Dinosaurs Without Bones

by Anthony J. Martin

  Every four-limbed animal has a baseline gait, or how it normally moves around on those limbs. In quadrupedal animals, such as canines, felines, bovines, or other domestic mammals, a few examples of gaits include: slow walking, normal (average) walking, fast walking, trot, lope, or gallop. For example, cats normally walk and dogs normally trot. When teaching these patterns to my students, I emphasize how gaits translate into distinctive track patterns, much like letters put together to form words. In these instances, trackway patterns read as “slow walk,” “fast walk,” “trot,” and so on. Once these students apply this knowledge to different animals’ baseline gaits, they then can more readily glance at and discern a trackway pattern, rather than stopping to measure track sizes and count toes, and much later saying “raccoon,” “coyote,” “deer,” or “grizzly bear.” (In my experience, the last of these is a very handy one to identify quickly, especially in a remote field area.) We also can get a better understanding of gaits by measuring distances between alternating feet (pace), between the same foot (stride), and the width of the trackway (straddle). Many other measurements can be taken from a trackway, but these three are essential and constitute a good start in their study.

  Can these same principles be applied to dinosaur trackways, in which you can just glance at a dinosaur trackway and excitedly shout “Theropod!” “Ornithopod!” or “Barney!” (whatever the heck he is)? The answer is, mostly, yes. Part of this identification is aided by the obviousness of some tracks, which are then confirmed by trackway patterns. For example, if you see bathtub-sized depressions that express themselves in an alternating diagonal pattern, you will probably not shout “Baby theropod!” Furthermore, sauropod tracks normally show a slow walking or “understep” pattern in which the rear foot did not quite fall in the same place as the front foot; appropriately, its stride is short, too. In a few instances, their rear tracks registered directly on top of (and hence wiped out) their front tracks, indicating a slightly less sluggish pace. So far, I have only seen one sauropod trackway in which the rear feet were placed ahead of the front feet on the same side, approaching what we might call a “trot.” This trackway was from a relatively small sauropod, which might have been a juvenile that didn’t know it wasn’t supposed to run.

  Speaking of sauropod trackways, their patterns can be further placed into two categories based on their widths: narrow gauge and wide gauge. These terms are borrowed from railroads, in which the rails are either narrowly spaced (light rail) or widely spaced (freight trains). For bipedal dinosaurs like theropods and most ornithopods, their normal trackway patterns show they were walking, although some have been interpreted as slow walking, fast walking, or running. Their trackways have an alternating right–left–right pattern, and most are probably narrower than the body width of the dinosaur that made them, especially for theropods. They did this by rotating their legs inward with each step forward, as if they were fashion models sashaying down a runway.

  To figure out the approximate size of a dinosaur from its footprint, you’ll need to use a formula. No worries, it’s an easy one. Take the length of a theropod or ornithopod track and then multiply it by four. The resulting number gives the approximate hip height of the dinosaur:

  H = 4 l

  where H = hip height and l = footprint length.

  For example, let’s say you find a definite theropod track—longer than it is wide, relatively thin toes, with sharp claw marks—and it is about 35 centimeters (14 inches) long. Let’s see: 35 cm 3 4 = 1.4 m, which translates to about 55 inches, or four and a half feet off the ground. That’s an intimidating height, especially if you think about it in human-meets-dinosaur terms, as a horizontally oriented predatory theropod would be staring directly in the faces of most people, and with the rest of its body behind it.

  Okay, now for a more complicated formula:

  V = 0.25g–0.5s1.67h–1.17

  In this, V is velocity, g is the acceleration of a free-falling object, s is stride length, and h is hip height, which you’ve already tackled. Originally devised in 1976 by a paleontologically enthused physicist, R. M. Alexander, this formula took into account the size (mass) of an animal as part of its forward momentum (the “g” in the equation relates to gravity), while also expressing the common-sense principle that, all other things being equal, short strides between tracks means an animal was moving slower, and longer strides means it was moving faster.

  But not all things are equal in this relationship, either. For instance, if a chicken were forced to race against an elephant, it has a decided disadvantage of its leg length being much shorter than that of a typical elephant. Chicken leg lengths are more or less proportional to their foot lengths, which can be readily seen in their growth from a small chick to a full-sized roaster. In other words, relatively long strides measured between tracks made by a small-footed and short-legged animal implies it was moving faster. Alexander’s formula was also based on modern animals, in which he used measured speeds, body masses, and stride lengths of many two- and four-legged animals to establish a baseline for comparing these to dinosaur trackways.

