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...continued
Dinosaur
tracks
On our third day in the field,
we drove a good distance to a
new area of low hills and buttes.
It looked like a great area for
fossils, but hours of prospecting
by the entire team only turned
up a few bone fragments.
"I found some
footprints over there," pointed
Dave. "They're actually on the
underside of that ledge." We all
headed over to the ledge, and
peered underneath.
Dave
looks upward to examine the footprint
cast.
This was an
amazing discovery. You had to
be crouched down, looking upward
to see it! "Dave, you know, most
of us look down when we prospect,"
Paul said, laughing and shaking
his head in amazement. "How could
you possibly have found this?"
Dave explained that when he saw
the ledge, it reminded him of
a ledge he had seen in Morocco
on a previous expedition. There
on a slope in the Sahara was a
layer of rock loaded with footprints.
It was a great
discovery, the first dinosaur
footprint recorded from all of
western Inner Mongolia. When we
think of dinosaur footprints,
we usually imagine leaf-shaped
depressions on a rock surface.
Dave's print was the reverse -
it stuck out from the rock surface.
It was a cast of the original
footprint. Some 90 million years
ago, a dinosaur made a footprint,
which then filled with mud. The
printed layer broke away, exposing
a beautiful cast of the footprint
hanging from the next mud layer.
It's almost like seeing the bottom
of the dinosaur's foot!
The print
was very well preserved, the joints
in each toe swollen slightly by
a foot pad. The resemblance to
a modern bird foot is very detailed.
For just such a comparison, Paul
saved one foot of the vulture
skeleton, beautifully mummified.
The
mummified foot of a vulture, with
fleshy pads under the joints of
each toe, is an excellent model
for the foot of our trackmaker.
Bird feet
are covered in scales. Heavy band-shaped
guard scales are located on the
front of each toe to take the
brunt of bumps and scrapes. Smaller
granular scales cover the underside
of each toe for grip. The stiff
upper part of the foot is covered
with polygonal scales. Dinosaurs
were very probably the same, the
granular scales sometimes preserved
as a fine dimpling in the footprint.
Like
this vulture foot, theropod dinosaurs
would have had scaly feet, with
downy feathers covering most if
the rest of the body.
The
trackmaker?
The footprint provides many clues
to help us determine the trackmaker
- or at least to help us eliminate
some from consideration! These
clues include the overall size
of the print, the number of toes,
the form of the claw, and the
length of each toe.
Size matters.
Our print is about 10 inches long.
Our trackmaker probably had a
body length of about 10 to 15
feet. It could not have been a
very small species, like the 3-foot
long theropod Eoraptor or the
slightly larger parrot-beaked
leaf-eater Psittacosaurus.
A small print, on the other hand,
could be made by either a small
dinosaur or a juvenile of a large
dinosaur; a hatchling T. rex,
for example, would leave a very
small 3-toed print even though
the largest 3-toed print we have
on record was probably made by
an adult T. rex. Our trackmaker
is a medium-sized dinosaur.
The print
has three toes -- comparable to
the second, third, and fourth
toes of our foot. Trackmakers
with more-or-less symmetrical
3-toed prints of this general
shape include ornithopods like
Iguanodon and theropods
like Allosaurus or Tyrannosaurus.
These are dinosaurs that walked
most or all of the time on two
legs. As a result, their hind
feet are positioned near each
other under their bodies, and
the footprint is nearly symmetrical.
Our trackmaker is two-legged.
The claws
of the print have pointed tips.
Pointed claws characterize theropods;
broader hoof-like claws characterize
large two-legged ornithischians,
such as Iguanodon. Our trackmaker
is a theropod. The toes are very
unequal in length. The middle
(third) toe is much longer than
the second or fourth toe. The
fourth toe is more complete than
the second toe, and its length
is less than half that of the
middle toe. Enlargement of the
middle toe (or reduction of the
outside toes) is usually associated
with speed (as in living one-toed
horses). There is one group of
medium sized, two-legged theropods
that have long middle toes that
suggest they were fast runners.
Our trackmaker may well have been
an ornithomimid, a theropod dinosaur
that resembles the quick-footed
ostrich in many ways.
New "kids"
on the block
As we climbed out of the cars
from a strenuous day of prospecting,
we were greeted by a pair of newcomers,
born earlier in the day. They
wandered about, staggering into
our sleeping quarters. As they
were getting acquainted with Camp
I, we were discussing plans for
Camp II, some 400 miles distant,
but soon to be our temporary home.
A pair of curious newcomers, barely
strong enough to stand,
were born early that morning.
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