Other scientists figured out how much musculature a T. rex would have needed to move such a massive body at high speeds and came up with numbers for the mass of muscles for its legs and around its tail. What they found was that a 45-mph-running T. rex would have required about 85% of its entire body mass concentrated in its legs and muscles around its tail, giving this dinosaur more than just a little “junk in the trunk.” Based on their calculations of more realistic proportions of muscle mass in those areas that correlated to speed, they instead proposed that T. rex moved at speeds of about 10 to 25 mph.
So as a result of this thought experiment and that of the running-stumbling-dying T. rex or a huge-booty T. rex, paleontologists are not expecting to find tracks of a running T. rex, Spinosaurus, Gigantosaurus, or other massive theropod anytime soon. But of course we would be delighted to be proved wrong and would be the first to applaud anyone who discovered such a trackway.
Sitting Dinosaurs
Watch nearly any documentary film that uses CGI (computer-generated imagery) to recreate dinosaurs in their natural Mesozoic habitats and you will almost never see a dinosaur sitting, lying down, sleeping, or otherwise taking it easy. This is understandable on the part of the director and animators, because the attention span of viewers would decrease in inverse proportion to the length of such a segment and they would quickly switch the channel to watch their favorite reality-TV stars. (Coincidentally, these “stars” will be mostly sitting, lying down, sleeping, or otherwise taking it easy.) Yet dinosaurs must have slept, rested, or paused, however briefly, in their daily activities.
How can we know for sure dinosaurs took a breather in their lives? Surprisingly, the skeletal evidence is scant, although two examples are exquisite. Both of these skeletons belong to the same species of dinosaur, Mei long (“soundly sleeping dragon”). Each was found with its long tail wrapped around its body, looking very much like a sleeping duck or goose. Even more amazing, the two specimens are mirror images of each other, one with its head turned back between its left arm and torso, and the other with its head to the right side.
A few other dinosaur skeletons, such as those of the theropod Citipati, have been found preserved in sitting positions, which also might be construed as “resting.” But these were positioned over nests with eggs, hence these dinosaurs were probably staying put to protect their eggs and died trying. As any expectant parent can tell you, though, taking care of your potential offspring should never be considered as “resting.” For instance, some modern flightless birds such as emus stay seated over their eggs for 50 to 60 days.
I suppose, then, that we must once again resort to dinosaur trace fossils to learn more about how they rested. Sure enough, we do know of some so-called “resting” or “crouching” traces—made when a dinosaur sat down—although these are quite rare. As of 2013, only ten had been found in the world, with three in Massachusetts, three in Utah, two in China, one in Poland, and one in Italy. For some unknown reason, these trace fossils are time-restricted, as they are only preserved in Early Jurassic rocks. Of the three from Massachusetts, two are from medium-sized theropods and one from a small ornithopod, whereas all the rest are from medium-sized theropods. So far, we do not know of any resting traces made by quadrupedal dinosaurs such as sauropods, ankylosaurs, stegosaurs, or ceratopsians.
Identifying a dinosaur resting trace is relatively straightforward. First of all, look for a side-by-side pairing of the rear feet. Most bipedal dinosaur trackways show a diagonal-walking pattern, meaning that any pattern deviating from that catches our attention and is examined more carefully. Hence, where dinosaurs stopped, they would have pulled up their trailing leg next to their lead leg, meaning their footprints will be parallel and adjacent, or slightly offset. Upon sitting down, the dinosaur will have lowered the long parts of its legs just above its feet, which consist of its metatarsals (equivalent to our heels). This pressing of its metatarsals onto the ground would have given its tracks elongated extensions. Once seated, the dinosaur didn’t stop moving and may have shifted its position as it settled in, causing multiple prints in a small area. Additional parts of the body might have made contact and left their marks too, such as the rear part of its anatomy—which is properly called an ischial callosity, and not the more appealing term “dinosaur butt”—as well as its tail and front-foot (hand) impressions. Of these, the ischial callosity is most likely to be preserved, although tail marks and hand imprints have been recorded in a few, too. Incidentally, dinosaur tail impressions are quite rare, with fewer than forty reported from the entire geologic record, and many of these are associated with resting traces.
The most recently discovered dinosaur-resting trace, and probably the best, is a spectacular one. Reported in 2009 in southwestern Utah, this Early Jurassic trace fossil not only shows where a theropod approached a sitting spot and sat down, but also got up and walked a ways afterwards. Just like how we would adjust our sitting position to a more comfortable one, this dinosaur shuffled forward twice after lowering itself to the ground, evidenced by repeated prints of the same feet. It also includes impressions of its metatarsals, ischial callosity, and two thin slices left by its tail.
An even more remarkable aspect of this sitting trace, though, is that the theropod put its hands down in front of it and left impressions of these. The traces showed the positions of the theropod’s hands with its “palms” turned inward toward the center of the body, almost as if it were measuring the width of the trackway. For too many years, paleontologists have cringed at reconstructions of theropods walking around limp-wristed, palms down: a posture sometimes derisively labeled as “bunny hands.” In fact, skeletal evidence indicates this was anatomically impossible, and that the hands must have been held with the palms turned inward, not downward. Thus these two handprints vindicated critics’ previous assertions of theropod hand positions. This combined resting trace and trackway, along with hundreds of other dinosaur tracks, warranted enough importance to have a building constructed around them for protection (the St. George Dinosaur Discovery Center), ably providing public education about the tracks in St. George, Utah.
Nevertheless, as wonderful as this trace fossil might be, my favorite dinosaur-resting trace is one made by an Early Jurassic theropod in what we now call Massachusetts. On display at the Beneski Museum of Natural History at Amherst College, this specimen, designated specimen AC 1/7 by paleontologist Edward Hitchcock in the 1850s, is a near-perfect record of where a human-sized theropod sat down on a muddy lakeshore just a little less than 200 million years ago. Unlike the St. George example, this trace fossil is quite limited in its area, preserved in an isolated slab of rock about the size of a coffee table. At some point after its discovery in the 1850s, it was framed like a work of art (which it is). It has two slightly offset pairs of feet and rear “heel” (metatarsal) impressions, and between those, an oval, apple-sized impression from a svelte part of its rear end. The detail associated with these traces is incredible, accompanied by wrinkle structures formed as the theropod shifted its weight from one side to the other when sitting down and getting up.
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 l
ooked 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.
Dinosaurs Without Bones Page 5
