Dance of the Dinosaurs

As geologist Erik Kvale and colleague Allen Archer walked across dusty badlands in northern Wyoming last summer, Kvale explained why they would never see dinosaur tracks in the rippled sandstone under their feet. The buff-colored Sundance Formation, he noted, was actually detritus from an inland sea that covered much of the interior West during Jurassic times. Fossil seashells might lurk in the stone, he said, but dinosaurs would have stayed on land.

Then the geologist glanced down at the rock and saw a three-toed footprint the size of a dinner plate that could only have been left by one creature:

A dinosaur.

As Kvale and Archer--accompanied by several of Kvale’s relatives-- looked more carefully, they realized that they were standing on a petrified tidal flat covered with thousands of dinosaur tracks as obvious as a dog’s muddy footprints on a clean white carpet.

What has since become known as the Red Gulch Dinosaur Track Site on federal land in Wyoming has shaken many suppositions about the Middle Jurassic and has begun to sketch in what had been a blank but pivotal chapter in the history of dinosaurs in North America.

“Dinosaur footprints, or even bones, are exceedingly rare for that period,” said Michael Brett-Surman, a Smithsonian Institution paleontologist who has done years of field work in Wyoming. Everybody thought this area was deep underwater. “It’s a slice of time we don’t know much about, especially when it comes to dinosaurs.”

From their bird-like footprints, it’s clear the dinosaurs that walked the Wyoming beachfront about 165 million years ago stood on two legs, making them a crucial evolutionary go-between in the reign of the dinosaurs on Earth. They roamed North America after the initial rise of the dinosaurs more than 200 million years ago in the late Triassic, but before the grand heyday of dinosaurs that began in the Late Jurassic and produced such mighty predators as razor-toothed allosaurus and, much later, Tyrannosaurus rex.

“Those later species didn’t just arrive--they evolved from the animals that lived at the time these tracks were made,” said University of Wyoming paleontologist Brent H. Breithaupt, kneeling on a sandstone expanse that forms the bottom of a natural arroyo and is dotted with so many ancient footprints that researchers have named it “the Ballroom.”

Few North American fossils of any kind date from the time of the enigmatic track makers, so scientists know little else about them. Kvale, Archer, Breithaupt, Brett-Surman and other researchers have joined with the U.S. Bureau of Land Management to pry as much detail as they can from the footprints, which are virtually the only sign the animals ever existed.

Earth was a much different place during the Middle Jurassic. Today’s major continents were just breaking up. Wyoming hung at about the same latitude as the Bahamas do now. And since an atmospheric greenhouse kept glaciers or polar ice caps from locking up water, sea levels remained high enough to overrun parts of the continents.

In North America, an inland sea fingered its way from the Arctic Ocean south through the western half of the continent and submerged what is today western Canada, along with much of Montana, Wyoming and Utah.

Or so most researchers thought because, clearly, the dinosaurs left their tracks on land.

More precisely, they left their tracks in shoreline sand so well sorted it might have been put through a sifter. Magnified many times, the sand looks like perfectly round pearls similar to sand seen today on warm, tropical coastlines like some in the Bahamas, where calcium carbonate in sea water solidifies somewhat like mineral scaling builds up on water faucets--into tiny spheres called oolites. Waves then sort the spheres into uniform consistency.

“We’re talking about clean, well-washed sand getting exposed a couple of times a day,” said Gary D. Johnson, a professor of geology at Dartmouth University and also part of the team studying the track site. “You had a pretty nice beach back then.”

A Walk Along the Sundance Sea

If landlubber dinosaurs frequented the tropical beach, it must have been part of an island, isthmus or other land mass in the ancient waterway known today as the Sundance Sea, after the Sundance Formation that it left behind.

As the dinosaurs walked, they sank their feet into sand that had the consistency of wet concrete, Johnson said. The air was probably warm and dry, since evaporation left salt ridges in the sand. Shortly after the dinosaurs traipsed through in what may have been only a few hours or days, something unusual happened that preserved these few tracks in Wyoming out of the multitudes the creatures likely left throughout their lives.

Just what that something was, nobody knows.

Maybe a storm or a Jurassic hurricane blew in fine sediments that settled like a soft comforter over the tracks without disturbing them, Johnson said. Then the sea level rose--or the land sank--and deeper water flooded the beach, leaving a diary of events in the layers of sediment atop the dinosaur tracks.

Inland, Brett-Surman says, a scattered subtropical forest of ginkgos, conifers and palm-like evergreens called cycads probably lined the horizon. Although that world vanished long ago, erosion in the millions of years since--assisted by a new culvert that inadvertently washed soil off the dinosaur tracks--has brought it back into view.

Nature at the time was reworking its original design for predatory dinosaurs. The ceratosaurs, lightweight carnivores that first stalked Earth about 225 million years ago, were bulking up, getting bigger and stronger to better face their primeval competition, Brett-Surman said.

Dinosaur species generally lasted 3 million to 5 million years before evolving into others, so the owners of the Wyoming tracks were one of many species along the evolutionary path from the early ceratosaurs to the more heavyweight carnosaurs such as allosaurus, a Late Jurassic predator that dominated North America once the Sundance Sea had receded.

Based on the varying track sizes, many individual dinosaurs cruised the beach--perhaps adults with adolescents in tow. The larger animals stood close to twice the height of a human and from the length of their strides were walking at a casual speed of a few miles an hour, Brett-Surman said.

As researchers use computer mapping to link the footprints of each dinosaur that stepped through the ballroom like a connect-the-dots puzzle, they hope the pattern will show them where the creatures were headed and whether they were traveling together in packs. It would take another 80 million or 90 million years of evolution before T. rex reigned, but the Wyoming dinosaurs were on their way there.

The U.S. Bureau of Land Management is seeking public input on how best to manage and protect the tracks. For more information, see the BLM web page at www.wy.blm.gov/whatwedo/tracsite/trac.html

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Tracking a Dinosaur

Newly discovered dinosaur tracks in Wyoming might give clues to a missing chapter in dinosaur history in North America. Paleontologists believe the three-toed tracks were made in the Middle Jurassic period (roughly 165 million years ago), even though it had been thought that much of the West was submerged beneath an inland sea during that time. Few other dinosaur remains from that period in North America are known.

Allosaurus Ancestor?

Paleontologists think the animal that left the tracks measured six to eight feet from nose to tail and may have been a smaller predecessor of the allosaurus, which evolved many millions of years later.

Sources: James Peterson, Gary D. Johnson, U.S. Bureau of Land Management, “The Complete Dinosaur” edited by James O. Farlow and Michael K. Brett-Surman, Dorling Kindersley Ultimate Visual Dictionary