270 Evolution? The Fossils Say YES!
To a casual observer, they look very similar, but to a paleontologist with anatomical training,
the differences are clear. Eoraptor, Staurikosaurus, and Herrerasaurus lack the highly special-
ized three-fingered hand (some still had the full five fingers), the relatively unspecialized
vertebrae (both sauropod and theropod vertebrae are very specialized and distinctive), a
sliding jaw joint, the fully recurved predatory teeth, and the distinctively theropod modifica-
tions of the ankle and foot found in Coelophysis and more advanced theropods. Finally, we
can trace creatures like Eoraptor, Staurikosaurus, and Herrerasaurus back to primitive nondi-
nosaurian archosaurs like Euparkeria (fig. 12.3A), which superficially resembles the earliest
dinosaurs but lacks the unique specializations that all dinosaurs have, such as the open hip
socket or the distinctive features in the skull.
Thus we can trace both sauropods (through the primitive prosauropods) and theropods
(through the primitive creatures like Coelophysis) to a common saurischian ancestor along
the lines of Eoraptor, Staurikosaurus, and Herrerasaurus, and from there to more primitive
archosaurs that were not dinosaurs, like Euparkeria. You could not ask for a nicer series of
transitional forms. Apparently, Gish never heard of any of these.
And there’s a cool final twist to this story of predatory dinosaurs. In 2005, my friend
Jim Kirkland and others announced the discovery of a remarkable new fossil known as
Falcarius utahensis from the Jurassic of Utah (fig. 12.5). This strange creature is a member
of an even stranger group known as therizinosaurs, whose exact position within the dino-
saurs has long been controversial. These beasts have many of the hallmarks of theropods
like Velociraptor, including long clawed fingers on the hands and a long neck and tail for
balancing. But they had toothless beaks and were apparently herbivorous. In recent years,
FIGURE 12.4. There are many transitional forms between the earliest dinosaurs like Eoraptor and the more
advanced groups. This is Coelophysis, one of the smallest and most primitive carnivorous theropod dinosaurs.
(Photo courtesy L. Taylor)