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Dinosaur Tracks Shed Light on Sauropod Evolution

John Roach
for National Geographic News
May 30, 2002
 
Dinosaur tracks made on the edge of a coastal plain 163 million years ago in middle England are providing a team of researchers with new insights into the evolution and behavior of sauropods.

Sauropods are the group of plant-eating dinosaurs distinguished by their long necks and tails. They include some of the largest creatures ever to walk on Earth.

An analysis of about 40 tracks of varying sizes indicates that they were made by a mixed herd of sauropods, concludes a group of researchers reporting in the May 31 issue of Science. One of the species represented in the tracks was a titanosaur, a dinosaur group that researchers didn't think had emerged for another 12 million years.



Track Investigations

The tracks, which were found in a limestone quarry near Oxford, can be divided into two distinct types: those that have a wide gap between the left and right feet and those that have very little space, if any at all, between the left and right feet.

Julia Day, a paleontologist at the University of Cambridge in England, and colleagues analyzed these tracks to try and determine which species made them.

"Different groupings of dinosaurs are defined on the basis of unique characteristics. This can also be applied to the trackways," said Day.

According to the researchers, each of the tracks contains characteristics that distinguish them as those made by sauropods. For example, the front footprints indicate the metacarpals—the hand bones on people—were held in a semicircular arrangement unique to sauropods.

While there is not enough detail among the narrow tracks to enable the scientists to discern what species of sauropod made what track, the wide gauge tracks were clearly made by titanosaurs, the researchers conclude.

The wide tracks do not contain an impression of a claw on the front foot, whereas the narrow tracks do as do another group of nearby tracks previously reported by Day's team to belong to theropod dinosaurs.

Other researchers have shown that this claw is reduced in brachiosaurs, the sister group to titanosaurs. The claw is believed to be completely lost in titanosaurs.

The absence of the claw in the tracks is thus not a sign that the tracks were poorly preserved, but rather corroborating evidence that the tracks were made by titanosaurs, the researchers note in their paper. "The titanosaurs are the only ones that make wide gauge tracks that don't have claw marks," said Day. "So we can be confident that they were made by titanosaurs."

The existence of titanosaurs at the quarry site 163 million years ago extends the record of when titanosaurs diverged from other sauropods by at least 12 million years, according to the researchers.

The earliest known skeletal remains of titanosaurs come from the Late Jurassic (154 to 144 million years ago) site of Tendaguru, Tanzania. Day hopes to confirm the earlier date for titanosaurs by finding fossilized bones in the limestone quarry where the tracks were found.

"We don't have any osteological evidence at this site," she said. "We found these footprints. Now the next stage is to look for the bones."

Moving as a Herd

Nearly all of the tracks at the quarry site point in a northeasterly, roughly parallel direction. Day and her colleagues suggest this is an indication that the sauropods traveled in a herd.

"Herding has been talked about before," said Day, who pointed out that evidence for herding has been reported from other track sites such as the Dinosaur Valley State Park site in Glen Rose, Texas. The difference, she said, is that there were at least two species of sauropods traveling together at the Oxfordshire site.

Geologic reconstructions conducted by Day's team indicate that in the time of the dinosaurs, the Oxfordshire site was a coastal plain 19 miles (30 kilometers) distant from the nearest landmass. The dinosaurs were walking directly where the sea meets the coast, so the tracks were probably left for only a few hours at a time between high tides.

"What were these animals doing out there?" asks Day. She suggests it's possible that the sauropods were migrating between landmasses in search of food and formed a mixed herd to defend against predators.

Erik Kvale, a research geologist with Indiana University in Bloomington who has studied dinosaur tracks in Wyoming, cautions against making any conclusion based on the interpretation of tracks alone.

"It is very difficult to time the passage of a series of animals," he said. "I have watched gulls on a beach repeatedly circle an area in search of food. The net result is a series of tracks which can parallel a shoreline made by a single bird."

According to Kvale, the tracks could have been made by a single dinosaur, or perhaps by dinosaurs moving singularly in one direction along a geographically restricted pathway spaced out by minutes, hours, or days.

"Their tracks may have nothing to do with migration but rather moving away from water or toward water or simply grazing or looking for food," he said.

The existence of theropod dinosaur tracks in the area, however, leaves Day speculating about why the sauropod herd might have been leaving the area and whether it was protecting itself against a threat. Theropods are the group of meat-eating dinosaurs that includes Tyrannosaurs rex.

"It is an interesting site," said Day. "Theropods were there and their trackways are all going in the same direction. The theropods might be tracking them."

Recent National Geographic News stories on dinosaurs:
Fossil of Dog-Size Horned Dinosaur Unearthed in China
Tyrannosaurus rex Was a Slowpoke
Researchers Rethink Dinosaur Die Off Scenario
Researchers Melt Polar Dinosaur Mysteries
Scientist's Finds Spur New Thinking on Dino Evolution
Dino-Era Vomit Fossil Found in England
Study Paints New Picture of Dinosaur's Nose
Skeleton of New Dinosaur "Titan" Found in Madagascar
"Tidal Giant" Roamed Coastal Swamps of Ancient Africa
"Feathered" Fossil Bolsters Changing Image of Dinosaurs
Oddly Angled Teeth Make Masiakasaurus Stick Out

Additional dinosaur resources from National Geographic:
Paul Sereno: National Geographic Explorer-in-Residence and Dinosaur Hunter
Dinorama
Wanted: Albertosaurus
Dinosaur Eggs
Destinations: Dinosaur National Monument
 

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