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Tracks made by a medium-sized theropod on a slab of rock just outside of Moab, Utah. Photo by author.

Two years ago, I visited the American West for the first time. I was immediately hooked. Seeing the morning sunlight hit the dinosaur-rich Jurassic rock of northern Utah’s Dinosaur National Monument was what really did it for me. When I saw that, I knew that I had to move out West, and a few weeks ago I settled in Salt Lake City to devote myself to writing about the prehistoric past. I now live right in the middle of dinosaur country—some of North America’s most productive and important dinosaur sites are within a day’s drive—and this past weekend I had the chance to visit a few located just a few hours from my new hometown.

At the southern tip of the series of highways making up the Dinosaur Diamond, Moab is right in the middle of dinosaur country. The geologic strata of the area is piled high with sedimentary rock from the heyday of the dinosaurs—from the Late Triassic through the Early Cretaceous in many places—and, at a few spots, vestiges left by dinosaurs can be easily seen. One such place is right along Potash Road, just outside Moab itself.

Left in Navajo Sandstone dating to about 190 million years ago, the Potash Road dinosaur tracks come from a time tens of millions of years before the famous Jurassic fauna of the Morrison Formation. The world was quite different then. Today the tracks rest in two slabs perched on a rocky hill within a stone’s throw of the Colorado River, but when the tracks were made the area was a sandy shore of a lake.

The tracks were left by at least three different size classes of theropod dinosaurs. Two slabs of rock contain relatively small tracks paleontologists have assigned the name Grallator, slightly bigger tracks known as Eubrontes and even larger footprints, according to an interpretative sign at the site, were left by Allosaurus. This last attribution is probably a mistake. Allosaurus lived later in the Jurassic—around 155 million to 150 million years ago—and, unless an animal dies in its tracks, paleontologists can’t be certain what species created them. That’s why tracks are given their own names. In fact, it is possible that at least some of the tracks were made by dinosaurs of the same species but belonging to different ages. We may never know for sure, but the Potash Road tracks are still wonderful monuments from a time when dinosaurs were at home in Utah. I can’t wait to visit more of them.

Modeling a dinosaur den: a half-size PVC model of a Oryctodromeus burrow used to tests scenarios for how they became preserved. From Woodruff and Varricchio, 2011.

Oryctodromeus isn’t exactly a household name. A small, herbivorous ornithopod found in the Late Cretaceous rock of western North America, it was the sort of dinosaur most often depicted as being prey for charismatic carnivores. But there was at least one aspect of Oryctodromeus that made it particularly interesting—this dinosaur may have lived in burrows.

Based on the context of the rocks they are found in, we know that dinosaur bodies were preserved in a variety of different environments. Some bodies were covered up by seasonal floods, other dinosaurs were washed out to sea, and dinosaurs even died in death-traps created by the footprints of even bigger species. But until Oryctodromeus, dinosaurs had not been found in fossilized dens.

The fact that the small dinosaurs had been buried within a burrow was made clear by the details of their den. At the end of an S-shaped tunnel was a large chamber that had been dug into three different layers of mudrock and later filled with sandstone. The fact that an adult and two juvenile Oryctodromeus were found in the sandstone confirmed that this was a den that had been flooded by a slurry of water and sandy sediment.

But were the dinosaurs buried inside their den, or had their bodies just been washed inside? The dinosaur bones were jumbled up rather than lying in articulated poses on the burrow floor. This left the details of their preservation unclear. In order to solve this mystery, paleontologists Cary Woodruff and David Varricchio created a half-scale model of the original burrow with PVC pipes and conducted experiments with rabbits to see what sort of scenario would best account for the way the dinosaur fossils had been preserved.

The paleontologists ran thirteen trials by filling their artificial burrow with a mixture of water, clay, and sand. Rather than using whole rabbits, though, Woodruff and Varricchio only used disarticulated skeletons. This is because no Oryctodromeus bones were found in their natural positions, hinting that the dinosaurs died, decomposed, and had mostly fallen apart before their preservation. By the time the den was flooded, the dinosaurs had already turned into piles of bones (regardless of whether their skeletons were inside or outside the burrow at the time of the event).

