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Psittacosaurus is not
as familiar to the general public as its distant relative
Triceratops but it is one
of the most completely known dinosaur genera. Fossils of over 400
individuals have been collected so far, including many complete skeletons.
Most different age classes are represented, from hatchling through to
adult, which has allowed several detailed studies of
Psittacosaurus growth
rates and reproductive
biology. The abundance
of this dinosaur in the fossil record has led
to its use as an
index fossil for Early
Cretaceous sediments of central Asia.
Description
Different species of
Psittacosaurus varied in
size and specific features of the
skull and skeleton, but
shared the same overall body shape. The best-known species,
P. mongoliensis, reached 2 meters
(6.5 ft) in length. Several species approached
P. mongoliensis
in size (P. major,
P. neimongoliensis,
P. xinjiangensis),
while others were somewhat smaller (P. sinensis,
P. meileyingensis).
P. ordosensis was
the smallest known species, 30% smaller than P.
mongoliensis. The largest were
P. lujiatunensis and
P. sibiricus,
although neither was significantly larger than
P. mongoliensis.
The skull of
Psittacosaurus was highly
modified compared to other
ornithischian dinosaurs
of its time. The skull was extremely tall in height and short in length,
with an almost round profile in some
species. The portion in front of the orbit (eye socket) was
only 40% of total skull length, shorter than any other known ornithischian.
The lower jaws of psittacosaurs are characterized by a bulbous vertical
ridge down the center of each tooth. Both upper and lower jaws sported a
pronounced beak, formed from the
rostral
and predentary
bones, respectively. The bony core of the beak may have been sheathed in
keratin to provide a
sharp cutting surface for cropping plant material. As the
generic name suggests, the short skull and beak superficially resembled
those of modern parrots.
Psittacosaurus
skulls shared several adaptations with more derived ceratopsians,
such as the unique rostral bone at the tip of the upper jaw, and the flared
jugal (cheek) bones.
However, there was still no sign of the bony neck frill or prominent facial
horns which would develop in later ceratopsians. Bony horns did protrude
from the skull of
P. sibiricus,
but these are thought to be an example of
convergent evolution.
Psittacosaurus
postcranial skeletons were more typical of a 'generic' bipedal
ornithischian. In P. mongoliensis,
similarly to other species, the forelimbs were only 58% as long as the
hindlimbs and their range of motion indicates that the hands could neither
be pronated nor used to generate propulsive force, suggesting that these
animals were totally bipedal in life. There were only four digits on the
manus ('hand'), as
opposed to the five found in most other ornithischians (including all other
ceratopsians). Overall, the four-toed hindfoot was very similar to many
other small ornithischians.
Species of Psittacosaurus
Psittacosaurus
was named in 1923 by
Henry Fairfield Osborn,
paleontologist and president
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Above: Restoration of P. mongoliensis (Source)
Below: Restoration of P. sibiricus (Source) |
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of the
American Museum of Natural History
(AMNH) in a paper published on October 19. Seventeen species have been referred
to the genus Psittacosaurus,
although only nine to eleven are considered valid today. This is the highest
number of valid species currently assigned to any single dinosaur genus (not
including birds). In contrast,
most other dinosaur genera are monospecific,
containing only a single known species. The difference is most likely due to
quirks of the fossil record. While
Psittacosaurus is known from hundreds of fossil
specimens, most other dinosaur species are known from far fewer, and many
are represented by only a single specimen. With a very high sample size, the
diversity of Psittacosaurus
can be analyzed more completely than that of most dinosaur genera, resulting
in the recognition of more species.
Classification
Psittacosaurus
is the type genus of the
family Psittacosauridae, which was also named by Osborn in 1923. Only one
other genus,
Hongshanosaurus, is
currently classified in this family alongside
Psittacosaurus. Psittacosaurids were
basal to almost all
known ceratopsians except
Yinlong and perhaps
Chaoyangsauridae. While
Psittacosauridae was an early branch of the ceratopsian family tree,
Psittacosaurus itself was
probably not directly ancestral to any other groups of ceratopsians. All
other ceratopsians retained the fifth digit of the hand, a
plesiomorphy or
primitive trait, whereas all species of
Psittacosaurus had only four digits on the hand.
