Dinosaurs Without Bones (6 page)

In essence, if a dinosaur track shows pad and scale impressions from the foot of its maker, only air separated the flesh of that dinosaur from the sediment preserving it.

Any dinosaur track we find in the geologic record, though, is at the end of its history. So we also have to throw in an additional dollop of skepticism when divining any given track found at the same earth’s surface we now occupy. Undertracks made by dinosaurs may have been spared weathering for as much as 200 million years, but once exposed they can quickly become blurred or erased completely by modern weathering. This is where paleontologists depend on the sharp eyes and good graces of the general public, who far outnumber paleontologists and perhaps are outside more often. Many a dinosaur track or trackway has been found by hikers, bikers, or other recreationalists who recognized their patterns and then did the right thing by reporting their locations. Maybe it will be your turn some day.

Dinosaurs on the Run

You might be thinking by now, enough about the hypothetical and anachronistic races with humans and talking about dinosaur tracks as theoretical objects. What was the fastest dinosaur ever interpreted from a dinosaur trackway? How fast were they, really, especially when compared to the speediest of humans or modern mammalian predators such as lions or cheetahs?

Most animals spend much of their time moving at a normal pace. Among humans, even marathon or ultramarathon runners normally do not spend more than 15% of any given day running and, let’s face it, they’re not sprinting throughout even that time. Hence, a reasonable reflexive prediction about dinosaurs is that whenever they were making tracks, they were walking. Running would have been quite rare and reserved only for emergencies, such as chasing down prey, not becoming prey, or avoiding other dire threats such as forest fires, floods, or storms. Sure enough, these expectations about dinosaurs mostly taking leisurely strolls are probably right. Whenever the Alexander formula or variations of it are applied to dinosaur trackways, most dinosaurs were simply walking.

Still, finding exceptions to a norm is one of the joys of science. Thus when paleontologists encounter tracks that indicate sprinting dinosaurs, they justifiably get really excited. So far, almost all track-ways interpreted as those made by running dinosaurs were made by bipedal ones, theropods and ornithopods. In contrast, only a few trackways of trotting quadrupedal dinosaurs are known. One of these, preserved in Late Cretaceous rocks of Bolivia, was made by an ankylosaur—a big, armored dinosaur—which must have looked like a living tank as it ambled along. Sadly, not one trackway yet shows that sauropods, ceratopsians, or other quadrupedal dinosaurs loped, galloped, pranced, or minced. However, this does not necessarily mean these animals only walked. Just remember that running is rare in all land animals, which means the likelihood of finding fossilized tracks of running quadrupedal dinosaurs is also quite small.

The first discovered running-dinosaur trackways were from a site in Texas, where at least three theropods moved at high speed. These dinosaurs had footprints ranging from 29 to 37 cm (11.4–14.5 in) long, which are not much longer than many people’s shoe sizes. Yet they were taking strides that measured 5.4 to 6.6 m (17–22 ft)! Once these numbers were crunched through Alexander’s formula, the tracks spoke of speeds of 8.3 to 11.9 m/s (27–39 ft/s), or 30 to 43 kph (19–27 mph). To put it into a bipedal-human perspective, the top speed recorded by Usain Bolt over 200 m (656 ft) during the 2012 Olympics was also 27 mph, meaning that these dinosaurs and he would have had a very good race, perhaps with an outcome only affected by who was pursuing whom. But we also have no idea if these running dinosaurs were actually reaching their top speeds or not on whatever day their tracks happened to get preserved for us to see millions of years later. Theropods and humans alike, though, would be humbled by the top speed recorded for a cheetah (
Acinonyx jubatus
), which from a standing start and over 100 m (328 ft) has been clocked at 98 kph (61 mph).

Nonetheless, a dinosaur tracksite in Queensland, Australia out-does the Texas tracksite for sheer numbers of running dinosaurs.
This tracksite is worth its own chapter, because it wasn’t just one dinosaur trackway but nearly a hundred, suggesting dinosaurs all with relatively small footprints skedaddling, scampering, booking, bolting, or otherwise traveling quickly (for them), which was at about 12 to 16 kph (7.5–10 mph). Even more significant, it was a mixed group of small ornithopods and theropods all moving in the same direction. However, this interpretation has undergone some recent scrutiny and reinterpretations, which we’ll take up in detail later.

To better understand what running-dinosaur tracks look like, we often turn to modern examples that can serve as search images for finding these, and not looking to cheetahs. Instead, we observe large bipedal flightless birds. For example, the fastest bipedal land animals today are ostriches (
Struthio camelus
), which have maximum speeds of 45 mph (72 kph). Less speedy—but still much quicker than humans—are emus of Australia (
Dromaius novaehollandiae
), which can move as fast as 40 mph (64 kph), and rheas of South America (
Rhea americana
and
R. pennata
) at about 35 mph (56 kph). And because all of these flightless birds are actually modern theropods, they act as marvelous proxies for how some Mesozoic theropods could have reached similar speeds when running.

Of course, modern animals have their limits when applied to the geologic past, especially once we start looking at gigantic dinosaurs, for which we have no modern analog. Hence, one of the more imaginative scientific questions applied to running dinosaurs has to be one posed of nearly everyone’s favorite dinosaur,
Tyrannosaurus rex
. The question was this: If
T. rex
could run at about 45 mph—as depicted so memorably in the first
Jurassic Park
movie—what would have happened to it if it tripped? One paleontologist, Jim Farlow, and two other colleagues figured out that given the average mass of an adult
T. rex
(about 6 tons) and a speed of 45 mph, its forward momentum, halted suddenly by a fall, would have instantly killed it. Given this, the notion of a flat-out running
T. rex
, however entertaining (or frightening), is not very likely if it or any other large predatory theropod died every time it tripped while running at full speed—which, let’s face it, is a poor adaptive strategy for passing
on genes. However, we should also keep in mind that animals run fast in terrains and substrates conducive to such behavior, but do not in, say, gooey mud or ice-covered lakes.

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.