Paleontologists had known about Massospondylus embryos and eggs since at least the 1970s, but the exact source of these fossils had not been documented until paleontologists followed up on a few clues much later, in 2006. The results of this exploration and discovery, summarized in a report published by Robert Reisz and four other paleontologists in 2011, were spectacular. The first embryos and eggs were discovered at this site in 1976 in loose rocks on the ground, next to where a highway crew had cut into the strata to allow better passage of a road in Golden Gate Highlands National Park. (Such exposures, which geologists worldwide revere and covet nearly as much as easy access to beer, are known simply as road-cuts.) This meant that people visiting this national park—unaware of what world-class paleontological secrets resided there—had driven by the undiscovered nests, tracks, and other fossils there for forty years before the rocks were scraped again.
The paleontologists working on this roadcut were elated to find that it held a mother lode of mother-dinosaur data. Contained within a vertical span of siltstone about 2 m (6.7 ft) thick and across a horizontal distance of 23 m (75 ft) were ten egg clutches; eight of these were apparently unmoved, implying these groupings of eggs marked the original sites of the nests. (Keep in mind, though, that this is only a two-dimensional sample, and many more nests might be inside the roadcut awaiting geological unveiling.) The nests were on four horizons and close to one another, which the paleontologists interpreted as good reasons to infer site fidelity and nesting colonies. If so, these were the oldest known examples from the fossil record for dinosaurs, or any other vertebrates for that matter.
One of the clutches had as many as 34 eggs, with perhaps a few more lost to erosion, a significant number of eggs compared to other dinosaurs. From an ichnological perspective, the paleontologists could not find any other evidence of actual nest structures that fulfill all of the previously mentioned criteria for dinosaur nests, but they inferred their presence from depressions that held the tightly packed eggs. The eggs, which formed discernible rows within each clutch, also may have been organized thusly by one of the parents after laying. Invertebrate trace fossils (burrows) in the sediments surrounding the eggs show that the clutches may have been partially buried in soil by one or more of the parents before being buried in a more lethal way by river floods. This assumption is strengthened by the close-fitting arrangement of the egg clutches. If these eggs had been left in an open nest when a nearby river flooded, they surely would have been scattered about like balls in the opening break of a billiards game.
Nonetheless, it was not only the nests and eggs that made this suite of fossils truly extraordinary, but also some startling dinosaur trace fossils in the rocks around the nests: teeny footprints. These four-toed tracks, preserved as natural casts on blocks of rock that fell from the roadcut, just happened to match the front and rear feet of a sauropodomorph like Massospondylus. Yet these tracks were only about 15 mm (0.6 in) long, just smaller than a U.S. dime. When compared to embryonic Massospondylus foot bones, the tracks were made by sauropodomorphs about twice as big as expected for hatchlings, and showed they were walking on all fours, in a manner similar to that interpreted for the adults.
This trace fossil evidence, along with the proximity of the tracks to the nest sites, suggested two points: the hatchlings had enough time to grow larger, and they must have been receiving parental care near the places they were born. These tracks, then, extend the evolution of dinosaurs as good parents more than a hundred million years farther into the geologic past than previously supposed, thus providing yet another example of why trace fossils tell us much more about dinosaur lives than just bones, eggs, or other body fossils.
CHAPTER 5
Dinosaurs Down Underground
Discovering a Digging Dinosaur
Big surprises often arrive in small packages; in this instance, it was in an e-mail message. On August 26, 2005, while preparing to teach my first classes of a new semester, I was glad to see the message from my friend Dave Varricchio. Although we had known each other since 1985, chatted in person at professional meetings, and had kept in touch via e-mail, at best we got together only once a year and our exchanges were infrequent. This seemingly unsocial behavior was not because we were less cordial with one another, but instead could be attributed to distance, time, and personal demands. Dave was busy earning tenure in his assistant-professor position at Montana State University in Bozeman, Montana, while also serving as a staff paleontologist at the Museum of the Rockies there. Meanwhile, I was at Emory University in Atlanta, Georgia, having just finished rewriting the latest edition of a dinosaur textbook and teaching full-time.
The longest span of time we had spent together since our graduate-school days—which was in the late 1980s—was a week of field work in northwestern Montana in 2000. While there, he and I took a close look at the Late Cretaceous fossil insect cocoons and burrows near the Troodon nest sites. Yet the resulting research paper from that investigation was dormant and nowhere near ready for submitting to a journal. So what could be the purpose of his message?
The message subject gave me no clue whatsoever. It simply said “specimen.” Dave’s message text was similarly cryptic, albeit enlivened by an injection of Freudian humor:
Hey, What do you think of this feature in the photo? Yes, it looks somewhat phallic, but that’s not what I’m asking. The central structure ranges from sandy mudstone to sandstone and crosscuts claystone. Beds dip to the left. Brush is one inch wide. DV [Dave Varricchio]
I opened the photo attached to the message and studied it. The top part showed a red mudstone overlying a gray mudstone. The latter had a big chunk removed—probably by shovels and picks—which freshly exposed a darker-gray rock underneath its weathered surface. There was indeed a “central structure” within that gray mudstone (what Dave called a “claystone”), so I focused my attention on that. It protruded slightly compared to the rock around it, so it was made of more resistant material, and was a lighter gray.