In addition, the antorbital fenestra,
an opening in the skull between the eye socket and nostril, was lost
during the evolution of
Psittacosauridae, but is still found in most other ceratopsians and in fact
most other archosaurs. It is
considered highly unlikely that the fifth digit or antorbital fenestra would
evolve a second time.
Provenance
Psittacosaurus
is known from over 400 individual specimens, of which over 75 have been
assigned to the type species, P. mongoliensis.
All Psittacosaurus
fossils discovered so far have been found in Early Cretaceous
sediments in Asia, from southern Siberia to northern
China, or possibly as far south as Thailand. The most common age of geologic formations
bearing
Psittacosaurus
fossils is from the late
Barremian through
Albian stages of the
Early Cretaceous, or approximately 125 to 100 Ma (million
years ago). Nearly all terrestrial sedimentary formations
of this age in Mongolia and northern China have produced fossils of
Psittacosaurus, leading
its use as an index fossil for this time period in the region, along with
the very common
pterosaur
Dsungaripterus.
Paleobiology
Daily
activity patterns
Comparisons between the
scleral rings of
Psittacosaurus and modern
birds and reptiles suggest that it may have been
cathemeral, active
throughout the day at short intervals.
Psittacosaurs had self-sharpening
teeth that would have been useful for cropping and slicing tough plant
material. However, unlike later ceratopsians, they did not have teeth
suitable for grinding or chewing their food. Instead, they used gastroliths,
stones swallowed to wear down food as it passed through the digestive
system. Gastroliths, sometimes numbering more than fifty, are occasionally
found in the abdominal cavities of psittacosaurs, and may have been stored
in a
gizzard, as in modern
birds.
Unlike many other dinosaurs, psittacosaurs had akinetic
skulls: the upper and lower jaws behaved as single units without internal
joints. The only joint was the jaw joint itself, and psittacosaurs could
slide their lower jaws forward and backward on the joint, permitting a
shearing action. Unlike most ceratopsians, their beaks did not form curved
tips, but were instead rounded and flattened. If the jaws were aligned, the
beaks could be used to crop objects, but if the lower jaw was retracted so
that the lower beak was inside the upper beak, the jaws may have served a
nutcracking function. A nut- or seed-rich diet would also match well with
the gastroliths often seen in well-preserved psittacosaur skeletons.
Growth rate
Several juvenile
Psittacosaurus have been
found. The smallest is a P. mongoliensis
hatchling in the AMNH collection, which is only 11 to 13 centimeters (4–5
inches) long, with a skull 2.8 centimeters (1 in) in length. Another
hatchling skull at the AMNH is only 4.6 centimeters (1.8 inches) long. Both
specimens are from Mongolia. Juveniles discovered in the Yixian Formation
are approximately the same age as the larger AMNH specimen. Adult
Psittacosaurus mongoliensis
approached 2 meters (6.5 ft) in length.
A
histological
examination of P. mongoliensis
has determined the growth rate of these animals. The smallest specimens in
the study were estimated at three years old and less than 1 kilogram (2.2
lb), while the largest were nine years old and weighed almost 20 kilograms
(44 lb). This indicates relatively rapid growth compared to most reptiles
and
marsupial mammals, but
slower than modern birds and
placental mammals. An
age determination study
performed on the fossilized remains of
Psittacosaurus mongoliensis
by using growth ring counts suggest that the longevity of the
basal ceratopsian was
between 10 or 11 years.
Integument
The
integument, or body
covering, of Psittacosaurus
is known from a Chinese specimen, which most likely comes from the Yixian
Formation of Liaoning. The specimen, which is not yet assigned to any
particular species, was illegally exported from China, in violation of
Chinese law, but was purchased by a German
museum and arrangements
are being made to return the specimen to China.
Most of the body was covered in
scales. Larger scales
were arranged in irregular patterns, with numerous smaller scales occupying
the spaces between them, similarly to skin impressions known from other
ceratopsians, such as
Chasmosaurus. However,
a series of what appear to be hollow, tubular bristles, approximately 16
centimeters (6.4 in) long, were also preserved, arranged in a row down the
dorsal (upper) surface
of the tail. However, according to some scientists "[a]t present, there is
no convincing evidence which shows these structures to be
homologous to the
structurally different [feathers
and protofeathers] of theropod dinosaurs." As
the structures are only found in a single row on the tail, it is unlikely
that they were used for
thermoregulation, but
they may have been useful for
communication through
some sort of display.
