272 Evolution? The Fossils Say YES!
In the case of the Ceratopsia, the transition is clear. All of the creatures with horns can
be traced back to the very well known Protoceratops, which has the frill over the neck and
the distinctive bones in the beak and lower jaw but lacks the horns (fig. 12.7). From the
very beginning, paleontologists pointed to Protoceratops as a nice transition between horned
ceratopsians and more primitive dinosaurs, but Gish apparently couldn’t figure this out. He
cites one out-of-context quotation from the first edition of Weishampel et al. (1990) about
the distinctiveness of the Protoceratopsidae (completely missing the point that this does not
make them any less a good transitional form), and he also mentions that they occur in the
Late Cretaceous, so they can’t be ancestral. First, paleontologists are not looking for ances-
tors but for sister groups (see chapter 5). Second, Protoceratops occurred in the early Late
Cretaceous, millions of years before all of its presumed descendants among the Ceratopsia
in the late Late Cretaceous.
And we have even better transitional forms. Bagaceratops (fig. 12.7) has a slightly smaller
frill and beak compared with Protoceratops, and its body is not fully quadrupedal. Archaeo-
ceratops has an even smaller frill and beak, and a body that is much lighter and most likely
bipedal. Psittacosaurus (the “parrot lizard”), which was so named because it had a parrot-like
beak (composed of the rostral bone unique to the Ceratopsia) and the beginnings of a frill
over the neck, is much more lightly built with a bipedal, gracile skeleton rather than the
heavier skeleton of Protoceratops. This creature not only shows the transition from frill-less
skull to one with a small frill to the larger frill of Protoceratops, but it also shows the transi-
tion from a light bipedal body (typical of nearly all the primitive dinosaurs) to the heavier
quadrupedal body of the more specialized groups.
Finally, the most amazing transitional fossil in this sequence was revealed with the
2004 discovery of Yinlong (fig. 12.8) from the much earlier beds of the Late Jurassic of
China (Xu et al. 2006). Its name means “hidden dragon” in Mandarin, in reference to the
FIGURE 12.6. Heterodontodosaurus, the most primitive known ornithischian dinosaur. Although it looked
superficially like Eoraptor and Coelophysis in its small size and bipedal stance, it had the unique predentary bone,
backward rotated pubic bones, and other hallmarks of the ornithischians. (Photo courtesy R. Rothman)