There is no mystery about why so many vertebrates burrow, either, with a number of perfectly fine evolutionarily based reasons to excavate a home and live in it. For one, parents can raise their young in a safe, quiet place, away from the prying eyes and noses of predators. In upland environments, where undergrowth and trees constitute fuel for wildfires, burrows act as shelters from those natural hazards. Burrows also maintain equitable temperatures year-round. Thus, in places with extreme daily or seasonal fluctuations in temperatures—especially in deserts or polar regions—burrows are not too hot, not too cold, but just right. For a few animals, such as lungfish or some toads and turtles, these burrows serve as aestivation or hibernation chambers in which they conserve energy and water during lean times, sometimes for months or years.
Given this burrowing norm in modern vertebrates, including the living relatives of dinosaurs—birds and crocodilians—along with the incredible diversity of dinosaurs during their 165-million-year history and the myriad complex behaviors paleontologists had inferred from their bones and trace fossils, one would think that someone, somehow, had thought of dinosaurs living underground. Turns out that, yes, a few people had thought of subterranean dinosaurs, although not in the formal sort of way that stood a chance of acceptance in the face of fierce skepticism by the paleontological community.
Indeed, the case for burrowing dinosaurs was not helped in that one of the first people to publicly propose it was the bombastically flamboyant and iconoclastic gadfly of dinosaur paleontology, Robert (“Bob”) Bakker. More P.T. Barnum than Barnum Brown, Bakker is well known as a great populist of dinosaur lore, with many of his views summarized in his still-intriguing and never-dull 1986 book, The Dinosaur Heresies. The popularity of this book stemmed at least in part from his gleefully sticking fingers, toes, and other appendages into the eyes of stodgy paleontologists who viewed dinosaurs as up-scaled, dull, and cold-blooded variations on lizards. As a result, Bakker is often credited for rebooting dinosaurs in the public imagination as active, hot-blooded animals that acted much more like birds or mammals. (Bakker was not original in this concept, though. Instead, paleontologists rightfully laud his Ph.D. advisor, John Ostrom, as the person who built the foundation for the “birds are dinosaurs” argument in the early 1970s.) Despite the hit-and-miss nature of Bakker’s assertions and conjectures about dinosaurs in his book, it nonetheless ensured him a near-permanent presence on dinosaur documentaries from then on, in which he disagreed with nearly every other paleontologist. After all, conflict sells.
Anyway, Bakker speculated that Othnielia and Drinker—both of which were small hypsilophodont dinosaurs that lived during the Late Jurassic Period—must have been living in burrows of their own making. He did not present much data to support this supposition—like, say, trace fossils of their burrows—but instead based it on how these dinosaurs were little and some of their carnivorous contemporaries were big. Thus, burrowing would have been a great way for these cute, innocent herbivores to avoid getting chomped by nasty, ravenous predators. Bakker mentioned this idea of burrowing ornithopods in a 1996 paper, and promoted it in talks afterwards.
As mentioned earlier, digging was proposed as early as 1985 to explain sauropod feet, which was confirmed about two decades later by their nests. Troodon nests found in the 1990s also showed that some dinosaurs could dig. Based on their anatomies, the small theropod Mononykus was mentioned as a possible burrowing dinosaur, as was the small ornithopod Heterodontosaurus. However, for Mononykus, its forearms seemed too short to do much more than break up soil.
Sure, all of this talk about whether dinosaurs lived underground or not is very interesting, but runs the risk of becoming an esoteric subject that is only appreciated by the most dedicated of dinosaur fan-boys or fan-girls. Ultimately, one has to ask that most underused but instructive of questions in science: So what? Other than being able to exclaim “We got ourselves a burrowing dinosaur! Yee haw!” what meaning did such a discovery have in a much larger sense, relating to the Mesozoic world and modern times alike?
