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Photo by Tadek Kurpaski. CC BY 2.0.

Museum of Western Colorado Unearths a Jurassic Record-Breaker

Apatosaurus was an enormous dinosaur. That’s something easily said, but can’t be understood without spending time in the shadow of the sauropod. To read that adult individuals of the Jurassic dinosaur reached lengths of 75 feet and had a heft of 16 tons is one thing. To actually be in the presence of those old bones – to see the parts of your own body totally dwarfed by those of the extinct herbivore – is quite another. So I can only imagine the awe paleontologists and volunteers at the Museum of Western Colorado felt as they excavated the largest Apatosaurus leg bone yet discovered.

Museum volunteer Kay Fredette found the first signs of bone poking out of the rock that would lead to the femur in western Colorado’s Mygatt-Moore Quarry. The site, overseen by the Bureau of Land Management, is a Jurassic treasure trove that contains the bones of dinosaurs who went for a drink and instead met their doom.

Julia McHugh, the new Curator of Paleontology at the Museum of Western Colorado, likens the site to “a watering hole in the African savanna.” The quarry’s geology is a major giveaway.

Like other Late Jurassic bonebeds scattered across the American west – such as Dinosaur National Monument, the Cleveland-Lloyd Dinosaur Quarry, and the Hanksville-Burpee Dinosaur Quarry – the Mygatt-Moore Quarry is in the upper part of the Morrison Formation, but with dinosaur bones scattered through a gray shale rather than the tough sandstone of other boneyards. Along with abundant plant material and the orientation of the bones, McHugh says, the shale suggests that the site was once “a continually wet seasonal pond.”

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Allosaurus, the most common carnivore in the Late Jurassic of western North America. Photo by Brian Switek.

The refreshing pool was a major dinosaurian attraction. “We have a lot of animals drawn to this pond to drink water, to eat plants,” McHugh says, “and Allosaurus and other predators were feeding on the herbivores.” That would explain the fact that most dinosaur bones found at the quarry are isolated elements attributed to Apatosaurus, with Allosaurus as a close second, not to mention the toothmarked bones that testify to theropod feeding.

But Mygatt-Moore bones aren’t found articulated with other elements. This isn’t a place to find intact skeletons. Water and dinosaur depredations broke up the bodies. The Mygatt-Moore Quarry, McHugh Says, is an “accumulation of bone that’s been broken up, disassociated, that’s been fed on, trampled down into the mud, and piled up. It’s a far cry from what people see in Jurassic Park.”

So when Fredette found the first signs of the huge femur in the summer of 2010, the fossil looked like an isolated and unremarkable Apatosaurus bone. Although what the paleontologists had discovered wasn’t immediately clear. Initially, McHugh says, the team thought the element was a vertebra – notoriously difficult to excavate due to their delicate anatomy – “and the first reaction was ‘Oh no, not another vertebra!'” But as Fredette and volunteer Dorothy Stewart excavated around the bone they found that this wasn’t part of a dinosaur’s backbone, but a femur. This was the thigh bone of one of the biggest animals to ever walk the Earth.

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Museum of Western Colorado field crew readies the Apatosaurus femur for extraction. Photo courtesy the Museum of Western Colorado.

It took the museum field crew five whole summers to uncover the whole extent of the Apatosaurus bone. When finished, the femur measured six feet, seven inches in length. There isn’t another Apatosaurus femur that can match it.

“The next closest we have is a reference to a six foot long femur” from another Apatosaurus, McHugh says, making the Mygatt-Moore specimen the largest known. And since paleontologists know Apatosaurus from relatively complete skeletons, as well as the relationship between the animal’s femur size and total length, McHugh and former Museum of Western Colorado paleontologist John Foster estimate that their big Apatosaurus stretched between 80 and 90 feet in length.

Removing such a massive bone is a monumental challenge. For starters, the excavators kept running into more bones as they worked to create a trench around the bone. The quarry is a “jackstraws pile” of bone, McHugh says, noting “We had to take out three other jackets just to get the femur ready.”

That was the easy part. The bone was so massive that the museum had to create special wooden struts to keep the bone from twisting during transport. “We literally built a ladder and plastered it into the jacket,” McHugh says, which kept the bone stable as a backhoe from the City of Fruita Public Works tugged the block out of the quarry and placed it onto a truck last week. The whole operation went smoothly. “It was really quite a dream roll,” McHugh says.

The crew weighed the truck before and after the jacket was placed on. The total weight – bone, rock, wood, plaster, and all – was 2,800 pounds.

Whether or not there’s more of the giant in the quarry is a mystery.  A lower leg bone of about the right size to match was found near the femur, McHugh says, but so far the quarry hasn’t turned up any parts that can be definitively attributed to the same dinosaur. “We think we have an isolated femur,” McHugh says, and the calling card of another dinosaur in the same jacket might explain why that’s so. “When we flipped the jacket over and put it in the trailer,” McHugh says, “there was an Allosaurus tooth in the bottom mud.” What may be the largest Apatosaurus ever, preserved in rock for 150 million years, comes down to us as leftovers.