Dinosaurs Without Bones (8 page)

Knowing that many mammalian herbivores today travel in sizeable groups, the preceding insights on the social lives of sauropods and ornithopods are probably not big surprises. But what about theropods? Do their tracks ever show that they hung out with one another, shared time in the same place, and hunted together like wolves or other social predators? Oh yes, and in some instances these trackways paint nightmarish scenarios for their intended prey.

At one site in Middle Jurassic (about 165
mya
) rocks of Zimbabwe, trackways of at least five large theropods, with calculated hip heights of more than 2 m (6.6 ft), indicate they were traveling together. At this site, some of the theropods stepped on each others’ tracks, further suggesting they had some sort of pack arrangement with one or more theropods taking the lead in a loose formation. An Early Cretaceous (about 125
mya
) site in China also shows six theropod trackways, equally spaced and pointing in the same direction, all of their tracks about the same size, 24 to 28 cm (9–11
in) long, and with only two toes on each foot. Yes, that’s right, these are dromaeosaurid tracks. The track sizes further indicate hip heights of about 1 to 1.1 m (about 3.5 ft), or slightly smaller than the fictional “
Velociraptors
” of the
Jurassic Park
movies. Yet these tracks reflect a chilling reality, one in which small or large prey would have been stalked and terrorized by sickle-clawed predators. Running dromaeosaur trackways—which I’ve seen at a spectacular tracksite in Utah—amplify this empathy, whether for the hunter or hunted.

Dinosaurs Who Stalk and Feed

Based on what you just learned about dinosaur pack-hunting behavior from tracks, along with other evidence, paleontologists have no doubt that some dinosaurs hunted other dinosaurs, birds, mammals, lizards, and additional animals. For example, gut contents in a few rare specimens tell us directly about a dinosaur’s last meal (which will be detailed in a later chapter). But what happened just before that meal was acquired? How did predatory dinosaurs hunt their prey? And how do tracks provide some insights on dinosaur hunting behaviors? All of these are good questions, and I’ll do my best to answer them through what dinosaur tracks tell us.

Probably the most famous and longest-known example of a possible “stalking theropod” trackway comes from near Glen Rose, Texas. This trackway, discovered by paleontologist Roland Bird in 1938, was in a limestone bed cropping out in the Paluxy River. He noticed the trackway in direct association with a sauropod trackway, and was thrilled to see how the theropod tracks at first paralleled and then intersected those of the sauropod; at this point, the theropod tracks ceased. Bird surmised that this was where the theropod leaped onto the left side of the sauropod to bring it down, just like a lion would with its prey.

“Wow, that sounds incredible!” you think. Yes, it does, which also means the story may not be so simple. For one thing, the sauropod tracks continue on past where the theropod tracks stopped, and show no alteration of gait or depth of tracks on the left side. One would think that the addition of a multi-ton predator on one side would cause some reaction, or at least a little bit of
imbalance. So now the more reasonable explanation is that, yes, the theropod might have been stalking the sauropod but did not jump onto it there. Instead, the tracks just weren’t preserved after the point where they disappear. So we don’t really know whether the theropod was going after this sauropod or not. (By the way, if you want to see this trackway, don’t go to Texas, unless you like looking at rectangular holes in riverbeds there. In 1940, Bird and many laborers extracted the trackway and took it to New York City, where it is now displayed in the American Museum of Natural History.)

Within a mile of this trackway, though, are elongated dinosaur tracks that may reflect evidence of stalking behavior, connecting to speculations mentioned previously that some theropods went fishing. Glen Kuban, a paleontologist who has studied and mapped the Paluxy River dinosaur tracks for more than twenty years, wondered whether a few of the theropod tracks there were a direct result of their stalking fish in shallow water, like modern grizzly bears seeking salmon. His reasoning was based on how a few trackways show where their makers went from normal digit impressions to those including metatarsal prints. This meant the dinosaurs were sinking more deeply into the mud, causing their footprints to become elongated.

