Traci Watson

Special for USA TODAY

Some 90 million years ago, three carnivorous dinosaurs sloshed through the mud, leaving footsteps still visible today. Now researchers have found clues hinting at the pack’s intent: to grab dinner, in the form of a fellow dinosaur.

If this idea is correct – and it’s far from confirmed – the tracks could provide valuable evidence for cooperative hunting by tyrannosaurs, the family of meat-eating, hind-leg-walking reptiles headed by the mighty T. rex.

Scientists have found fossils and other tracks suggesting dinosaurs hunted cooperatively, but evidence for dinosaur behavior “is still relatively rare, because we’re talking about something we can’t see,” says Brent Breithaupt, a Wyoming-based regional paleontologist for the U.S. Bureau of Land Management who was not involved with analyzing the tracks. And whatever happened at this spot one day in the Cretaceous, “tracks are the first good step – excuse the pun – into learning about what animals were there.”

The predator tracks were first discovered a decade ago in a rugged corner of New Mexico overseen by the land-management bureau. The researchers found 13 prints left by three meat-eating dinosaurs: small, medium and large.

The proportions and shape of the three-toed prints point to tyrannosaurs as the culprits, says Douglas Wolfe of the White Mountain Dinosaur Exploration Center, who found the tracks. Fossils of a tyrannosaur-like dinosaur – a new species still under study -- lie nearby, bolstering the case that the tracks are the footwork of tyrannosaurs.

Many of the tracks parallel each other, indicating “a family group, maybe moving in concert,” Wolfe says. What stumped him was the pattern of the tracks. The prints march across the ground in a relatively straight line. Then they suddenly change direction and veer off.

A few years ago, Douglas Wolfe’s wife Hazel noticed something no one else had seen: a round footprint exactly where the tyrannosaur tracks swerve. Little piles of sediment hint the round-footed animal kicked up sand as it scrambled away.

The maker of the round track was probably a plant-eating horned dinosaur of the Triceratops family, Douglas Wolfe says. One candidate is Zuniceratops, a smallish horned dinosaur. Wolfe thinks the tyrannosaurs closed in on the horned dinosaur’s right side before their quarry fled. The Wolfes reported their findings at a recent meeting of the Society of Vertebrate Paleontology.

A joint tyrannosaur attack may seem far-fetched, but dinosaurs were not the loners of the popular imagination. At one Canadian site, a collection of fossils from tyrannosaurs of different ages suggests the animals died together, says James Kirkland of the Utah Geological Survey. “These were sophisticated animals,” Kirkland says.

Scientists not involved with the New Mexico find say it’s too early to know its significance. Breithaupt calls the site “intriguing” but wonders whether the meat-eaters and the plant-eater actually crossed paths, rather than trekking across the same spot days or weeks apart. He’d like to see more data, including digital imaging of the tracks.

Douglas Wolfe says the animals would not all have left tracks if they hadn’t shown up at the mud patch at roughly the same time. In any case, he says, it’s useful simply to find prints of tyrannosaur-like animals: the tracks indicate tyrannosaurs once stalked this patch of ground in the Cretaceous, and that they did so in a herd.

The set of tracks also “shows there are many, many unique sites still out there” waiting to be discovered, Breithaupt says. “Each one of these sites has a different story … that will provide us with a better understanding of the life and times of the animals that were roaming around back in the Age of Dinosaurs.”