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by Paul
Sereno
Photo
by Jeff Wilson
Assembled like a puzzle from more
than
20 pieces that lay on the side
of a hill,
the skull of a psittacosaur emerges,
the rest safely protected in a
jacket.
Parrot-beaked
Dinosaurs
Most dinosaurs have long snouts,
whether they eat plants or meat.
Think of Tyrannosaurus
or the horned Triceratops.
There is always a snout of good
length. But there is one group
of extremely short-snouted, plant-eating
dinosaurs that populated the Asian
continent for millions of years
during the Cretaceous. They are
the parrot-beaked dinosaurs-the
psittacosaurs. They have the shortest
snout of any dinosaur, which gives
the skull a parrot-like profile.
Their nose opening is small and
positioned high on the snout,
and their beaks are anchored by
special bones-above the rostral
bone, and below the predentary.
They were consummate herbivores-ate
nothing but plants. They walked
normally on two legs but clearly
could reach the ground with their
three-fingered hand. They were
good runners but would have set
no speed records for dinosaurs.
In general, they were extremely
successful in Asia anout 100 million
years ago.
The
parrot-beaked dinosaur Psittacosaurus
from Outer Mongolia is the
standard species
for comparison.
Skull
puzzle
White pieces of bone drew Jeff
to the very top of a butte, where
he saw the edges of a skull diving
below the surface. It was a strange
short skull. He recognized it
instantly as belonging to a parrot-beaked
psittacosaur. He spotted pieces
that included the unusual pair
of special bill bones, the predentary
and rostral bones. But there were
so many pieces, perhaps all that
would remain intact is the part
still in the ground.
That was how
the pieces arrived back in the
library at Camp I. First Jeff
fit the lower jaw together. Then
the race was on. It was clear
that this was a paleontological
puzzle. Erosion over many years
had washed the front margin of
the skull and the lower jaws down
the hill. Could we reverse that
breakdown? Paul and Dave joined
the effort. Critical in this endeavor
is how careful Jeff was when he
first found the skull. Did he
look for all of the pieces?
After a few
hours and with nearly all of the
bones glued back into place, the
anterior margin of a psittacosaur
skull lay on the lab table to
our amazement. It had a relatively
long skull that is shaped a lot
like its cousin from Outer Mongolia,
Psittacosaurus mongoliensis.
Stomach
stones
Psittacosaurs are most common
in the Early Cretaceous of Asia,
about 135 to 110 million years
ago. Paul led the team to rock
suspected to be of this age. We
thought we would give the area
a closer look.
"I think it
could be good. Looks like the
bones are going in. Plus there
is this bone-I don't know-I have
never seen anything like it,"
remarked Fabrice. He had found
a psitacosaur skeleton all right.
As we gathered around to work
on the skeleton, the strange bone
was found to be connected to a
small skull-it was the lateral
horn of a psittacosaur! This small
skull is quite a bit shorter in
its proportions, almost round,
than Psittacosaurus mongoliensis.
We were terribly thrilled to realize
that we had in hand a complete
skull of this unusual plant-eater.
Photo
by Fabrice Moreau
The big discovery of the day,
a complete skull
of a psittacosaur, garners everyone's
interest in the field.
As Paul edged
around the skeleton, he chipped
a layer of mud off, revealing
a cluster of polished stones.
He knew in an instant what it
was-the "stomach stones" or gastroliths
of the psittacosaur. There were
more than a dozen, possibly more
than two dozen. Paul had worked
on Psittacosaurus mongoliensis
for many years and knew that finding
as many as 40 or more of these
gastroliths in a single skeleton
is not unusual.
How do you
tell a gastrolith from a common
pebble. Actually it can be quite
difficult! We had the huge advantage
that the matrix around the skeleton
was fine-grained mud. In other
words, the gastroliths really
stuck out.
Photo
by Fabrice Moreau
Rounded and polished from grinding,
the "stomach stones" or gastroliths
of Fabrice's psittacosaur lay
under the skeleton.
Desert
Graveyard
It may come as a surprise for
some to realize that a dinosaur
skeleton is composed of the same
basic set of bones as that of
a camel. By studying the bones
of many different animals, paleontologists
come to appreciate how subtle
changes in design, size, shape
or number translate into basic
adaptations. The camel, for example,
has a number of specializations.
It nis plants by clamping a sharp
set of incisors in the lower jaw
against a hard pad at the front
end of the upper jaw. We examine
dinosaur teeth and the margins
of the bones of their jaws for
similar features - wear facets
on tooth crowns, attachment surfaces
for horny bills.
Dinosaurian
descendants
Some animals we might find in
a desert, however, are closer
to dinosaurs than others. Birds
we now know, are dinosaurian descendants
- "living dinosaurs" you might
say. . Their skeletons are remarkably
similar to those of Cretaceous
raptors, like the dromaeosaur
Deinonychus. For a dinosaur
specialist - and there are several
on the team - a well-preserved
bird skeleton would be great to
have for comparisons. But they
are so fragile, they rarely survive
for long.
With
its horny bill in place and its
bony eye ring still in its eye
socket. the skull of a vulture
makes a superb study specimen.
Paul
was the lucky one. A large vulture,
with wing bones as long as a human
forelimb bones crashed into the
side of a hill and was partially
buried in sand. A predator ran
off with the skull an part of
the neck, which Paul found a couple
of hundred yards away. The skeleton
was amazingly complete - what
a treasure for the comparative
anatomist!
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