The tracks of a previously unknown, two-leggedswimming dinosaur have been identified along the shoreline of anancient inland sea that covered Wyoming 165 million years ago,according to a University of Colorado at Boulder graduate student.
Debra Mickelson of CU-Boulder's geological sciences departmentsaid the research team identified the tracks of the six-foot-tall,bipedal dinosaur at a number of sites in northern Wyoming, includingthe Bighorn Canyon National Recreation Area. "It was about the size ofan ostrich, and it was a meat-eater," she said. "The tracks suggest itwaded along the shoreline and swam offshore, perhaps to feed on fish orcarrion."
Mickelson will present her findings at the Geological Societyof America's annual meeting Oct. 16-19 in Salt Lake City. Shecollaborated on the project with researchers from CU-Boulder, IndianaUniversity, Dartmouth College, Tennessee Technological University andthe University of Massachusetts.
Mickelson said scientists have previously reported evidenceof swimming dinosaurs in other parts of the world and at other times inthe geologic record. But the new findings by the team are the onlyknown evidence of any dinosaurs in the Wyoming region during the middleJurassic, she said.
The dinosaur does not have a name, although Mickelson iscontinuing to look for bones and other remains that could be used toidentify and name the new species. "This dinosaur is similar to aCoelosaur," she said. "It is a dinosaur with bird-like characteristicsand is a possible ancestor of birds. It lived in a much earlier timeperiod and was very different from larger dinosaurs like T-Rex orAllosaurus."
The tracks are embedded in a layer of rock known as the MiddleJurassic Bajocian Gypsum Spring Formation, a 165- to167-million-year-old rock formation that contains fossilized remains ofa marine shoreline and tidal flats. Geologists believe an inland sea,called the Sundance Sea, covered Wyoming, Colorado and a large area ofthe western United States during the Jurassic period from about 165million years ago to 157 million years ago.
Mickelson said the sea might have been warm and relatively shallow, much like the Gulf of Mexico today.
"The swimming dinosaur had four limbs and it walked on its hindlegs, which each had three toes," Mickelson said. "The tracks show howit became more buoyant as it waded into deeper water -- the fullfootprints gradually become half-footprints and then only claw marks."
Mickelson explained the tracks are found among the traces leftby many animals, including ancient crocodiles and marine worms. "Thetracks of the ancient crocodiles are very different," she said. "Theywalk on four legs and have five digits."
Since summarizing preliminary findings last spring,Mickelson and the research group have expanded their study area, whichshe said contains millions of dinosaur tracks in a number of GypsumSpring Formation rock outcrops in northern Wyoming.
The tracks are of different sizes and were deposited at aboutthe same time, according to Mickelson, revealing that the dinosaurslikely traveled in packs and exhibited some variation in overall size."Further research into the geologic record and depositional history ofthe region supports our conclusion that the dinosaurs wereintentionally swimming out to sea, perhaps to feed," she said.
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