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
Destinations: Dinosaur National Monument
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