Dinosaurs Without Bones (21 page)

With the removal of each piece of mudstone, the structure became better defined and took shape. Once done with our handiwork, we stood up and then walked around it to gain a fuller perspective. It was a curious object. For starters, the horizontal segment joined with a gently inclined one above, and another inclined one below, making it just more than 2 m (6.7 ft) long. The last segment joined with the enlarged part of the sandstone where the dinosaur had been. Sure enough, just as I had visualized earlier, the whole thing was semi-helical, close to an “S” or a “Z” lying on its side. Looking at it from above, we could see it made an abrupt right turn, and then a left, ending at the spot where the dinosaur had been taken out. The first two segments were about equal in length, 60 to 70 cm (24–28 in), and their widths were within a narrow range of 30 to 38 cm (12–15 in), a little bit taller than wide. The gentle inclines for each of the segments added up to a vertical drop of about 50 cm (20 in), which was about the length of my lower leg.

The field crew had truncated the lowermost part of the structure when they took out the expanded mass of rock and its dinosaur in the supposed burrow chamber at its end. Despite the absence of this chamber, we drew its approximate outline on the ground, which helped complete our understanding of the entire structure. This was a real burrow all right, and with a form and dimensions that made sense as a burrow. We started to talk about how this geometry would have worked for a small dinosaur moving through it, tail included. We also wondered whether the dinosaur collected from here, once prepared, would turn out to be too large, too small, or just right for this structure as the burrowmaker.

But this is what really got me excited: the structure had small projections of sandstone poking from each of the two corners of the bending tunnel. On the upper turn, a bundle of a half-dozen horizontally oriented narrow sandstone cylinders—each about the width of a pencil—suggested that something else had been added on to the main structure. A horizontal sandstone cylinder about the width of a baton stemmed from the lower turn. “There they are,” I said to Dave. “Commensal burrows.” These were likely small
burrows that also had been hollow spaces connected to the main burrow and filled in by the sandstone. Insects could have made the upper cluster of burrows, whereas the lower burrow was more appropriately sized for a small Mesozoic mammal. The study of modern traces—also known as
neoichnology
—had been kind to us, with the gopher tortoise burrows of Georgia supplying a sensible explanation for these seeming aberrations.

We took as many measurements and photos as we could manage, and each of us drew labeled sketches in our field notebooks. Neither of us planned to ever come back there, so we made sure to gather enough information to analyze what was there and later write a coherent story about it. Once satisfied with our data collection, we placed plastic sheeting over the structure and buried it as best we could. By interring the structure instead of leaving it out in the open to weather, we increased our chances of it being there for us to study again someday if necessary. Furthermore, given the GPS coordinates and our descriptions of how to find it, future generations of other paleontologists could investigate it, no doubt aided by new technologies or knowledge that might lend further insights on what we so crudely described in 2005. “Leave No Trace” took on a duplicitous meaning as we covered this fossil burrow and prepared to say good-bye to it.

During the drive back to Bozeman, Dave and I discussed and debated more about what we had just seen, felt, and studied. By the end of the day, though, we were nearly convinced that this sandstone-cast feature was indeed a large burrow. Yet we also knew that we needed much more information before declaring that this was the world’s first known dinosaur burrow. It was time to take a closer look at the dinosaur found in the burrow, and find out whether it had died in a burrow of its own making or not.

Documenting the Discovery of a Digging Dinosaur

Meanwhile, the jacketed dinosaur was still in its presumed burrow chamber, surrounded by plaster and sitting in the basement of the Museum of the Rockies in Bozeman. After I left Montana and
went back to life in Georgia—which did not involve excavating dinosaur burrows—I inquired every month afterwards about when the dinosaur skeleton was supposed to be freed from its jacket and sandstone matrix. My impatience was understandable. I felt as if Dave and I had done a thorough job diagnosing the burrow and we really needed the dinosaur to complete the study. Nevertheless, the Museum of the Rockies is a very busy place for dinosaurs, and our little ornithopod was sitting in line behind a number of other, much larger dinosaurs, which accordingly take much longer to prepare.