Here is one word that should warrant sitting up and paying attention: extinction. As discussed earlier, most scientists now accept that a large meteorite hit the earth about 65 mya, landing in the area of what is now the Yucatan Peninsula. Apart from those that died from the direct hit, this impact likely had dire consequences worldwide for life in the oceans and on land, including a dust cloud—mixed with soot from forest fires—that would have blocked out sunlight for several years. Take away sunlight and terrible things happen to most ecosystems. For example, many photosynthesizing algae in Cretaceous oceans were adapted to full sun year-round, as well as land plants that grew and reproduced best when illuminated. Most of these species would have died within a year or two. Plummeting global temperatures also would have contributed further to this massive die-off of photosynthesizers, as the dust cloud would have acted like the ultimate sunscreen, preventing infrared radiation (heat) from making it to the earth’s surface.
Far fewer photosynthesizers meant less food was being produced at the base of nearly all ecosystems, which meant animals that ate these photosynthesizers died too. After that, animals that ate the animals that ate photosynthesizers died. (Although I also imagine scavengers very briefly having enjoyed an end-of-the-world party, greedily chowing down on a sudden bounty of dead animals.) Think of the food web holding together an entire ecosystem having one thread after another ripped apart. We witness such trophic cascades today in ecosystems that undergo a precipitous shock such as a major forest fire, volcanic eruption, or tsunami.
Yet the end-Cretaceous was far worse because it was global in scale, affecting the food supply for animals in the seas and on the continents alike. Also, animals living at the end of the Cretaceous with the misfortune of having large body sizes had greater caloric needs, meaning bigger was not better for them in this situation. So large animals likely died the soonest after the impact, and yes, I am talking about dinosaurs, including much beloved ones such as Triceratops, Tyrannosaurus, Hadrosaurus, and Ankylosaurus, to name a few. All non-avian dinosaurs had no way to survive such terrible conditions, even the ones living in polar environments. Thus they went extinct, while some of their somehow luckily adapted descendants—birds—survived and thrived, giving dinosaurs a second chance in a radically transformed world. Among land-dwelling vertebrates, mammals, crocodilians, turtles, lizards, and amphibians also got tickets to ride on the Cenozoic Express; their lineages evolved and had their own more modest extinctions since.
Now throw a post-Cretaceous monkey wrench into this scenario in the form of burrowing dinosaurs. Burrowing was a strategy that at least a few small non-avian dinosaurs could have used to survive an end-Cretaceous “meteorite winter.” Just recall a few of the advantages of a burrowing lifestyle, and suddenly an apocalypse becomes ever so slightly more optimistic: equable temperatures year-round, protection from hungry predators (made more desperate from a lack of food), a room to hibernate, and—most important from a delaying-extinction standpoint—a safer place to raise offspring.
It is not a coincidence that people of the 1950s and 1960s, to cope with nuclear-tinged fears of the Cold War, built underground bomb shelters in an attempt to assure themselves they would survive a nuclear exchange between the Soviet Union and the U.S. More realistically, though, such shelters only would have delayed the inevitable, and perhaps small burrowing dinosaurs lived only a few years longer than their larger, non-burrowing relatives before succumbing to the wretched after-effects of a massive meteorite hitting their planet. Nonetheless, knowledge that at least a few dinosaurs had the means to live just a few more years bolstered credence to a concept of dinosaurian resilience, defying long-held and unfair stereotypes of maladapted ineptitude summarized in the phrase “dead as a dinosaur.”
Lastly, burrowing is a behavior present in every major group of vertebrates that made it past the mass extinction: birds, mammals, crocodilians, turtles, lizards, and amphibians. Could burrowing have b
een one of the essential tools in a Swiss Army knife of adaptations possessed by terrestrial animals, which came in handy once disaster struck? Is it possible that an ability to burrow has allowed certain lineages of animals, such as crocodilians, to make it past multiple mass extinctions in earth’s history? How might burrowing aid future generations of modern animals to survive human-influenced extinctions? All such questions swirled around the revelation that at least maybe one dinosaur burrowed well before the last breaths of the rest of its kin.
Digging up the Diggings of a Dinosaur
In September 2005, one month after learning about this dinosaur and its possible burrow, my friend Dave thought it would be a good idea to have me come out to Montana to look at the burrow with him. Despite my hesitation—thinking about it for as long as three seconds—I found no reason to disagree with him. Fortunately, he had research funds available to buy me an airline ticket, so I flew from Atlanta to Bozeman, and within a day of my arrival he drove us to the field site. It was about three hours away in southwestern Montana, an area of the country I had never before seen.
