Dinosaurs Without Bones (7 page)

In 2004, I studied this specimen intensively with a colleague, Emma Rainforth, in which we tried to figure out the sequence of movements made by the theropod that would have produced such a trace fossil. We were also testing an audacious idea that some of the wrinkle marks near the edge of the leg impressions were actually from feathers. This was an extraordinary claim at the time because feathered theropods, although then-recently discovered in Early Cretaceous rocks of China, were completely unknown from the Early Jurassic anywhere in the world. Yet other paleontologists who had examined the trace fossil just a few years before us concluded that the odd wrinkle marks were “feathers.” The surface preserving the trace fossil also had little pockmarks, which had been interpreted as “raindrop impressions” imparted by a Jurassic shower.

We wanted these structures to be feathers, too; but in science, reality does not always live up to our wishes. Once we looked at the trace fossil more carefully, we realized that a thin algal film covered the original muddy (but firm) surface. This film acted like plastic wrap covering a dish: any pressure exerted laterally against it caused the film to deform and wrinkle. Debunking further, we also concluded that supposed “raindrop impressions” were more likely gas-bubble escape structures. These were made when the theropod stepped onto the surface and pressed its full body weight on the mud when it sat down; this in turn caused trapped gas in the mud from underneath to bubble up to the surface. In short, this specimen records a full sequence of movement by the theropod and how it altered the ground beneath it, recorded in exquisite detail because it was preserved under the right conditions.

As fascinating as these dinosaur-sitting traces might be, though, we are still puzzling over why they only seem to be in Early Jurassic rocks. Did dinosaurs just stay upright through the rest of the Jurassic and Cretaceous periods, too busy to sit down or otherwise take it easy? This scenario seems absurd, although it also poses a good question as to how some of the largest of dinosaurs, especially those with small arms, would have managed to both lie down and get up (I’m looking at you,
T. rex
). As many of us experience each morning, getting up is the hardest part following our resting. Still, the Middle and Late Jurassic, as well as the Cretaceous, abounded with small dinosaurs, too, which would have had no problem stopping and becoming supine. So perhaps it’s only a matter of time before paleontologists start recognizing more such trace fossils that record when a dinosaur took the pause that refreshed.

Swimming Dinosaurs

Dinosaurs and water have had an odd back-and-forth relationship in our imaginations. At some point in the initial studies of sauropod and “duckbill” dinosaurs (hadrosaurs), paleontologists started wondering how such large animals kept themselves upright on land without also placing incredible stress on their muscles, bones, and joints. So all
paleontologists needed was a little bit of suggestive evidence to nudge these big animals into the water, where their weights would have been supported through buoyancy.

For hadrosaurs, this evidence was scanty but persuasive for those who wanted these dinosaurs to be aquatic. For example, one hadrosaur trace fossil specimen had skin impressions around its hand that stretched between its fingers. This led paleontologists to conclude that this skin was webbing that aided it in paddling around in bodies of water. Only later did paleontologists realize this “webbing” was actually a result of skin drying around its bones after the dinosaur had died. Another hadrosaur,
Paralophosaurus
, also had a tall hollow crest on its skull, which was explained as a “snorkel” that allowed the dinosaur to breathe while most of its body was hidden underwater from predators. A major flaw in this seemingly marvelous adaptation was that the hollow tube in the center of the crest, once studied in more detail later, actually makes a U-turn which would have constituted a perfectly inept snorkel. (If you don’t believe me, try making one like this and let me know how that worked out for you.) Yet another anatomical trait was an elongated snout that led to the nickname of “duck-billed dinosaurs” for hadrosaurs, which imagines them as favoring soft aquatic plants as food. Again, a reexamination of their teeth and jaws as well as their trace fossils (coprolites and microwear on their teeth, explained in a later chapter) revealed that hadrosaurs could eat all sorts of land plants. In short, just calling a hadrosaur “duck-billed” doesn’t make it a duck.

