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Diggin' For Dinosaurs - Egg Mountain

Updated on March 30, 2010

Egg Mountain, Montana

Every afternoon from late June to late August, dinosaur fans arrive in the small town of Choteau (SHOW-toe) and detour west 12 miles on a good gravel road to get a glimpse of this site popularly known as Egg Mountain. It was here in 1978 that dinosaur hunter John R. "Jack" Horner and colleagues uncovered the world's first nest of baby dinosaurs and, the next year, the first clutches of dinosaur eggs found in North America. Every year since, Horner's crews have mined a two-by-two-mile square of eroded hills and gullies on an ancient geologic wrinkle known as the Willow Creek Anticline, amassing clues that have revolutionized our view of dinosaur behavior. Each summer, crews from the Museum of the Rockies in Bozeman and The Nature Conservancy, which now owns the site, return to dig, conduct field schools, and lead the daily two-hour public tour.

The first stop is the pit, atop a low ridge overlooking the 20-teepee field camp. Inside lies an exposed section of the densest, most extensive dinosaur graveyard known. Perhaps 30 million fossil fragments from 10,000 animals, all duckbills of a species Horner christened Maiasaura peeblesorum, fill a bone bed that's more than a mile long and a quarter mile wide. The site was dubbed "Camposaur" because it was literally discovered in the camp by an uncomfortable volunteer who couldn't stake his tent or sleep comfortably on the bone-pocked ground. One day 76 million years ago, some catastrophic event - most likely a blast of volcanic gas and ash - overtook the duckbill herd, leaving clear evidence that these ancient grazers lived in social groups just as bison or wildebeests do today.

The Rocky Mountains, which shimmer blue in the heat haze only 30 miles to the west, were young and dotted with active volcanoes during the Cretaceous. To the east today, the Great Plains stretch for 400 miles. But the Cretaceous continent was split down the middle by an inland sea that lay 200 miles east of this site. These arid plains were shrubby uplands (grasses had not evolved) laced with streams and with shallow alkaline lakes, milky green and devoid of fish.

It was on these uplands that maiasaurs, small but fearsome Trosdons, and at least one other unidentified dinosaur came to lay their eggs - the first two in colonial nesting grounds.

A few hundred feet from the camp, the guide shows visitors a ravine full of bare mudstone knobs. This was the first of three maiasaur nesting grounds discovered here, from which dozens of eggs, hatchlings, and babies have been removed. These are the finds that led Horner to propose that Maiasaura took care of nestbound babies just as modern birds do.

Visitors can also caravan to Egg Mountain, a low hill three-quarters of a mile away where the first intact dinosaur eggs - those of the six-foot meat eater Trosdon formosus - were found.

The fossil eggs and babies have all been removed to the security of museums. You can see them at the Museum of the Rockies, or in a smaller but worthwhile display at the Old Trail Museum in Choteau. Only the roped-off rim of a Trosdon's mud nest remains, and bits of blackened eggshell that the guide passes around, then carefully retrieves. But from this hill, visitors can usually see crews on hands and knees hoping to uncover some new detail of dinosaur life.

Continued in: Diggin' For Dinosaurs - John Day Fossil Beds

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      Bhaskar 4 years ago

      Hi Paul,I just want you to know that this is the absolute best post I've read from you ever, ealscielpy for newbies. I am still struggling along with no website (I do have a domain hosted with HostGator and on a limited income, just can't get a decent WP site up.) I have foolishly bought several things that I can't work with because I didn't know enough to see that graphics made in PSD format won't work for me. I'm not whining, still plugging, but it helps when the marketers explain things in a way that some of us can understand. I do subscribe to the Real Guys but so much is over my head still.All that to say, THANK YOU for today's blog post. Keep them coming. And, please remember keep it simple sometimes. Thanks

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      Yogendar 4 years ago

      Erin,This image is stunning. Love how peefrct that light shines between them and the adorable pregnant belly. So wish I had one like this when I was pregnant!!! Love it!

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