Geoscience Reference
In-Depth Information
holes in
Triceratops
skulls, leaving us with intriguing scenarios of ceratopsian con-
flict.
Pachycephalosaurs,like
Triceratops
,arealsogreatcandidatesforhavingmade
trace fossils caused by their own species and having those trace fossils left in their
skulls. Pachycephalosaurs, such as
Pachycephalosaurus
,
Stegoceras
, and others,
are relatively rare dinosaurs and only found in Cretaceous Period rocks. They are
distantrelativesofceratopsians,havingsharedacommonancestorbeforetheCreta-
ceous. Although their limbs are poorly known, they were bipedal, and their teeth
look perfectly adapted for a plant-eating lifestyle. So far, no one has interpreted
pachycephalosaur tracks, which is completely forgivable as their feet are unknown.
Fortunately, pachycephalosaurs are well represented by their opposite ends, which
are the tops of their heads. These skullcaps are incredibly thick, and in
Pachyceph-
alosaurus
can be nearly 25 cm (10 in) thick. They are composed of parietals and
closely associated skull bones, which are sometimes accompanied by crowns of
spikes and horns. In many instances, such bones are the only evidence of pachy-
cephalosaurs in a given time and place.
Paleontologists have long wondered why these dinosaurs were such bone-
heads. Growing bone is energetically expensive and must have meant this trait
hadsomeadaptiveadvantageduringtheevolutionaryhistoryofpachycephalosaurs.
How could this have helped them, whether to survive, to have sex, or survive to
have sex?
Nearly everybody agrees that these robust skulls must have been used for but-
ting,asin,theseweretheMesozoicequivalentofcrashhelmetsusedtoprotecttheir
heads as they smashed them against something. What reasonable people disagree
on, though, is what kind of butting? Did pachycephalosaurs use their heads for de-
fenseagainstpredators,suchasrunningfull-tiltintotheflankofatheropod,leaving
it to limp away and make small mental notes not to attack pachycephalosaurs? Did
they use them to drive off other herbivores from their favorite plants? Or did they