Woodruff and Varricchio modeled the different ways the bones could have found their way into the den by running a variety of tests. In some trials the bones were placed in the burrow, while in others they were included in the sediment mix used to fill the artificial den. Each setup produced a different distribution of bones in the PVC chamber.

Six different trials with differing conditions all created the kind of elevated, dispersed assemblage of bones found in the Oryctodromeus burrow. Bones were initially inside the chamber for four of these trials, but were outside the burrow and contained within the sediment, respectively, in the other two. While this evidence supports the idea that the dinosaur bones may have been inside the den when it was flooded, it remains possible that the bones were washed in from outside.

If the dinosaur skeletons really were washed into the burrow from outside, however, Woodruff and Varricchio argue, it is strange that the bones of an adult and two juveniles should be found together. Furthermore, bones transported by sediment-filled floods are often broken and abraded, and there are no signs of such destructive transport on the Oryctodromeus fossils. The hypothesis that the Oryctodromeus bones were already inside the den remains the best-supported idea. Woodruff and Varricchio caution that further investigations are required to understand how these dinosaurs—and other den-dwelling fossil vertebrates—became preserved.

References:

WOODRUFF, D., & VARRICCHIO, D. (2011). EXPERIMENTAL MODELING OF A POSSIBLE ORYCTODROMEUS CUBICULARIS (DINOSAURIA) BURROW PALAIOS, 26 (3), 140-151 DOI: 10.2110/palo.2010.p10-001r

Tracks made by an Iguanodon-like dinosaur at the 100 million year old Dinosaur Ridge tracksite. Image from Flickr user Matthew Saunders.

The Cretaceous dinosaur tracks scattered along Morrison, Colorado’s Dinosaur Ridge have persisted in the fossil record for 1o0 million years, but they are now in danger of being lost forever. Exposed on the surface, the tracks are being eroded away bit by bit, and a local controversy over the aesthetics of the Colorado landscape has complicated efforts to preserve these tracks.

The fossil sites of Dinosaur Ridge come from three different time periods. There is a 150-million-year-old dinosaur bone quarry, a 100-million-year-old track site, and a 68-million-year-old track site. It is the 100-million year-old set of tracks, dominated by footprints made by an Iguanodon-like dinosaur, that is at the center of the debate. Regular freeze-thaw cycles and exposure to the elements have been gradually destroying the tracks. According to an article in the Denver Post, the nonprofit group Friends of Dinosaur Ridge has proposed that a canopy of high-tech fabric be erected over the site to help prevent further damage. The trouble is that this proposal runs counter to Jefferson County’s official Front Range Mountain Backdrop policy which forbids structures that would obscure or detract from views of the mountains. An article on the debate from LJWorld.com reports:

“The plan that they came up with includes structures and it just doesn’t work,” said Kathryn Heider, a spokeswoman for Jefferson County, which owns the land where the tracks are located 15 miles from Denver. “It doesn’t mean we don’t want to preserve the footprints. It just means we don’t want structures on the backdrop.”

Discussions about what can be done to save the tracks are ongoing, but there is not much time left. Based upon damage already done to the tracks, the Friends of Dinosaur Ridge project that the tracks have only about 10 to 15 years before they are lost. Their destruction would rob a natural treasure from scientists and the public alike. I hope that an amenable solution to this dilemma can be found soon.

A snapshot of the world during the Cretaceous, about 90 million years ago. From Wikipedia.

Paleontologists are constantly reminding themselves of the incompleteness of the fossil record. What has been preserved is only a small fraction of all the organisms and environments that have ever existed. This makes detecting evolutionary patterns a bit of a challenge. In a presentation given at this year’s Society of Vertebrate Paleontology conference, Smithsonian paleontologist Matt Carrano dug into the long-standing question of whether changes in sea level triggered changes in dinosaur diversity.

