Dinosaurs Evolve—and Fly 271the consensus was that therizinosaurs were indeed theropods that had somehow returned
to plant eating. The discovery of Falcarius provided the “missing link” in this dietary tran-
sition because it retains many “raptor” dinosaur features yet is the most primitive therizi-
nosaur with a toothless herbivorous beak. Thus it was a classic transitional form not only
in linking therizinosaurs anatomically to raptors but also in showing how they made the
remarkable transition from carnivore back to herbivore.
The other main branch of dinosaurs is the Ornithischia, which includes nearly all the
herbivorous dinosaurs (except sauropods): the duckbills, the iguanodonts, the turtle-like
armored ankylosaurs, the spiky stegosaurs, the bone-headed pachycephalosaurs, and
the frilled and horned ceratopsians. The earliest ornithischians include primitive Triassic
forms such as Lesothosaurus, Fabrosaurus, and Heterodontosaurus, which are small bipedal
dinosaurs that look superficially like Eoraptor or Coelophysis (fig. 12.6). But on closer inspec-
tion, they show all the hallmarks of the ornithischians: part or all of the pubic bone in the
hip is rotated back parallel to the ischium; the cheek teeth are deeply inset inside the jaw,
suggesting that they had cheeks to confine their food in their mouths while they chewed;
and they have a unique extra bone at the tip of the lower jaw known as the predentary
bone. All these features are unique to the Ornithischia, yet we can see the transitional forms
like Heterodontosaurus already had them in the Triassic while they still resembled the other
primitive dinosaurs of the time.
The other example that Gish always trotted out is Triceratops, one of the last of the
horned dinosaurs or ceratopsians. Even in his 1995 book (pp. 119–122) he talked about it
and a few other ceratopsians, claiming there are no transitional forms—and completely
missing the point of everything he has read! Ceratopsians provide yet another classic case
of transitional forms between highly specialized forms, such as the horned and frilled cera-
topsians, and much more primitive forms that resemble the common ancestor with other
dinosaur lineages.
FIGURE 12.5. Transitional fossils among the dinosaurs. This is the therizinosaur Falcarius, which shows the
transition between carnivory and herbivory in this peculiar theropod group, shown next to Dr. Jim Kirkland,
who discovered and described it. (Photo courtesy J. Kirkland)