These observations and inferences of mine synced with Dave saying it was a sandy mudstone or sandstone, which would have contributed to its bas-relief appearance. It also had a horizontal segment toward its top, which turned abruptly downward into a vertical segment. In my imagination, I converted this part into three dimensions, and a spiral shape came to mind. The vertical part connected directly to a white oval near the bottom of the photo, which had a piece of shiny metal poking from it. Even though I almost never worked with fossil vertebrate skeletons, I nonetheless recognized the white oval as a plaster jacket, which must have been placed around the remains of a fossil vertebrate. The metal was likely aluminum foil the field crew used to cover part of the rock around the skeleton before slapping burlap strips soaked in wet plaster of Paris around the fossil specimen, the latter a time-honored practice in vertebrate paleontology since the 1880s.
And yes, there was a small field brush as the only scale in the photo, off to the left. Assuming a width of about 2.5 cm (1 in), I used that to extrapolate its length as about 15 cm (6 in). I then applied this brush length to figure the approximate length and width of the plaster jacket. Using this seat-of-the-pants reckoning, it came out to 55 to 60 cm (22–24 in) long and 40 cm (16 in) wide. The central structure kept a consistent width throughout its length, and was also about 30 to 35 cm (12–14 in) wide, flaring some as it connected with the rock holding the plastered specimen.
I waited until the next day to write and send Dave my assessment of what was depicted in the photo. It went like this:
Hypotheses:
Large burrow and terminated with the burrowmaker (the latter inferred from expansion at the end and your nicely jacketed specimen)
Large burrow and terminated with some critter that was not the burrowmaker
Collapse feature, but with some hapless critter at the end of it that fell into/drowned in it
Impact structure, with aluminum from the alien spaceship still preserved (and the spaceship safely conceal
ed from prying eyes of the guvmit [government])
Not enough information, you speculative ichnologist!
I ended my reply with a summary interpretation: “… it would most likely be a bank burrow (adjacent to a channel) going into a fluvial [river] overbank deposit … I’ll guess that you got a crocodilian at the end of its burrow.”
My thinking at the time was influenced by what I knew about big modern burrows I had been studying on the Georgia barrier islands, namely alligator dens. Alligators are represented by two species that live in widely separated places: the southeastern U.S., which has the American alligator (Alligator mississippiensis), and eastern China, with the Chinese alligator (Alligator sinensis). Both are burrowers, especially the Chinese alligators, which dig tunnels as long as 20 m (67 ft) into soft sediments next to water bodies. The American alligator burrows are not so extensive, but still impressive structures. These can be more than a meter (3.3 ft) wide at their entrances, are 4 to 6 m (13–20 ft) long, and, like their Chinese relatives, are made next to ponds or intersect the local water table.
Dens are used by alligators for a variety of reasons, such as keeping their skin from drying out from sun and heat, maintaining more reliable “indoor” temperatures during cold winters or hot summers, or providing safety from droughts or fires. Most important, though, these burrows serve as a refuge for young, newly hatched alligators, which are watched closely for more than a year by the ultimate of overprotective mothers. In my studies on the Georgia barrier islands, I had already experienced a few tooclose-for-comfort encounters with alligator mothers and babies near their dens and learned to proceed with extreme caution near them, treating all such large burrows as if they were loaded guns.
In their evolutionary history, Chinese and American alligators shared a common ancestor not too far back in the geologic past, but they also connect to all other crocodilians, a lineage that extends well into the Jurassic Period. However, despite this ancient heritage, no crocodilian burrows had been interpreted from the fossil record. This made the structure in Dave’s photo potentially exciting, but not knowing the age of the rocks in the photo, or even their location, I could not allow myself to get too enthused about it. Still, I knew that Dave often prospected Cretaceous-age rocks in Montana and other parts of the western U.S., so I held some hope that he had found a Cretaceous example of a crocodilian burrow, and one with a crocodilian at the end of it. Such a discovery would have been very pleasing to me, because it would have linked directly with my Georgia-coast interests in alligator dens, while also documenting a much deeper history of burrowing behaviors in crocodilians.
My mild curiosity concerning whether I was right or not about my crocodilian-in-a-bank-burrow hypothesis was addressed later the same day, but in a completely unexpected way. It turned out that I was both right and wrong. Dave’s next message had an intriguing title: “hoping you say ‘yes’.” Here are the first three sentences of text from his note:
You have correctly inferred that there is a vertebrate at the end of this structure. But it’s not a [ancient] crocodile. It’s a dinosaur, a hypsilophodont.
Never had I been so happy to be mistaken. It was a dinosaur in a burrow. Sure, lots more scientific questions had to be asked before jumping to any more conclusions, but this revelation warranted a happy dance (as demonstrated by Snoopy in A Charlie Brown Christmas), followed by popping the cork on a champagne bottle, and concluded by a happy dance while swigging vigorously from that champagne bottle. Lacking champagne, though, I merely pried off the caps of whatever beers were in my fridge at the time and toasted this discovery with my wife Ruth, who was promptly sworn to secrecy about Dave’s discovery.
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 titanosa
urs 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).
Dinosaurs Without Bones Page 14