Finally, at one point, Dave told me that only a
Triceratops
skull preceded our specimen in the queue. Having this big, sexy, and geologically younger dinosaur prepared before ours seemed a little unfair to me, like the doorman to an exclusive club lifting a rope to let it in while all of the not-so-hot dinosaurs, including our little ornithopod, stood out in the rain. Also, the preparator assigned to our dinosaur was well known for her precise and meticulous craft in separating dinosaur bones from rocks. Thus we began to talk about the amount of time it would take her to complete the extraction of a
Triceratops
skull, which changed the meaning of “
Triceratops
skull” to a unit of time in our minds.

Our patience was tried but rewarded, and big-time. Preparation of the dinosaur yielded a few shocks, but the good kind. As I told people later, it was like celebrating Christmas on your birthday. The first shock came when Dave sent me an e-mail message in which he relayed the news from Montana:

There’s another dinosaur in there, a juvenile
.

The adult dinosaur had a youngster in the presumed burrow chamber with it, and it was of the same species. Although not as complete as the adult, some of its limb bones were there and they matched the adult’s bones in form. Based on relative proportions of their limb bones, the juvenile was about 55 to 60% the size of the adult, the equivalent of a growing teenager who was still hanging out in the burrow with Mom or Dad. This discovery thus had huge
implications related to dinosaur behavior. An adult with a half-grown juvenile in a burrow was not only evidence of denning—a previously unknown behavior in dinosaurs—but also of extended parental care. The latter synced beautifully with previous work done by Dave’s advisor, Jack Horner, who had proposed that
Maiasaura
raised its young in their nests until they were large enough to fend for themselves.

That was exciting just by itself. So you can imagine how it felt to receive additional news a week later from Dave that said this:

There’s a
second
one [juvenile].

This called for another happy dance. It is hard enough to find one dinosaur, let alone three of the same species, and possibly from the same family, all together. Although this specimen was also incomplete, enough of its bones were there to distinguish it from the other, and their matching bones were the same size. These dinosaurs could have been brother and sister to one another, and might have been entombed together with one of their parents in a burrow. This revelation strengthened our denning hypothesis, albeit while also conjuring sad thoughts of parental loss.

More pertinent information came in as Dave studied the bones, looking for anatomical clues that might tell him whether the adult dinosaur was capable of burrowing. Once he finished his study, three relevant traits stood out. First, one of the bones integral to its shoulder—the scapulocoracoid—had attachment sites for large muscles. This meant it had the right musculature for using its forelimbs in burrowing, similar to that of modern burrowing mammals like armadillos. Second, the bone on the front of its snout—the premaxilla—was fused, an unusual feature in a dinosaur. This reinforced its nose, making it easier to use as a small spade to augment the dinosaur’s busily digging hands. Third, when compared to its closest relatives (other small ornithopods), its hip had an extra vertebra. For example,
Orodromeus
had six vertebrae in its hip, whereas this newcomer had seven. Dave surmised that this
additional vertebra strengthened its hip, which the dinosaur used as a brace while digging. As anyone might notice while watching his or her dog dig under a neighbor’s fence, dogs anchor themselves into digging position with their rear legs while scratching enthusiastically with their front feet; only later are the rear feet used for excavating. It was now easy to imagine that this dinosaur, with three adaptations for a burrowing lifestyle, would have behaved similarly.

So, like a burrowing dinosaur cutting through substrata, we uncovered even more evidence that connected the remains of this dinosaur to its probable trace fossil. Using the bones to reconstruct the size of the dinosaur, Dave calculated that it was just more than 2 m (6.7 ft) long, with much of that taken up by its tail. Its torso (from hips to shoulders) length was about 70 cm (28 in) long; its body was 26 to 30 cm (10–12 in) wide, and its mass (weight) somewhere in the range of 20 to 30 kg (44–66 lbs). Take away its tail, and it was about the size of a collie. When documenting the burrow at the field site, Dave and I had noted the regular lengths of tunnel segments, and that it was slightly higher than it was wide. This got us to thinking about “burrow fit.” In other words, how easily could this dinosaur squeeze into it and turn in the tunnel?

