Dinosaurs Without Bones (18 page)

The Oldest Dinosaur Day Care

Dinosaur body fossils make up the bulk of evidence for documenting the earliest years of dinosaurs, consisting of eggs, embryos, hatchlings, and half-grown juveniles. Although this wealth of information provided by unborn and baby dinosaurs has contributed to our understanding of dinosaur life cycles, paleontologists
still have to face the reality that these all represent lives that were snuffed out before they behaved much. As just discussed, what do we know about the lives of hatchlings freeing themselves from their enclosing eggshells? How would we know through trace fossils whether newborn dinosaurs were altricial (depending on their parents for extended care), or precocial (able to leave mom and dad soon after hatching)? Furthermore, what clues can trace fossils give us about when such behaviors first evolved in dinosaurs?

One of the most impressive dinosaur nest sites with other trace fossils to address these questions is in Early Jurassic rocks of South Africa from about 190
mya
. Because of their antiquity—these are the oldest known dinosaur nests in the world—this site also tells us much about the early evolution of nesting in dinosaurs, and is unparalleled for the sheer amount and quality of body- and trace-fossil goodness in a relatively small place. At this site are nest structures, eggshells, embryos, and juvenile footprints, all identifiable to the sauropodomorph
Massospondylus
. (Sauropodomorpha is a clade that includes sauropods and their relatives, among which were “prosauropods” like
Massospondylus
, lineages that only lived during the Late Triassic through the Early Jurassic, about 230 to 180
mya
.)
Massospondylus
was not a massive dinosaur, probably weighing about 130 to 150 kg (285–330 lbs); this made it much smaller than
Maiasaura
or the nesting titanosaurs, but about three times bigger than
Troodon
. All in all, these fossils provided a rare and intimate look at dinosaur nesting from a time when this behavior might still have been relatively new in dinosaurs. To put this in perspective, nearly all other nests interpreted thus far, such as those of
Maiasaura
,
Troodon
, and titanosaurs, are from about 80 to 65
mya
, meaning the South African nests are more than twice as old as these.

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.

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
:

1. Large burrow and terminated with the burrowmaker (the latter inferred from expansion at the end and your nicely jacketed specimen)
   
2. Large burrow and terminated with some critter that was not the burrowmaker
   
3. Collapse feature, but with some hapless critter at the end of it that fell into/drowned in it
   
4. Impact structure, with aluminum from the alien spaceship still preserved (and the spaceship safely concealed from prying eyes of the guvmit [government])
   
5. 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.