This explanation of body fossil evidence favoring aquatic lifestyles for dinosaurs was even extended to dinosaur tracks. In 1938, paleontologist Roland Bird of the American Museum of Natural History learned that the area around Glen Rose, Texas, had lots of dinosaur tracks. Once he investigated, he confirmed the presence of exquisitely preserved three-toed theropod tracks, but also made an astonishing discovery: the first known sauropod dinosaur tracks from the geologic record. These huge tracks faithfully matched the size and anatomy of sauropod feet: five toes in the rear, and a
rounded pad in the front. However, among these sauropod track-ways were ones in which only the front feet registered. Why would the weightiest part of a sauropod—its rear end, with long tail—not connect with the sediment surface? Bird surmised that this was a result of a sauropod floating along, only touching the bottom with its front feet.

Later, a closer look at these tracks showed that the missing tracks in the sequence of steps could be attributed to differences in track preservation. If these sauropods had applied more pressure in the front while walking on land, these would have been more likely to be preserved as undertracks than the rear feet. Hence, Bird had not been looking at tracks from the original surface where sauropods placed their feet (or not), but more at the ghostly prints below. Once this alternative explanation caught hold, people realized that Bird was likely wrong about “swimming sauropods” at the Texas site.

Ironically, Bird’s recognition of sauropod tracks in the first place led from an initial view of sauropods as aquatic dinosaurs that, with more such discoveries, shifted them onto the land. Once paleontologists had the right search images for sauropod trackways, they started finding them outside of Texas. In the U.S., sauropod tracks are also in Colorado, New Mexico, and Utah, as well as in Argentina, Australia, China, France, Korea, Mexico, Morocco, Switzerland, the United Kingdom, and Zimbabwe, among other places. These tracks are also in rocks from near the start of sauropods in the fossil record (Late Triassic) to their very end (Late Cretaceous). Something noteworthy about these sauropod trackways found thus far, though, is that nearly all show these massive animals walked on emergent surfaces, such as along coastlines, lakeshores, or river floodplains.

Still, paleontologists wondered: What if dinosaurs other than hadrosaurs or sauropods went for a swim? How would we know from looking at their bones? For example, even the most skilled anatomists would be hard pressed to demonstrate from an elephant’s skeleton that these multi-ton animals are capable of swimming long distances. Yet Indian elephants (
Elephas maximus
) can
swim as far as 25 miles (40 km), a feat far better than most humans are capable of. In fact, elephant swimming abilities show one of the probable ways mammoths dispersed to islands during the Pleistocene Epoch, where some isolated populations lasted until only about 4,000 years ago. (These elephants also became much smaller after generations of living on these islands, leading to the oxymoronic condition of becoming “dwarf mammoths.” But that’s another story.)

Just in case you were wondering whether trace fossils might come to the rescue again to solve this dinosaurian mystery, you would be right (again). First, as early as 1980, a paleontologist interpreted swim tracks from Early Jurassic rocks of Connecticut as made by theropods, and provided a fine argument as to how such dinosaurs would have made these tracks while partially buoyed by water. More than twenty years later, in 2001, paleontologists working in separate studies and places (Wyoming and the U.K.) interpreted Middle Jurassic tracks as possible dinosaur swim tracks. Soon after that (2006), hundreds of much better examples were discovered and documented by Andrew Milner in Early Jurassic rocks of southwestern Utah at and near the St. George site that also has the dinosaur-sitting traces mentioned earlier. The next year (2007), dinosaur swim tracks were again interpreted from long linear marks on an expansive surface of Early Cretaceous rock in Spain. In 2013, yet more dinosaur swim tracks were reported from another Early Cretaceous site in Queensland, Australia. Suddenly, dinosaurs seemed to be swimming everywhere.

How would you know whether a dinosaur was swimming by looking at its tracks? Well, for one thing, you wouldn’t know it at all unless its feet touched the bottom of the water body it crossed. If the water were too deep, buoyancy would have kept dinosaur bodies—along with their feet—above any sediment surface that would have recorded their tracks. But through a combination of legs long enough to reach the bottom and water shallow enough to allow this, they would have made tracks.

Why should a dinosaur swim at all? Or as an actor might ask, what was their motivation? Getting from one place to another is a
likely reason, instead of walking around a shallow lake or stream, or the old “to get to the other side” answer. Yet another argument relates to their attraction to aquatic environments as great sources of food. For theropods, this might have been fish, but other aquatic animals also might have served as tasty treats. For hadrosaurs and sauropods, though, which were (as far as we know) all herbivores, this is not such a good explanation. Not surprisingly, recreational purposes have never been suggested for swimming dinosaurs, but who knows whether an occasional dip might have also relieved any dinosaurs suffering from skin parasites or a hot day in the Mesozoic.