The nearest town of any notable size near the field site was Lima, which has a population of just more than two hundred. Travel from the claustrophobic overpopulation and traffic of metropolitan Atlanta to the unpeopled expanses of “Big Sky” country requires some mental adjusting for me, but is always a welcome kind of different. During the drive, Dave and I caught up on what had been happening in our lives the past few years, but he also filled me in on some of the details about what led up to my seemingly improbable presence there with him, united by an ichnologically significant find.
This region of Montana had outcrops of the Blackleaf Formation, which was composed of mudstones and sandstones formed by rivers during the middle of the Cretaceous Period, about 95 mya. Few fossils of any kind had been reported over the years from the Blackleaf, but among those were some scrappy and vaguely identified dinosaur bones. Apparently, this was enough incentive for Dave to decide it was worth prospecting for more dinosaur material. So during the summer of 2005, he gathered a group of experienced dinosaur-bone spotters and pickers to accompany him to places with extensive exposures of the Blackleaf Formation, some of which were on U.S. Forest Service land.
During one of their searches, one of the crew, Yoshi Katsura, spotted a few bones protruding from a hillside. Once they determined that these were from a small ornithopod and that more remains were probably just underneath the surface, they decided to collect them. They excavated around the potential skeleton, not knowing exactly what was there, but leaving plenty of rock around the exposed bits so that the specimen would be as complete as possible. The field crew was happy about the find because it meant that the Blackleaf might have dinosaur skeletons after all. Even better, because it had been explored so little, it might even hold species previously unknown to science.
It was during their digging around the specimen, and Dave taking notes about the sedimentary rocks immediately around the bones, that he began to notice something peculiar. What he experienced is a feeling shared by many field-oriented scientists, expressed succinctly in a quote by famed science fiction writer Isaac Asimov: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but ‘That’s funny …’” Dave noticed that the bones were concentrated in an egg-shaped mass of sandstone, which also was better cemented than the surrounding and overlying mudstone. He looked above where the bones were located, and saw that the sandstone continued upward as a vertical structure, and then turned to the right. That’s funny, he thought. This structure had to have been a significantly sized hole that was later filled by sand from above, making a natural cast of it. What would have made such a hollow form during the Cretaceous Period, and one that also had a dinosaur at the end of it? Even better, the dinosaur was a small ornithopod. Although Dave could not yet identify its species, it reminded him of Orodromeus, another small ornithopod dinosaur he had seen often in northwestern Montana. As mentioned before, he had suspected Orodromeus was a burrowing dinosaur, but lacked the key supplementary evidence for it: namely, a burrow. But now it looked like he might have both a trace fossil and its tracemaker together.
Fortunately for him, and for vertebrate paleontology in general, Dave is a careful observer and cautious with his interpretations. He knew that much more evidence needed to be gathered to address a few key questions. For example, was this odd structure really a burrow, or just a hole in the ground made by some other process, such as erosion along a riverbank? Even if it were a burrow, was the dinosaur in it the same one that made it? If this dinosaur was not the burrowmaker, what was it doing there, and how did it become entombed in the burrow? Did it die in there, or did it die somewhere else and was later placed in the burrow, whether by river currents or a predator that used the hole as a cache?
Dave shared a few of these thoughts with the rest of the field crew at the time, and once the specimen was jacketed and detached from the earth, they carried it out on a stretcher, a victim of Cretaceous circumstances undergoing its last journey. He and everyone else then had to wait for the skeleton to be prepared at the Museum of the Rockies in Bozeman, with the hope that its bones would tell Dave more secrets.
However, before this specimen was separated from its entombing sediment in a preparatory lab, Dave had used me—in an intellectually sordid way, I might add—to test his hypothesis. His exclusion of much information about the find, such as its age, location, interpretation of the original environment, and the oh-so-minor detail that it involved a dinosaur, was purposeful and sneakily effective. Still, it reduced the possibility of bias in my assessing the photo and ensured that I would interpret it solely on its face value as a possible trace fossil and not as something I was hoping to see.
