In the Tracks of the Cretaceous
A giant, meat-eating dinosaur called Acrocanthosaurus atokensis left its tracks on a muddy coastal plain in southwest Arkansas. The tracks are mingled with those of large plant-eating dinosaurs similar to the ones depicted here. © Julius T. Csotonyi (http://csotonyi.com)
Newly discovered dinosaur footprints in southwestern Arkansas offer scientists a peek back in time.
Beads of sweat form on geoscientist Steve Boss’s face and trace tracks through sunscreen as they travel down his neck. From beneath his hard hat and behind his safety glasses, he surveys the grey field of rock surrounding him, heat radiating off bleached limestone in the southwest Arkansas landscape under the midday sun. He and his colleagues have been laboring since before sunrise at the research site, and the thermometer now reads 106 degrees Fahrenheit. He takes a swig of water from a plastic bottle, working hard to stay hydrated; yesterday one of the researchers almost succumbed to the heat, so the team has redoubled its efforts to stay cool.
As they did the previous day, the scientists will work for another six or seven hours in the humid, hot temperatures of a July day. But the scientists aren’t thinking about the uncomfortable conditions or the long hours – their minds are focused 115 million years in the past, back when dinosaurs walked through Arkansas.
“This looks like a claw,” says graduate student Brian Platt. He points to the end of one of three distinct grooves in the rock as he and Boss lean closer to the giant impression in the chalky ground. The footprint is large enough that a collie could curl up in the center of it. It is one of hundreds of footprints of different shapes and sizes that pepper the terrain around the now prone scientists. The tracks face in all directions, criss-crossing back and forth, some on top of one another.
Boss sets the scene, for the writer, his hand sweeping to encompass the barren landscape: 115 million years ago, this area bordered the edge of the continent, and the whitish ground we stand on was part of a shallow coastal mud flat. Boss likens the climate and surroundings at that time to the Persian Gulf today – hot and arid conditions with very salty water and no real vegetation to speak of.
“It was a place that you wouldn’t want to spend much time in,” Boss says.
Despite the harsh conditions, many dinosaurs walked through this area. Prints found include those of sauropods – giant, long-necked plant-eating dinosaurs – as well as predatory theropods, probably Acrocanthosaurus atokensis, one of the largest carnivorous dinosaurs to ever walk the earth.
“We’re not sure what the animals were doing here, but clearly they were here in some abundance,” Boss says. “There is a remarkable diversity of tracks here. We have some large predatory dinosaurs that have never been described in Arkansas before. We’ve speculated for a long time what kind of dinosaurs lived in this area, and now we have direct evidence.”
The discovery of dinosaur footprints provides as much excitement to scientists as skeletons do, says Platt. Scientists can use footprints to learn about animal behavior as well as biology – where did an animal go, how did it walk, how much did it weigh.
“Tracks are trace fossils. They represent the behavior of ancient organisms,” Platt says. “An animal has only one body to donate to science, but it can make thousands of tracks in its lifetime. Tracks represent a page in history that tells about what was going on here 115 million years ago.”
How can scientists guess the height and weight of an animal that lived so long ago? To do this, Platt and others study modern-day living creatures.
“I’ve worked with elephants walking in different types of sediments to see how they deform the sediments,” Platt says. “This is a good model for the sauropod dinosaurs.”
While Platt examines the print of a plant-eater, Boss follows a set of three-toed tracks made by a giant predator. The space between the tracks spans such a distance that a person can’t jump from one footprint to the next. After following the steps for a few hundred yards, Boss stops and points to two tracks side by side.
“It looks like this animal stopped here,” he says.
The researchers’ excitement at seeing the tracks firsthand is tempered by the need to act quickly. Their access to the site is limited to two weeks, and they will not be able to return. The team of researchers, which includes scientists from the University of Arkansas, the University of Kansas and Boise State University have a fast-track grant from the National Science Foundation to collect as much information from the site as they can before time runs out.
To accomplish this, the researchers have brought an array of tools with them: picks and whisk brooms, measuring sticks, notebooks and pencils, and laser scanners.
