Geoscience Reference
In-Depth Information
roamed the environment. However, the armor may also have played a
role in helping these dinosaurs recognize members of their own
species and potential mates. Because we cannot observe the behavior
of these animals in the wild, again, we simply cannot be certain.
The duckbills, horned dinosaurs, and dome-headed dinosaurs
comprise a group called Cerapoda. Unlike the thyreophorans, most of
these dinosaurs retained the hind-legged gait of their early ancestors.
Only within the horned dinosaurs did a four-legged gait evolve, illus-
trating a common theme that often recurred in the evolution of
large, heavy groups of dinosaurs. Cerapods evolved from a common
ancestor that had an uneven covering of enamel on the inside and
outside surfaces of their teeth, which, like other reptiles, they
replaced continually throughout their life. In many cerapods, a com-
plex mosaic of teeth grew one on top of the other in the jaws. Pre-
sumably, the uneven covering of enamel helped to maintain a rough,
rasplike surface on top of the batteries of teeth to shred and crush the
vegetation that they ate.
Duckbills, which are also called hadrosaurs, represent an extremely
diverse and abundant lineage on the family tree of cerapods. They
arose from a common ancestor in which the hingelike joint between
the upper and lower jaws lay below the level of the tooth rows. This
arrangement resulted in an extremely powerful crushing action as the
chewing musculature clamped the jaws together. The lower posi-
tion of the jaw joint, along with the rasplike chewing surface on top
of the tooth rows, provided duckbills with an ability to grind up
tough vegetation that was unmatched in other groups of dinosaurs.
Undoubtedly, this specialized feeding adaptation played an important
role in the evolutionary success of duckbills. In North America, these
dinosaurs are among the most commonly found. Their fossil remains
are sometimes preserved in large accumulations known as bone beds,
which presumably represent death assemblages caused by the cata-
strophic demise of huge herds that contained individuals of all ages.
Large herds of duckbills probably roamed the floodplains along major
rivers that ran across the continent toward the end of the Mesozoic.
In spite of their distinctive jaw apparatus, however, duckbills are
most commonly recognized by the elaborate bony crests that adorned
the top of the skull in many species. The function of these crests has
intrigued paleontologists for more than a century. Some have specu-
