "cursorial" animals,
but only a few herbivorous dinosaurs seem to have done so (including
hypsilophodontids, dryosaurids, and early ornithischians).
Theropods are perhaps the best-studied group of dinosaurs,
largely because their ranks include the earliest birds.
The transition from terrestrial, "reptilian" non-avian
dinosaurs to flying, feathered birds has occupied paleontologists
for decades. New fossils of feathered dinosaurs have helped
to clarify many of the steps in this complicated evolutionary
sequence, but much remains to be learned. It is now clear
that feathers must have evolved prior to the origin of
flight, and therefore were first adapted for some other
purpose. This may have been insulation, display, or some
as-yet-unknown function. Many other "avian" features
appear to have evolved relatively early in dinosaurs and
and were also probably not directly related to the origin
of birds or flight. For example, the specialized wrist
of birds, which is used in the complex wing-stroke during
flight, was present in many non-avian coelurosaurs. Components
of this wrist were present in theropods such as Allosaurus.
The furcula (or wishbone) appeared in theropods as primitive
as the Triassic Coelophysis, while many features
of bird legs are holdovers from the earliest dinosaurs
ancestors. Thus, although the origin of birds from within
theropods seems clear, the details of this transition are
still being uncovered.