Description
Ceratopsians are easily recognized
by features of the skull. On the tip of a
ceratopsian upper jaw is the
rostral bone, a unique
bone found nowhere else in the animal kingdom. Along
with the
predentary bone, which
forms the tip of the lower jaw in all ornithischians, the
rostral forms a superficially parrot-like beak. Also,
the
jugal bones below the
eye are very tall and flare out sideways, making the skull appear somewhat
triangular when viewed from above. This triangular appearance is
accentuated, in later ceratopsians, by the rearwards extension of the
parietal and
squamosal bones of the
skull roof, to form the neck frill.
History
of study
The first ceratopsian remains known
to science were discovered by
Fielding Bradford Meek
during the U.S. Geological and Geographical Survey of the Territories led by
the American
geologist
F.V. Hayden. In 1872,
Meek found several giant bones protruding from a hillside in southwestern
Wyoming. He alerted
paleontologist
Edward Drinker Cope,
who led a dig to recover the partial skeleton. Cope recognized the remains
as a dinosaur, but noted that even though the fossil lacked a skull, it was
different from any type of dinosaur then known. He named the new species
Agathaumas sylvestris,
meaning "marvellous forest-dweller."
Classification
Ceratopsia was coined by
Othniel Charles Marsh
in 1890 to include dinosaurs possessing certain characteristic features,
including horns, a
rostral bone, teeth
with two roots, fused neck
vertebrae, and a
forward-oriented
pubis. Marsh considered
the group distinct enough to warrant its own suborder within Ornithischia.
As early as the 1960s, it was noted that the name
Ceratopsia is actually
incorrect linguistically and that
it should be
Ceratopia.
However, this spelling, while technically correct, has been used only rarely
in the scientific literature, and the vast majority of paleontologists
continue to use Ceratopsia. As the
ICZN does not govern
taxa above the level of
superfamily, this is
unlikely to change.
Taxonomy
Following Marsh, Ceratopsia has
usually been classified as a suborder within the order Ornithischia. While
ranked taxonomy has largely fallen out of favor among dinosaur
paleontologists, some researchers have continued to employ such a
classification, though sources have differed on what its rank should be.
Most who still employ the use of ranks have retained its traditional ranking
of suborder,
though some have reduced to the level of infraorder.
Possible ceratopsians from the
Southern Hemisphere include the Australian
Serendipaceratops,
known from an
ulna, and
Notoceratops from
Argentina is known from
a single toothless jaw (which has been lost).
Paleobiology
Biogeography
Ceratopsia appears to have
originated in Asia, as all of the earliest members are found there.
Fragmentary remains, including teeth, which appear to be neoceratopsian, are
found in North America from the Albian stage (112 to
100 million years ago), indicating that the group had dispersed across what
is now the Bering Strait by the
middle of the Cretaceous Period. Almost all leptoceratopsids are North
American, aside from
Udanoceratops,
which may represent a separate dispersal event, back into Asia. Ceratopsids
and their immediate ancestors, such as
Zuniceratops, were unknown outside of western
North America, and were presumed
endemic to that
continent. The traditional view that ceratopsoids originated in North
America was called into question by the 2009 discovery of better specimens
of the dubious Asian form
Turanoceratops, which
confirmed it as a ceratopsid. It is unknown whether this indicates
ceratopsids actually originated in Asia, or if the
Turanoceratops immigrated
from North America.
Individual
variation
Unlike almost all other dinosaur
groups, skulls are the most commonly preserved elements of ceratopsian
skeletons and many species are known only from skulls. There is a great deal
of variation between and even within ceratopsian species. Complete growth
series from embryo to adult are
known for
Psittacosaurus
and Protoceratops,
allowing the study of
ontogenetic variation
in these species. Significant
sexual dimorphism has
been noted in Protoceratops
and several ceratopsids.
Ecological role
Psittacosaurus
and Protoceratops
are the most common dinosaurs in the different Mongolian sediments where they
are found.
Triceratops
fossils are far and
away the most common dinosaur remains found in the latest Cretaceous rocks
in the western United States, making up as much as 5/6ths of the large
dinosaur fauna in some areas.
These facts indicate that some ceratopsians were the dominant herbivores in
their environments.
Some species of ceratopsians,
especially Centrosaurus
and its relatives, appear to have been gregarious, living in herds. This is
suggested by bonebed finds with the
remains of many individuals of different ages. Like modern migratory herds,
they would have had a significant effect on their environment, as well as
serving as a major food source for predators.
Although ceratopsians are generally
considered herbivorous, a few
paleontologists have speculated
online that at least some ceratopsians may have been opportunistically
omnivorous.
Posture
and locomotion
Most restorations of ceratopsians
show them with erect hindlimbs but semi-sprawling forelimbs, which suggest
they were not fast movers. But recently some paleontologists have argued that at
least the later ceratopsians had upright forelimbs and the larger species
may have been as fast as
rhinos, which can run
at up to 56 km or 35 miles per hour.
Daily
activity patterns
A nocturnal lifestyle has
been suggested for the primitive ceratopsian
Protoceratops.
However, comparisons between the
scleral rings of
Protoceratops and
Psittacosaurus and
modern birds and reptiles indicate that they may have been
cathemeral, active
throughout the day at short intervals.
Paleopathology
Activity-related bone fractures have been documented in
ceratopsians.
Return to the
Old Earth Ministries Online Dinosaur
Curriculum homepage.
