Dinosaurs Without Bones (19 page)

Just to give you an idea of the depth of my wrongness in not thinking about a dinosaur as a possible burrowmaker, as well as the consequences of Dave and his field crew finding a dinosaur in a burrow, the following is a definitive statement I had made in the second edition of my then-newly revised dinosaur textbook. As you read this statement, keep in mind that I had just extensively rewritten the book, after blistering peer review and editing, and it was already on its way toward publication a few months later:

Furthermore, no convincing evidence has revealed that dinosaurs lived underground because no dinosaur has ever been found in a burrow
,
nor have any burrows been attributed to them
.

By the way, did I mention I was wrong? Of course I did not think “dinosaur” when looking at the photograph earlier in the day, as all of my ideological blinders had been switched on. Was I studying modern alligator dens at the time, biasing me to think “crocodile” instead of “dinosaur”? Check. Had I previously thought about burrowing dinosaurs but rejected their existence on the basis of what was then known? Check. Had I let these biases overrule the fact that Dave studies dinosaurs almost exclusively? Check. Was I an idiot? Check. But I was pretty damned delighted to be one in this instance.

Dave’s message had more to say, including a brief confession of his long-held suspicion that other small dinosaurs such as “hypsilophodonts”—a non-clade grouping of small ornithopod dinosaurs—might have burrowed. This informed guess was based on his observation of the small Late Cretaceous hypsilophodont
Orodromeus
near his
Troodon
nest sites in northwestern Montana. In more than a few specimens, Dave had noticed that the
Orodromeus
skeletons were unusually complete and concentrated in compact masses, encased in well-cemented rocks. The simplest explanation for this unusual preservation, he thought, was that these small herbivorous dinosaurs might have been in chambers at the ends of those burrows, then either were buried after dying or buried alive in their burrows. Nonetheless, he had no other evidence for this idea, such as an actual burrow connecting to these masses of bones. So it remained just a nagging conjecture awaiting more persuasive evidence. And now he had it: a probable burrow, attached to a probable burrow chamber which held the body fossil of its possible maker, which just happened to be a hypsilophodont dinosaur. The last paragraph in Dave’s message neatly summarized and resonated with my feelings at the time:

Obviously … some interpretation is dependent upon the actual specimen within … but still … a cool specimen. I think that really only hypothesis 1 and 2 are suitable
. The sequence includes paleosols not too unlike the Egg Mountain area.

When he referred to “hypothesis 1 and 2,” he meant what I had enumerated previously: (1) a large burrow that had the burrow-maker at one end, which in this instance was a dinosaur; and (2) a large burrow with a dinosaur at its end, but one that was not the maker of the burrow. In this second scenario, the dinosaur could have lived in the abandoned burrow of another unknown animal—dinosaurian or otherwise—or had its remains washed into the burrow where they were subsequently buried and fossilized. This would have been quite a geological coincidence, but it still had to be considered as a possibility. Regardless, Dave was right about one thing: it was a cool specimen.

My reply to him the same day was understandably animated, and I wrote my responses in between his quoted text to approximate a call-and-response dialogue. Following his quotation “It’s a dinosaur, a hypsilophodont,” I said:

[Colloquial expression—censored here—denoting excitement and enthusiasm, containing several profane words, and comparing the discovery to the application of a foot to one’s posterior.]

What I meant, of course, was that is a most interesting observation. Congratulations on a noteworthy find, perhaps warranting further investigation.

[Repetition of colloquial expression—also censored here—denoting excitement and enthusiasm, containing several profane words, and comparing the discovery to the application of a foot to one’s posterior.]

Discoveries are only the beginning, though. It was time for us to do a lot more science.

I Have No Shovel, and I Must Dig

Only in the past fifteen years or so have paleontologists realized that a few dinosaurs dug into the ground underneath them. As we learned previously, scratch-digging sauropods were one such
example, in which titanosaurs used their specially adapted feet to gouge the earth before laying their eggs.
Troodon
was another, and it might have used both its hands and feet to excavate and shape its rimmed nests. A few other dinosaurs had anatomical attributes that could have helped them to dig, such as the small Late Cretaceous theropod
Mononykus
of Mongolia, which had weird shortened and pointy forearms that looked like pickaxes. These appendages have been inferred as tools for breaking into termite or ant nests, gouging these nests to gain access to their goodies, sort of like a dinosaurian anteater. Moreover, one specimen of the Early Cretaceous ceratopsian,
Psittacosaurus
, was found with 34 juveniles just like it, all collected in a bowl-like depression. Paleontologists speculated that all of these animals may have died together in a collapsed burrow, but no one could tell for sure if this is what happened, nor whether or not the adult made the depression.

More recently, in 2010, trace fossils attributed to predatory dromaeosaurs—such as
Deinonychus
—were interpreted as dig marks made by claws on their rear feet as they tried to unearth small mammals, whose preserved fossil burrows were just underneath these dig marks. But these and other dinosaurs were only scraping the surface, so to speak. What about dinosaurs that went deeper and actually spent extended periods of time underground?

Before jumping into our time machines and going back to the Mesozoic Era, a much better idea to ask ourselves first is what modern vertebrates might tell us about burrowing behavior. After all, the variety of today’s burrowing vertebrates is astounding, ranging from lungfish to spadefoot toads to skinks, alligators, naked mole rats, puffins, aardvarks, and more. Furthermore, just limiting your list to mammals that live part or most of their lives underground will result in hundreds of species, and they range in size from Oldfield mice (
Peromyscus polionotus
) to grizzly bears (
Ursus arctos
).

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.