Wait, metatarsal impressions? Where have we heard about those before? That’s right, these are associated with trace fossils where theropods voluntarily squatted. So these elongated tracks could have been made either through the dinosaurs walking from spots with firm mud to spots with squishier mud (involuntary), or they could have occurred from these dinosaurs lowering their bodies while walking (voluntary). When fishing without a pole or net, the best way to catch fish is to get your tools closer to the water surface, and for a theropod that would have meant lowering its arms and mouth. Although this is a difficult hypothesis to test further, it is nice to have a more dashing alternative to one that simply states, “They waded into squishy mud and kept going.”

Have paleontologists ever discovered theropod tracks connected to a kill site? Not yet, but some Late Jurassic theropod tracks and
other circumstantial evidence of feeding are associated directly with sauropod bones. I was lucky enough to see these trace fossils during the summer of 2005 while passing through Thermopolis, Wyoming. My wife Ruth and I had stopped to see the Wyoming Dinosaur Center there, but also took a guided tour of a nearby dinosaur dig site. The site not only had an impressive array of sauropod bones exposed but also included (more significant) dinosaur tracks.

There, clustered around the skeleton of a juvenile
Camarasaurus
, were three-toed tracks. Researchers who worked on this site had also found sauropod tracks, showing they also lived in the area, but found ominous toothmarks on the bones along with shed teeth of juvenile and adult
Allosaurus
. The tracks were also about the right size and shape for this formidable predatory dinosaur. Additional trace fossils in the sauropod skeleton included large rounded stones. These were interpreted as gastroliths originally within the body cavity of the sauropod, exposed by the theropods as they chowed down on the
Camarasaurus
. As a result, the researchers concluded that this was a feeding ground, perhaps after a kill or—perhaps less exciting to most people—following some other cause of death for the sauropods, which brought in allosaurs to scavenge. More compelling, it conjures a scene of an adult
Allosaurus
and its offspring dining together on the remains of a young and then-recently departed sauropod.

Other Cool Stuff You Probably Didn’t Know about Dinosaur Tracks

Because there are so many dinosaur tracks recorded in Late Triassic through Late Cretaceous rocks worldwide, the chances are good that a few unusual or otherwise stimulating insights might be conveyed by some of them. Sure enough, some dinosaur tracks tell amazing stories. The following is a short selection of what I think are provocative perspectives on dinosaurs bestowed by their tracks, and that we might not have ever figured out from bones alone.

Imagine you are a small theropod dinosaur, out on a walkabout one fine Jurassic day, on the prowl for food or mates, and not necessarily in that order. As you stride over some slightly bumpy terrain,
you smell something interesting in a meter-wide mud-filled pit. As soon as you jump into the pit to investigate, your feet start sinking into the saturated mud and you suddenly realize you’re stuck. Even worse, as you struggle to free yourself, you’ve further liquefied the mud and caused it to envelop you even more. At some point you just stop flailing because you can’t move. You’ve just been buried alive.

During the next few days, several others of your species make the same mistake. A few make it out, but most others, including additional species of dinosaurs, likewise become stuck and die. Meanwhile, the sauropods that unwittingly made these traps are already miles away, having no idea that they contributed to the asphyxiation and fossilization of these small theropods. Their deep tracks, pushing down into the moist substrate, had created new hazards for smaller animals, which fell in and became mired in the squishy track interiors.

Oddly enough, the preceding scenario is exactly what three paleontologists proposed to explain how some exquisitely preserved small theropods in Middle Jurassic rocks of western China were unwillingly ushered into the fossil record. In a 2009 journal article with the beguiling title of “Dinosaur Death Pits from the Jurassic of China,” the researchers—David Eberth, Xu Xing, and Julia Clark—proposed that specimens of the small theropod
Guanlong wucaii
had fallen into 1 to 2 m (3.3–6.6 ft) deep mud-filled pits made by large sauropods similar to the Late Jurassic
Mamenchisaurus
.
Guan-long
wasn’t the only animal to suffer such a fate at the feet of these sauropods, either, as the theropod
Limusaurus
and about twenty other species were entombed in these tracks. What all of these dinosaurs had in common was that they were small and bipedal, meaning they lacked sufficient limb strength to yank themselves out of their morasses. More than 160 million years later, those dinosaurs’ misfortunes became paleontologists’ treasures, as this unusual mode of death and fossilization resulted in beautifully complete skeletons of scientifically significant species. All thanks to the sauropod tracks.