The Not-So-Secret Social Lives of Dinosaurs

Tracks also tell us about dinosaur social lives, and thanks to these trace fossils we are confident that many dinosaur species moved together as herds, packs, flocks, congresses, murders, or whatever group name seems appropriate. Assemblages of dinosaur bones composed of many individuals but only representing one species also support this idea, and we now take for granted that the stereotype of the “lone dinosaur” is not so likely in many species. Because of this combination of trackway and skeletal evidence, we now nonchalantly discuss the ecological effects of vast herds of sauropods, hadrosaurs, and ceratopsians, or the pack-hunting behavior of theropods, on Mesozoic ecosystems. “Strength in numbers” is a strategy used by many animals today, from schooling in fish to herding in caribou to pack hunting in wolves.

So let’s say that paleontologists find a dinosaur tracksite with hundreds of tracks preserved in it. They can then test whether these dinosaur trackmakers moved together as a large group or not. This is based on whether the following questions receive an answer of “yes” or “no”: (1) Do the tracks look alike? (2) If the tracks look alike, do they also show variations in sizes? (3) Are the tracks all heading in (more or less) the same direction? (4) Do the tracks show any other harmony of movement, such as staying parallel to or following one another? (5) Do the tracks seem to have been made at about the same time?

Here’s what those questions are testing: The first—do the tracks look alike—is examining whether they belong to the same dinosaur species or not. If they do have the same basic form but also show a range of sizes (question 2) from small to large, then they also could represent growth stages of the same species, from babies to full-grown adults, and perhaps gender differences as well. The third question is key, then, as this sorts out whether the dinosaurs were truly moving together or not, and not just randomly milling about. The fourth question further clarifies the third, as it asks about more nuanced behavior such as whether dinosaurs in the group maintained a consistent “personal space” from one another or whether they were following leaders. Finally, the fifth question addresses whether these tracks were all made by a sizeable group of dinosaurs moving through the area, as opposed to, say, a dinosaur family consisting of just two adults and two juveniles that neurotically walked through the same spot every day for several weeks.

Given this idealized dinosaur tracksite in mind, do any fulfill all of the criteria? I think you suspect the answer to that, but let’s look at a few examples anyway. The first recognized example of herding behavior shown by dinosaur tracks, and still one of the best, stems from a series of Early Cretaceous sauropod tracks at Davenport Ranch in Texas, found by Roland Bird in 1941. This site has trackways of more than twenty sauropods walking in the same direction and apparently made at about the same time. A Late Jurassic sauropod tracksite near La Junta, Colorado, also shows the tracks of five sauropods moving in the same direction, spaced at regular intervals, and turning in harmony along the length of their trackways. One time, when visiting this site with students, I asked them to walk alongside the tracks. By observing them this way, the lesson became much more visceral for these students as they could experience the subtle changes in movement made by the sauropods, rather than just gazing at them from afar or listening to me babble on about them.

Sauropod tracks, in fact, provide the best evidence that these dinosaurs herded. So far, sauropod trackways indicating group behaviors are documented from Jurassic–Cretaceous rocks of the
U.S. (Arkansas, Colorado, Texas, Utah), Brazil, Bolivia, China, Portugal, and the U.K. The tracksite in the U.K., preserved in Middle Jurassic (about 165
mya
) rocks, was likely made by dozens of sauropods moving together, a truly awesome spectacle to imagine. Even better, some of the same sauropod tracksites show smaller tracks along with larger ones, suggesting that sauropods of different ages were traveling with one another, perhaps with multiple families.

For ornithopods, similar sorts of trackways also point toward their herding. These include several sites from the Cretaceous of Korea, one of which has tracks of about twenty large ornithopods heading in the same direction, and another from the Cretaceous of Canada, which had 10 to 12 ornithopods. A few ankylosaur tracksites have parallel trackways, suggesting that at least two ankylosaurs were traveling together at the same time. So far, stegosaur tracks are so rare we don’t know if these animals were social or not. Ceratopsian tracks are also uncommon enough to withhold judgment on that aspect of their lives too, although rocks bearing hundreds of bones of the same ceratopsian species tell us these dinosaurs were likely group-oriented also.