On the Loose: Fossils and Footprints
Made of mud 90 million years ago, the cast of a dinosaur footprint provides many clues about the track maker.
Closing the quarry
Our relentless pursuit of the small theropod into the depths of the hill will pay a big paleontological dividend-many months from now. It will become the best known theropod from Inner Mongolia and perhaps from the entire Gobi Desert. We will know this animal from head to toe. The skeletons and clues from the exceptional bone horizon will tell us about how the animal moved, what it’s social life might have been like, what it ate, and, perhaps, how it died.
Andy’s gloves tell of hard work in the quarry.
Such information is hard won and requires backbreaking work and meticulous records. There was no shortage of either at the quarry, despite the help of a bulldozer. We cleaned up the site, leaving no stray wrapper or piece of burlap. As we pulled out, we gazed a little remorsefully at the now familiar hills and mountains of a quiet and beautiful place in the Gobi, whose secrets from the distant past will eventually be told.
Prospecting ‘til you drop
We poured over the geologic maps, planning an aggressive week of prospecting. After many days chipping away at the same square meter or two, we were really looking forward to the long and strenuous hikes that fossil prospecting calls for.
There’s art and science to prospecting. The artsy side is akin to having lopsided luck. You just happened to be in the right spot at the right time. You slipped on a rock that dislodged another, which exposed a fossil.
Jeff uses his eyepiece to have a closer look.
But you’d be surprised how much of fossil discovery isn’t plain serendipity. Some find more of their share of discoveries. Some always find the small fossils. Others find the best ones. Some teams do much better than others in the same general field area. This side of prospecting is called having the “nose.”
Andy, on his first dinosaur expedition, has developed a keen awareness for fossils. “The most important thing is to learn the color and textures of the fossils in the search area. Then you can really key in.” He added quickly, “Also, you gotta be willing to get down close to the rock, to stop and check, all of the time.”
“For a team, I think it’s legs that are most important,” suggested Paul. “We often need to cover an immense area. Some of our best fossils are well exposed, but you got to be able to find them. You need a great pair of legs, because an hour of prospecting time can involve miles of walking over rough terrain.”
Dinosaurs large and small
The prospecting yielded results on our first day. A new predator, a dromaeosaur, was hunted down by our microfossil specialist, Fabrice. His background – years of searching for shark’s teeth – has given him a keen eye for small fossils.
“I found these small vertebrae just over there and the rest of the bones nearby,” remarked Fabrice, pointing to a very ordinary slope on the side of a very ordinary hill. “They are so tiny,” exclaimed Mike. “This is the tip of a small claw,” remarked Jeff, pointing to a small triangular fragment dwarfed by the palm of his hand.
Two characteristic vertebrae from a strongly inclined neck record the presence of a new small raptor.
We decided to double back to the hill with sieves the next day, as the pile of tiny hand-picked bones from the site included many from an unknown small predator — a dromaeosaur or relative ofVelociraptor.
Dave sorts through the seives catch, plucking fossil pieces that otherwise would have gone unnoticed.
More vertebrae and other bones appeared in the sieves, as we went through the sediment on the side of the hill with a fine-tooth comb. The most amazing piece was the base of a small claw, to which we reattached the tiny claw tip, found the day before.
The sickle-shaped claw was from the hand of a very small dromaeosaur. “This was a vicious predator, perhaps only three or four feet long as an adult,” remarked Paul.
Small enough to balance on a fingertip, the tiny hand claw of a raptor was found by the sharp-eyed team.
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 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 likeIguanodon 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.