Wait – why bring a laser scanner to a field of footprints? Cue Jackson Cothren and Malcolm Williamson of the Center for Advanced Spatial Technologies at the University of Arkansas. Boss asked Cothren and Williamson, who have used laser-scanning technology to map World Heritage sites including Machu Picchu, to apply their work to the dinosaur tracks. The researchers are using a high-resolution scanner perched atop a cherry picker to create a precise digital map of the location of the prints within the landscape. The technology not only allows measurements of the prints themselves within six millimeter accuracy, but it also maps the prints in relation to one another – something that could take months to do using old-fashioned techniques.
“If you are trying to understand where the tracks go and the relationship between the various tracks, it is better to see everything at once,” Cothren says. “The laser scan will give you the ability to look at the entire site from an aerial perspective, zoom in on the areas that you are interested in and map out features that you may have missed because they are too difficult to see in the monochrome limestone here.”
The scans will be made available to paleontologists and other interested individuals wanting to study the site so they will be able to make a virtual visit to the site without having to travel to the site itself.
“Scientists can use these data to look at how the animals were moving through the area,” Boss says. “These scans will produce a comprehensive map with precise measurements of the whole site.” From this, the University of Arkansas researchers and others will be able to study various aspects of the site related to the movement of the animals over time and in relationship to one another.
A glance at the computer screen under the shade of a tent reveals a surprising array of prints – more than are seen looking at the site through safety glasses. The scan on the screen confirms that the site contains layers of tracks from different generations of the animals.
“The computer images of the site show detail not visible to the naked eye,” Cothren says.
While the scanner takes images, the researchers remove dirt from the footprints and record their observations in field notebooks. They create a grid and photograph the site for a future photo mosaic. They sketch some areas with a marker and a plastic sheet, looking at the size of the footprints, the distance between footprints and the angles of the tracks themselves.
“We don’t always know what data might be the most important,” Boss says.
Later they also will take plaster casts of the footprints, a messy process, but one that will allow them to preserve the imprints back in the laboratory. Together, these processes will give the scientists a comprehensive set of data to work with.
In addition to learning more about the dinosaurs that walked through this area from the footprints, the scientists will be able to learn more about the environment they lived in. The researchers will take small samples of the rock bed where the footprints were found and look at the chemical composition of these rocks to learn about the Cretaceous.
“These tracks were made in shallow water, so we know that 115 million years ago this surface was exposed,” says Celina Suarez, a postdoctoral researcher at Boise State University who will begin working as a faculty member at the University of Arkansas in the department of geosciences in fall 2012. She points to a rock covered in wavy lines that look like ripples in the sand that have been frozen in time. “By looking at the chemical composition of rocks like these, we can learn about the climate at that time.”
By determining the temperature and the intensity of evaporation in this region by looking at the chemical composition of the rocks, scientists can compare the information found in these rocks to those found at other sites and develop a global “snapshot” of the climate during the early Cretaceous.
“There’s a lot of post analysis that needs to be done,” Boss says. But before that can happen, the researchers must finish the tasks at hand under the scorching July sun.
To accomplish this, research associate Joann Kvamme plunges her arms elbow-deep into white plaster. She moves rapidly, together with geosciences graduate student Terryl Daniels, to mix the powdery plaster with water in a bucket, stir the mixture and transport it to the designated print. Once they have it there, Honors College student Alex Hamlin and geosciences major Ryan Shell dip strips of burlap into the gooey white liquid, pull them out and slop them into the greased dinosaur tracks before the plaster begins to harden. The researchers repeat this process until they have plaster casts of about 30 different tracks. They choose these tracks on the basis of particular characteristics of the track – its size, shape or location, for instance.
Once the plaster casts have dried, a team of four or five people carefully pries them up. Finally they load the back of the research van with the plaster casts, the tent and the other equipment. The two weeks are over, and it is time for the hours-long drive back to campus. For the researchers, these two weeks of hot, sweaty work in the sun represent just the beginning of the work. Back at the laboratory, they will begin to find out what they can learn from the information they collected.
Even as they leave the site, excitement about the find remains high among the scientists. This particular site expands the knowledge about the geographic range and biodiversity of dinosaurs during the early Cretaceous and provides additional insights into the puzzle of global climate change.
Photos by Russell Cothren