Limb
function
Studies by Phil Senter in 2007
conducted on Psittacosaurus neimongoliensis
and Psittacosaurus mongoliensis
concluded that the forelimbs of these taxa (and likely those of other
Psittacosaurus species)
were too short to reach the ground and could neither be pronated nor
generate propulsive force for locomotion, suggesting that
Psittacosaurus was
entirely bipedal. The forelimbs were also too short to be used in digging or
bringing food to the mouth, and Senter suggested that if
Psittacosaurus needed to
dig depressions in the ground it may have used its hindlimbs instead. The
forelimbs could be used for two-handed grasping of objects or scratching the
body, but due to their extremely limited flexibility and reach, they could
have only been used to grasp objects very close to the belly or sides of the
animal and could have scratched only the belly, flank and knees. Even though
the hands couldn't reach the mouth,
Psittacosaurus could have still used them to
carry nesting material or food to a desired location.
Parental
care and gregariousness
An extremely well preserved specimen
found in the Yixian Formation of Liaoning Province,
China provides some of the best evidence for parental care in dinosaurs.
This specimen consists of an adult
Psittacosaurus (not assigned to any particular
species), which is closely associated with 34 articulated juvenile
skeletons, all preserved in three dimensions. The
young
Psittacosaurus,
all approximately the same age, are intertwined in a group underneath the
adult, although all 34 skulls are positioned above the mass of bodies, as
they would have been in life. This suggests that the animals were alive at
the time of burial, which must have been extremely rapid, perhaps due to the
collapse of a burrow.
The juvenile bones are very small
but are well-ossified.
This has been taken as evidence of extensive parental care, as the young
must have been in the nest long enough for their bones to become ossified. A
separate find of a herd of six Psittacosaurus
individuals killed by a volcanic mudflow indicates the presence of at least
two age groups from two distinct clutches. This find has been taken as
evidence for group fidelity and gregariousness extending beyond the nest;
the earliest such evidence for any ceratopsian. Even very young psittacosaur
teeth appear worn, indicating they chewed their own food and may have been
precocial, although
this does not rule out continued parental care.
Possible
aquatic behavior
Ford and Martin (2010) proposed that
Psittacosaurus was
semi-aquatic, swimming through the use of its tail similar to crocodiles,
and through paddling and kicking. They based their interpretation on
evidence including: the lacustrine (lake) depositional setting of many
specimens; the position of the nostrils and eyes; interpretations of the
motions of the arms and legs; tails with long
chevrons (and with the
bristles on the tail interpreted as possibly skin-covered, forming a fin),
providing a propulsive surface; and the presence of
gastroliths,
interpreted as ballast. They further suggested that some species of
Psittacosaurus were more
terrestrial than others.
Predation
Another fossil from the Yixian
Formation provides direct evidence of
Psittacosaurus as a prey animal. One
skeleton of
Repenomamus robustus,
a large triconodont mammal, is
preserved with the remains of a juvenile
Psittacosaurus in its abdominal cavity. Several
of the juvenile's bones are still articulated, indicating that the carnivorous mammal
swallowed its prey in large chunks. This specimen is notable in that it is
the first known example of Mesozoic mammals
preying on live dinosaurs. Heavy predation on juvenile
Psittacosaurus may have
resulted in
R-selection, the
production of more numerous offspring to counteract this loss.
Pathology
Out of over 400 known
Psittacosaurus specimens,
only one has been published with any sort of
pathology. The specimen
in question, consisting of a complete adult skeleton and tentatively
assigned to P. mongoliensis,
was found in the lower beds of the Yixian Formation of China. There is no
sign of a bone fracture, but very
clear signs of an infection can be seen
near the midpoint of the right
fibula. The bone
exhibits a large round pit, evidence of
necrosis due to a lack
of blood supply to the
region. The pit is surrounded by a massive amount of swelling along the
lower third of the bone. This large amount of bone deposited around the
injury indicates that the animal survived for quite a while despite the
injury and subsequent infection. As psittacosaurids were bipedal animals, a
similar injury to a weight bearing bone in
the leg would likely have been fatal. However, unlike the
femur and
tibia, the fibula is
not a weight-bearing bone, so this animal would still have been able to walk
to some extent. The source of the injury remains unknown.
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