But let’s say your timing was even worse, and you just happened to be in the same place where a dinosaur foot—with the rest of a
living dinosaur attached to it—happened to land. If the dinosaur was
Microraptor
or some other similarly puny feathery theropod, then no big deal, unless you were a small insect. But if the foot belonged to a tyrannosaur, ceratopsian, ankylosaur, stegosaur, or sauropod, then that was a problem for any animal smaller than that foot. Sure enough, a few fossils, such as snails and clams, found in dinosaur tracks were crushed underfoot, although we can’t tell for sure whether they were alive or already dead when stomped.

Dinosaur tracks can also sometimes tell us if a given dinosaur had any health problems. We know a fair number of dinosaur bones show evidence of having been broken and later healed, but it’s difficult to say whether these had an effect on an animal getting around, either while healing or afterwards. Fortunately, dinosaur trackways are abundant enough that some with asymmetrical gait patterns pop out. In these trackways, one side of the body stepped less distance than the other, resulting in noticeably unequal strides. The easiest explanation for these imbalanced patterns is that these dinosaurs were compensating for a weak leg, such as one injured from an accident or combat.

One theropod trackway in a Late Jurassic stratum in Utah leaves no doubt that its maker had a tough time walking, showing alternating long and short steps that demonstrate how it was hobbling along with just one good leg. Because this trackway is relatively easy to access and on public land, I’ve taken university students to it, given them a tape measure, and said, “Measure this, then tell me what you think this dinosaur was doing,” without saying anything more about it. One time there, within about fifteen minutes one of the students, after writing down a few measurements, looked up from the trackway with a huge grin on her face. “It was limping!” she shouted triumphantly. (It was one of those proud moments that, as a teacher, I’ll never forget, and hopefully the students have not either.)

Once the students had all of the information they needed to further confirm and test this hypothesis, I then asked them, “Okay, which leg was injured?” It took a few more minutes for them to
figure this out, but they did. The students reasoned that the strong (non-injured) leg of the theropod enabled it to take a longer step, whereas the hurt leg could not as easily bear the theropod’s weight, so it would have taken a shorter step when pushing off with that leg. Using this logic, they figured the left leg was hurt, whereas the right leg was normal.

Other maladies show up in dinosaur tracks, such as missing or deformed digits. Paleontologists have to be careful when making such judgments, of course, because many, many dinosaur tracks did not faithfully preserve a record of every digit on the original feet, especially those made as undertracks. Nonetheless, once in a while we notice that a trackway consistently shows all expected toes on one foot, but one less on the other foot. One such trackway is from the Early Jurassic of Massachusetts, in which three tracks in sequence—right, left, right—have a perfectly fine three-toed theropod track on the left, but a two-toed one on the right, missing its innermost digit.

Expanding from a single toe to entire ecosystems, dinosaur tracks are also incredible tools for showing us exactly where dinosaurs lived, which seems to have been nearly every lowland environment and with occasional forays into water bodies. Their tracks are interpreted from geologic deposits formed in deserts, river and delta floodplains, swamps, lakeshores, and tropical shorelines, and from formerly equatorial regions to near the North and South Poles. Because of the preservation problems of mountainous areas—their rates of erosion are too high to preserve tracks very well—we don’t know whether dinosaurs lived in alpine environments or not. Still, the nearly ubiquitous occurrence of their tracks elsewhere implies that they would not have been restricted from occupying higher-altitude places, either. Dinosaurs, according to their tracks, covered the landscape.

The Last Steps of the Dinosaurs

So what about at the other end of dinosaur history, when they all died out? Do their tracks help to answer the seemingly
unanswerable? Well, sort of. As nearly every child can tell you, non-avian dinosaurs all went out with a cosmically delivered bang from a meteorite impact at the end of the Cretaceous Period and the start of the Paleogene Period, around 65
mya
. This impact left an ash layer enriched with a rare element—iridium—that is only found abundantly in meteors. It also left lots of other evidence for its impact, not least of which is a huge crater of the right size and age in the Gulf of Mexico next to the Yucatan peninsula of Mexico.