Once in Lima, Dave had prearranged a meeting with a friend and his wife who were passing through Montana that week. Both were non-geologically inclined but able-bodied and eager to help us, so the four of us went to the field area. Like many dinosaur discovery sites in the western U.S., getting there involved getting off pavement and going down a dirt road, with cow pastures on either side. Once on U.S. Forest Service land, we parked, unloaded shovels, picks, and other human-powered earth-moving tools from Dave’s car, and began walking across a sparsely grassed expanse with rolling hillocks and minor gullies.
I scanned the ground as we traveled, looking for pieces of sandstone weathering out of the mostly weathered mudstone bedrock in the area, stopping to pick up and look at the more interesting samples. Some of these sandstone bits contained small holes that led to longer tubes, which I recognized as burrows. These trace fossils were made by invertebrate animals—probably insects—living with the dinosaurs in this area about 95 mya. Other than these ichnologically motivated pauses, the stroll to the discovery site was surprisingly short, taking only about twenty minutes. We stopped when Dave said “This is it” and started digging.
In accordance with their permit for working on U.S. Forest Service land, the field crew had followed a “Leave No Trace” edict by burying the discovery site and restoring it to an approximation of what it looked like before their arrival. I found this amusing on many levels. For one, the disturbed soil there betrayed the crew’s former presence, as did the dried flecks of white plaster on the ground, which had persisted despite whatever weather had taken place between their jacketing the dinosaur in July and our visit in September. I also pondered the metaphysical implications of the Forest Service imploring us to minimize our tracemaking, which involved digging, while we investigated a trace fossil from a digging dinosaur. I further wondered whether such thoughts might generate a self-absorbed poem, read aloud later at an urban coffeehouse and accompanied by clicking fingers of appreciation. Fortunately for much of the world, this ambition slipped away as soon as our shovels chopped the ground and we started to pile weathered mudstone around us.
At some point in our careful excavation, we reached the surface where Dave and t
he others in the field crew had placed plastic sheets above the possible burrow and the not-so-final resting spot of the dinosaur skeleton. From here we became even more vigilant and began to clear away some of the softer reddish-green mudstone that enveloped the more solid whitish sandstone filling the presumed burrow. However, this work was cut short as the sun thwarted our objective by falling toward the western horizon. We hastily covered our previous efforts with a shallow layer of sediment. Dave’s friends had to leave the next morning, but he and I would come back to see what lay beneath. That night, we enjoyed a meat-and-pie-laden dinner at a restaurant in nearby Dell, Montana—The Dell Calf-A, with its name written cursively with a neon “lasso”—and we slept at a cheap motel with running water, electricity, and an intact roof overhead. In other words, we were experiencing the height of luxury for field paleontologists.
In the morning, Dave and I said good-bye to his friends, who were off to see other parts of Montana, and he and I returned to the field site to resume our handiwork. The fresh familiarity of the previous day made the walk seem shorter, although we also moved a little more quickly in anticipation of learning something new that day. Once at the site, we first used shovels to uncover the thin layer of sediment dumped on top of our objective, but soon set these aside once the white sandstone emerged. Replacing the shovels with rock hammers—pointy ends first—and small hand picks, we began breaking the mudstone closest to the sandstone. Our goal was to reveal this mystery structure in all of its full three-dimensional glory, which would answer many of our questions or, more likely, generate a lot more.
While we worked, I kept comparing the whitish structure in front of us to the flat digital image Dave had sent me the previous month. It definitely was a big, sandstone-filled tube, cutting across the mudstone around it. Starting from above, it turned into a horizontal segment, and then twisted down to where the dinosaur bones resided until just recently. Despite this torsion, its width stayed constant throughout, at about 30 to 35 cm (12–14 in). If emptied of its sandstone fill and turned into a hollow tube, it would have been too tight of a fit for Dave or me to crawl into (especially after the previous night’s meal). But it would have been perfect for a five-year-old child, or a small dinosaur.
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