Over the past few decades, paleontologists have produced a number of graphs depicting dinosaur diversity through time. They show a general trend toward increasing diversity from the Late Triassic through the end of the Cretaceous, but with a few fluctuations in between. The rise and the fall of the seas has been proposed as one of the drivers of these changes. Perhaps, it has been hypothesized, high sea levels might have favored dinosaur diversity by fragmenting some terrestrial habitats or isolating one area from another while simultaneously creating more environments where dinosaurs might be preserved. Then again, it has also been suggested that dinosaur diversity might go up when sea levels are low since there would be a larger land area. In order to detect whether any such trends existed, the scientists looked at the occurrence of about 749 dinosaur species through time and space, noting where paleontologists have gone looking for their bones, as well.

What the Carrano and his colleagues found was that the fluctuations in sea level did not influence dinosaur diversity as we know it today. Our perspective of dinosaur diversity is significantly shaped by where paleontologists have gone looking for fossils, the amount of effort expended there, and also by places that have yet to be extensively studied. Dinosaurs might be more plentiful and easier to find in Cretaceous rocks than Triassic ones, for example, which would account for why dinosaur diversity differs between the two time periods. Any scientific work proposing to look at dinosaur diversity has to take these sampling biases into account.

This is not to say that sea level change did not or could not have influence dinosaur diversity, though. Rising sea levels could have created island chains and other geographical pockets that could have driven dinosaur speciation, or low sea levels might have allowed dinosaur species to range more widely. (We know, for example, that the Western Interior Seaway caused Cretaceous dinosaurs to evolve in different ways in the eastern and western parts of North America.) Detecting these signals from the fossil record, however, will require in-depth sampling and a recognition of the way in which our search for dinosaurs skews the picture of their diversity. As stated by the authors of the paper that was the basis for the SVP presentation: “Considerable future work is required to establish how sampling biases may affect proposed long-term diversity trends and mass extinction events in the terrestrial realm.” If paleontologists want to get at the big picture of dinosaur diversity, they need to look at these biases and get digging at places which are still poorly known.

References:

Butler, R., Benson, R., Carrano, M., Mannion, P., & Upchurch, P. (2010). Sea level, dinosaur diversity and sampling biases: investigating the ‘common cause’ hypothesis in the terrestrial realm Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.1754

The shell of the fossil snail Lioplacodes embedded in the coprolite of an herbivorous dinosaur. From the Lethaia paper.

One of the many reasons I love paleontology is that every now and then I stumble across a paper on some aspect of ancient life I had never considered before. There is much more to the science than descriptions of new species, and one of the studies that most recently caught my eye carried the title “Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana.”

As reported in the 2009 study, paleontologists digging at a 76-million-year-old site within the well-known Two Medicine Formation have found more than 130 snail specimens closely associated with—and sometimes even within—the fossilized feces of herbivorous dinosaurs. Scientists had long recognized that the snails were present in the same deposits as the dinosaurs, indicating that they shared the same habitat, but no one had systematically documented interactions between the large vertebrates and the small gastropods. In fact, up to seven different snail taxa were found in close association with the dinosaur coprolites. Apparently dinosaur poo was a regularly-used resource by many species of snail.

The occurrence of snail fossils within the dinosaur dung was also used by the scientists behind the study to reconstruct what kinds of habitats the animals were living in. Since the most common snails on and within the coprolites were terrestrial snails, the authors of the study propose that the dinosaurs left their droppings on dry land before their feces were subsequently flooded (which would have filled in dung beetle burrows also seen in the coprolites). Although they noted that some of the snail shell fragments within the coprolites could have come from snails that were accidentally ingested while the dinosaurs were eating leaves and rotting wood, at least half of the snail fossils were intact and show no signs of being digested. This suggests that the snails made their way to the dino pats after they were deposited, with the dinosaur feces providing warm, wet, food-rich mini-environments that the snails could comfortably exploit.

CHIN, K., HARTMAN, J., & ROTH, B. (2009). Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana Lethaia, 42 (2), 185-198 DOI: 10.1111/j.1502-3931.2008.00131.x