If the burrow was too small for this dinosaur, then we had to come up with a very complicated (but still possible) scenario in which one adult and two juveniles of the same species had their bones deposited and buried together in some other unknown animal’s burrow. If the burrow was too big, then that suggested the adult and its young were squatters, taking over a burrow made by another, larger animal: a Cretaceous version of the “Occupy” movement, 95 million years ahead of its time.

So let’s look at those numbers again. The torso length of the reconstructed dinosaur was about 70 cm, and the tunnel lengths were 70 cm. The width of the burrow was 30 to 38 cm, and the width of the dinosaur was 26 to 30 cm. So far, so good. But was there a way to calculate the weight of an animal based on its burrow? Why, yes, there was. In a study published the same year the dinosaur was discovered (2005), a zoologist compared the
cross-sectional areas of burrows made by a wide variety of modern burrowing animals to the weights of their makers and found a positive correlation between these. He had even provided a handy formula, which we applied to the cross-sectional area of our burrow. Taking into account the slight variations of tunnel width along its length, we came up with a range of 22 to 32 kg (48–70 lbs), which overlapped with the mass Dave calculated for the adult dinosaur on the basis of its bone. The burrow dimensions matched the dinosaur.

Granted, the burrow was a close fit, but this situation also applies to many modern burrowing and denning mammals, such as coyotes (
Canis latrans
), striped hyenas (
Hyaena hyaena
), and aardwolves (
Proteles cristatus
). It also makes good evolutionary sense for burrows to be narrower rather than luxuriously spacious. For one, tight spaces limit who else can go down the burrow, including predators of either the adults or juveniles. Also, slender burrows are better able to retain heat and humidity, maintaining better climate control that is less influenced by surface conditions and making it more like a cave. The twisting Z shape of the burrow would have helped both of these factors, while also discouraging or confusing predators.

For the latter, put yourself in the place of a predator and imagine looking down the burrow entrance and only seeing the short tunnel segment to the first turn, not the next segment burrow, let alone the final burrow chamber with its potentially yummy, tender juveniles. Gopher tortoise burrows display exactly the same sort of strategy, in which these at first run straight down, but then turn abruptly to the right or left within a meter or so from the entrance.

What made everything even more groundbreaking than the burrowing nature of the dinosaur was that the dinosaur itself was a new species. Based on its anatomical traits, it was sufficiently different from other hypsilophodont dinosaurs that it warranted its own unique name before we revealed it to the rest of the world. This took much deliberation of Greek and Latin roots, which we discussed via e-mail with the discoverer of the dinosaur and our co-author, Yoshi Katsura (who was in Japan), and a paleontologist friend of mine
who was an expert on the rules of nomenclature, Andy Rindsberg (who was in the faraway and exotic land of Alabama). Eventually, Dave, Yoshi, and I agreed on the name
Oryctodromeus cubicularis
, which literally translates to “running digger of the den,” a unique name that would signify how this dinosaur was capable of digging and denning, but also could run about on two legs.

So just to summarize, we had the following:

• A new species of dinosaur;
  
• Anatomical evidence in that dinosaur, showing it was adapted for burrowing;
  
• A burrow that fit the reconstructed dimensions of the dinosaur;
  
• A burrow that had smaller burrows attached to it, showing a commensalism like that seen today in large vertebrate burrows;
  
• Two half-grown juveniles of the same species, indicating denning and extended parental care.
  

In our estimation, then, this discovery did indeed fulfill the earthy expression of applying a foot to one’s posterior. Clearly it was time to do the logical next step in the scientific method, which was to hold a press conference.

Just kidding. What scientists are supposed to do after a momentous discovery, well before scheduling interviews on talk shows, dating Hollywood stars, or uploading self-promoting videos to YouTube, is to put all of our observations and analyses into a coherent report that states our argument. The report is then sent to a scientific journal, where our peers—other paleontologists—give it a beady-eyed critical review. The very first step of this peer review happens with a journal editor. She or he decides whether the report deserves to be sent on for peer review, or whether it should be kicked back immediately to the authors with a pithy and dismissive “not worthy of our journal” notice. If this report gets past the editor, it is sent to at least two experts in the appropriate field for review.