Ostrich-size dinosaurs walked on two legs from a sandy shore into a Wyoming sea about 165 million years ago, waded deeper and eventually pushed off the seafloor to swim, according to a new study.
A University of Colorado at Boulder graduate student found strange “swim tracks” of previously unknown dinosaurs across northern Wyoming.
The biggest of the dinosaurs found were about 9 feet tall, walked on two legs and had four limbs, the tracks indicate.
The discoveries have geologists scrambling to explain why there was a shoreline in what they thought was the middle of a prehistoric ocean.
“We start with the full footprints, and then, as the animal becomes more buoyant, we see scrape marks and then toe dinks,” CU-Boulder graduate student Debra Mickelson said.
“They were definitely heading offshore; we know that from the geology, and invertebrate traces also tell us we’re in deeper water,” she said.
Mickelson worked closely with colleagues from Indiana, Massachusetts and Tennessee and is presenting the team’s research this week during the Geological Society of America’s annual conference in Salt Lake City.
The dinosaurs’ footprints – from a group of meat-eating animals called therapods – speak to both geology and the social behavior of the animals, she said.
Researchers have found land and swim tracks from the three- toed creatures across northern Wyoming, from Cody to Shell and Ten Sleep.
Those tracks indicate the region was not entirely covered by the Sundance Sea, as once thought, Mickelson said.
The tracks also suggest the animals were social, traveling in groups that headed offshore together, probably to hunt fish.
“If it is proved, it would be pretty significant,” said Martin Lockley, a dinosaur-tracks expert at the University of Colorado at Denver who hasn’t yet seen the new tracks.
Paleontologists have long debated the significance of scratchy swim marks in ancient rock, he said.
Lockley himself helped disprove one of the first set of tracks purported to represent a swimming dinosaur, found in Texas in the 1930s.
Other sites, like one identified recently near St. George, Utah, give stronger evidence that some land-dwelling dinosaurs took to the water, Lockley said.
At the Utah site, researchers have even found the munched bones of fish the dinosaurs were apparently hunting.
Dinosaur-track discoveries often “shake things up,” Lockley said.
It was through tracks that paleontologists proved that big, plant-eating dinosaurs weren’t limited to shorelines, for example, he said.
Land tracks indicate the plant-eating giants moved easily on land and did not have to stay partially submerged in water, Lockley said.
Mickelson’s Wyoming tracks disprove another classic dinosaur tale – that plant-eaters could escape nonswimming predators by wading into water, Lockley said.
Staff writer Katy Human can be reached at 303-820-1910 or khuman@denverpost.com.
