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A big find

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World’s smallest longneck dinosaur tracks discovered in Morrison

By Emile Hallez

A baby Apatosaurus was trotting through a trickling streambed 150 million years ago in what is now Morrison, leaving tiny footprints that were eventually preserved in the sandstone surface. A boulder containing the tracks was broken off during construction of the Alameda Parkway in the 1930s, sitting for seven decades alongside the road as daily commuters unwittingly drove by.

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That changed five years ago, when the Morrison Natural History Museum brought the sample back to its building. Though the boulder, one of many, was suspected to hold fossilized dinosaur bones, only upon close examination did researchers recently notice the curious, repetitive shapes on the rock’s surface.

“During the collection we began to find fossil footprints on these blocks,” said museum director Matt Mossbrucker, noting that adult-size prints were more readily obvious. “Bones and tracks in the same beds are really unusual. Once we were certain of what we were looking at with the big tracks, we started to find smaller and smaller tracks.

“We’ve been carefully cleaning the blocks. We were overwhelmed with data at first. Everything that we’ve been finding, we’ve been carefully processing.”

Categorizing the tracks as small may be an understatement. The prints, roughly the diameter of softballs, are the smallest by far ever identified for an Apatosaurus and indicate the 40-pound, 6-foot-long baby was about the size of a border collie. By contrast, a full-grown adult of the species, Apatosaurus ajax, reaches about 80,000 pounds and 100 feet in length.

“This is the big announcement: These footprints are the world’s smallest longneck dinosaur tracks. We can tell that by the shape of the footprints. The shapes of longneck dinosaur feet are distinctive,” said Mossbrucker, who presented the groundbreaking findings Monday at the Geological Society of America’s annual meeting in Denver. “The only longneck dinosaur that has a claw that’s this shape — a squared-off claw — is Apatosaurus. … The adaptation is so that the animal can use the claws for digging. Our fossil evidence and fossil soils evidence suggests that Morrison was a fairly arid place in the Jurassic. And this adaptation allowed Apatosaurus to dig out holes so that it could drink water that was just below the surface.”

The Dakota Ridge area is well-known as a paleontologist’s playground, and though findings of Apatosaurus fossils have been plentiful, the recent baby tracks present a handful of new possibilities. Aside from being about half the size of the previous baby Apatosaurus footprints on record, the Morrison tracks indicate the young dinosaur was running — most likely on its hind legs.

“The first thing you notice is the spacing between the footprints. … You have about double distance between foot strike to foot strike,” Mossbrucker said. “That suggests an increase in pace, a functional run.

“What we also don’t see is any evidence of a front paw strike. Now, that could be that when this animal was running, the back paw eclipsed the track made by the front paw. I don’t think it was quite moving fast enough. … It just looks like the animal was running on its back legs,” he said. “There are a lot of firsts right in front of you.”

Also lacking on the sandstone surface is a line bisecting the left and right tracks — evidence that the animal did not drag its tail. Other fossilized tracks have similarly lacked a tail mark, leading paleontologists to reconstruct large sauropod models with the creatures’ tails waving in the air.

“The suggestion that the animal might have been running on its back legs is especially intriguing, because there’s been a debate in sauropod paleobiology as to whether or not these animals kind of rear up on their back legs,” Mossbrucker said. “Apatosaurus has a back leg that’s almost twice as tall as the front leg. … There could be 10 times more muscle mass back here.”

And due to the small size of the prints, researchers are also being dealt a chip of insight into the reproductive habits of the massive dinosaurs.

“By finding these diminutive tracks, we’re able to understand where Apatosaurus chose to reproduce and chose to have its offspring,” Mossbrucker said, explaining that whether the creatures reproduced in another location and later migrated to the Morrison area was previously unknown. “I wouldn’t be surprised at all if some day we dug up a pelvis of an Apatosaur with embryonic remains in it. … They might have given live birth.”

Remains of the species in question have been found in locations over the western U.S. The dinosaurs, popularly known by the obsolete moniker Brontosaurus, had a unique body structure, including lofty necks spaced with air cavities to help reduce weight. And their brains, despite a gargantuan body mass, are estimated to have been about the size of an adult human’s thumb.

“They’re slowly moving, ponderous stomachs, just going from grove of conifer to grove of conifer,” Mossbrucker joked. “That kind of devalues the dinosaurs. … Their bodies are very sophisticated. Their brains just happen to be very small.”

Though the tiny prints mark a unique finding, they are not the first baby dinosaur prints discovered by the museum. Several years ago the world’s smallest Stegosaurus tracks were found, indicating an infant of about 6 pounds. Fully grown Stegosauruses reached about 12,000 pounds.

Mossbrucker presented the Stegosaurus track findings last year at a conference in Switzerland, and the discovery was also featured in Smithsonianmagazine.

The Morrison Natural History Museum, which opened in 1989, is supported largely by funds from the Scientific and Cultural Facilities District.

“If it wasn’t for Jeffco SCFD, we wouldn’t know about these tracks,” Mossbrucker said. “Tax dollars that do something — it’s kind of nice.”

Mossbrucker, 31, has a background in geology and museum education. Also the chief curator, he was hired as the museum’s director in 2006. He worked previously as a volunteer at the museum.

“We have a lot of hungry minds come through here of all ages. And they’re curious about natural history, and we get to show them fossils they can’t see anywhere else on the planet,” he said. “Folks walk away from the museum with a brand new appreciation for the ancient life that’s paraded before us here in Jeffco.”

 

Contact Emile Hallez Williams at emile@evergreenco.com or 303-933-2233, ext. 22. For updates, check www.ColumbineCourier.com.

 

Morrison Natural History Museum

501 Colorado Highway 8

Morrison 80465

Open daily at 10 a.m. Ticket sales for museum tours end at 4:30 p.m.

For more information, visit www.mnhm.org.