February 12, 2005

In the footprints of dinosaurs

[San Diego] North County Times

In a remote northeastern corner of the Mojave Desert lie slabs of sandstone embedded with the footprints of dinosaurs that walked there some 200 million years ago. Dating from the Jurassic period, they are the only known dinosaur tracks in California.

Alan Trujillo, associate professor and chairman of the Earth Sciences department at Palomar College in San Marcos, has traveled to the Mojave Desert many times to photograph, trace and measure the footprints since he and his students first discovered them in 1992.

"We saw that it said 'dinosaur tracks' on the very detailed Bureau of Land Management map, but when we asked Steven Spear (also an Earth Science professor at Palomar College) about them, he said that he had never seen them. So we thought maybe it was just legend."

The group decided to spread out to best search the high desert terrain that "was definitely a four-wheel drive kind of place," Trujillo said. "I told the students to look for tracks about the size of a dog; I was just guessing.

And before too long, one of the students found trackways ---- the term for a sequence of footprints ---- in the Aztec sandstone. They turned out to belong to about eight species of dinosaurs that roamed the region when it was coastal sand dunes.

The time period of the tracks is determined by the age of the surrounding sedimentary rock.
"Sand buried into the depressions left by their feet; they were covered and hardened into the rock," Trujillo explained."

After that, they were uplifted by thrust faults and only recently exposed. It is (a) mineral-rich area." Flagstone quarries and extensive mining operations are located nearby.

The footprints are believed to be those of three bipedal (two-legged) coelurosaurs and about six quadrupeds (four-legged animals).

"The quadrupeds are probably mammal-like reptiles ---- not dinosaurs," said Robert E. Reynolds, a paleontologist at LSA Associates in Riverside, who has studied the trackways. "One resembles the tracks of a desert lizard."

Also found in the area are trails and tracks of invertebrates ---- probably worms and tarantulas ---- from the same period.

In some ways, the footprints pose more questions than offer answers.

"Every time we go there, we find new questions," Reynolds said. "What is different from finding fossil bones is that these trackways show what the animal was actually doing at the time. And since no fossil skeletons have been found in the Mojave, much of the information we have about them must be surmised from only the footprints, and there is much educated guessing involved.

"A number of dinosaur skeletons have been found in Orange and San Diego counties, but they are primarily from the Cretaceous period," Reynolds said. "These are one of the few (trace) fossils from the earlier Jurassic period.

"Researchers use the length and width of fossil footprints to estimate the dinosaur's size, including leg length, posture, gate, foot structure and in some cases, even social behavior. The spacing of the prints also reveals whether the animal was walking along or running and at what speed they may have traveled.

Indeed, the top speed for the most famous bipedal dinosaur of all, the tyrannosaurus rex, has been the subject of discussion in paleological circles. Scientists from UC Berkeley reported a couple of years ago that rather than traveling at the speed of a car, as they did in the movie "Jurassic Park," T Rex's maximum speed based on trackways was probably closer to 27 mph.

"That's still faster than Olympic sprint champions," Trujillo said, "and like a sprinter, they probably didn't sustain that speed for long, either.

"Of course, how energetic dinosaurs were also is a matter of debate. "If they were warm-blooded, like birds, then they may have been more active," he said.

Of the 116 tracks found in the Mojave, the two-legged tracks are thought to be made by coelurosaurs. And since those are distinguished by only their footprints, two have been assigned the ichnogenera "anchisauripus" and "arallator." The third remains unnamed. "When the animal did not die in its tracks," Reynolds said, "we give them the ichnogenera, which means 'footprint group.'

"Researchers have compared the dimensions of the feet to coelurosaur skeletons found in other Western states. Coelurosaurs had three toes and claws and probably ate a variety of things, including vegetation and meat. "Their teeth are for catching animals," said Reynolds, who added that their footprints are 4 to 6 inches in length and the stride is about 3 feet. "We think they were ostrich size and may have run quite rapidly," he said.

The grallator's footprint is notably asymmetrical. Trujillo has estimated its leg length at 3 feet ---- about the size of a human ---- and its speed (based on left and right stride of the prints) to be about 3.6 mph. "That's about half our walking speed, because walking 3 miles an hour is a good clip for most people," he said.

Though the trackways in the Mojave have not been preserved per se, the Bureau of Land Management regularly patrols the area containing where they have been found. They have also been inventoried and studied, and replicas have been made of the prints by Reynolds and Ted Weasma, a geologist with the Mojave National Preserve in Barstow.

For Trujillo, though, learning more about the dinosaurs and their trackways is just a hobby. His real work, he says, is writing oceanography textbooks and teaching at Palomar full time, but he certainly understands the mystique dinosaurs hold for modern man.

"Finding the dinosaur trackways was really exciting," he said, smiling with enthusiasm. "Walking in the same footprints of a dinosaur? It is very cool."

What are trace fossils?

Fossils can be divided into two general groups ---- body fossils and trace fossils.

Body fossils are the preserved anatomical parts of the plant or animal and provide direct evidence, while trace fossils are produced by the animal's activities. A trace fossil, therefore, is indirect evidence of ancient life and provides information on the behavior of the organism.

There are many different types of trace fossils. Dinosaur tracks and trackways are perhaps the best known. Often, animals' burrows become filled with sediments and are preserved. Nest structures are another type of trace fossil.

Evidence of feeding can be preserved as trace fossils, such as insects chewing on leaves. Tooth marks on bones may be left by a predator while feeding on its prey or by rodents chewing on bones for the minerals. Eggs, gizzard stones and dung are also considered trace fossils. The study of trace fossils is called ichnology.

----Park Paleontology, published by the Geologic Resources Division of the National Park Service, summer 2002