  Fortunately, there is a simpler way to express this equation and get a quick-and-dirty sense of whether a dinosaur was walking slowly, trotting, or running. This is to look at stride length versus hip height as a ratio, called relative stride length. As an example, let’s take our previously mentioned theropod with the 35-cm long footprint and 1.4 m hip height. Let’s say its stride was measured as 2.8 m (about 9 feet). So its relative stride length is 2.8 m/1.4 m, which = 2.0. Basically, Alexander proposed that relative stride lengths of 2.0 or less reflect walking, 2.0–2.9 trotting, and >2.9 running. This means our hypothetical theropod was likely walking. Using the full formula, this corresponds to a calculated speed of about 3 m/s, or 10.6 kph (6.6 mph).

  How fast is that in practical everyday terms? Olympic racewalkers regularly exceed 15 kph, which they can keep up for 20 km (12.4 mi). However amusing it might be to visualize, a good racewalker would cross the finish line of a 20-km race a half hour before our imaginary dinosaur. Even more entertaining, human racewalkers would have even outpaced huge dinosaurs—such as sauropods—with leg lengths more than double the heights of those people, as their trackways also indicate slow speeds for these dinosaurs, too.

  Nonetheless, let’s go back to that theropod trackway and follow it for a while. Along the way, you might notice its stride increased to a maximum of, say, 4.0 m. Consequently, its relative stride increases to 2.9. Now we’re talking “run,” and the calculated speed would be 5.3 m/s, which is about 19 kph (12 mph). Our Olympic racewalkers would badly lose a race of any distance to this theropod, and many well-conditioned runners would too.

  Since Alexander first came up with this formula, it’s been prodded, probed, tweaked, and otherwise tested to see how well it works, or not. Not surprisingly, then, other paleontologists have come up with their own formulas for estimating dinosaur speeds. A few have even tried to say that some dinosaurs were not capable of running at all, a supposition based on analyses of dinosaur skeletons, probable ranges of motion, and muscle masses that would be required to propel a multi-ton animal forward at high speed. Nonetheless, Alexander’s original formula still endures and is normally the first that paleontologists reach for when they find a dinosaur trackway and want to know how quickly that dinosaur was moving.

  This is probably where the gentle reader, who has seen supposedly sophisticated mathematical models undergo complete failures, might wonder if other features in a dinosaur trackway tell whether a given dinosaur was speeding up, slowing down, or otherwise varying its pace. For one, the width of the trackway should get narrower as a dinosaur increased its speed, and wider if it slowed down. Think about how a full stop by a Triceratops would show all four feet planted at the width of the shoulders and hips; in contrast, a full gallop would have registered the feet more toward the centerline of its body. A general rule for both bipedal and quadrupedal track-ways: the narrower the trackway, the more
likely the trackmaker was moving quickly.

  Don’t believe me? Fine with me, as science thrives on disbelief and testing. If you have a dog, have it walk, trot, and then run down the beach, and you will see its trackway width become visibly narrower along the way. The reverse will happen as your dog slows down to a stop, feet planted at shoulder and hip widths, tail wagging happily.

  For another independent way to test dinosaur speed, you can look at how each footprint is a record of how much ground beneath a dinosaur was disturbed by its movement. This is an experiment that can be readily performed on the same sandy beach you used for your dog experiment, and you can use your dog again, or even yourself. Tracks made by walking show little disturbance of the sand around and inside of the tracks, but jogging results in ridges, mounds, and plates of sand caused by each foot as it pressed against the sand. Sprinting imparts more prominent structures, and maybe sand kicked completely out of the track. In other words, tracks and these structures caused by the applied and released pressure—which some trackers call pressure-release structures, pressure releases, or indirect features—can be applied as another type of speedometer and used as independent checks of relative stride lengths.

  These structures, however, depend on many factors for their formation and preservation, such as the type of sediment (mud, sand, pebbles?), moisture content (dry, slightly moist, saturated?), packing (loose sand, hard-packed sand?), slope angle (flat surface, heading uphill, going downhill?) and not just the speed of the track-maker. Imagine if a theropod ran across moist and slippery mud, and then dry sand; then visualize how different its tracks would look in each type of sediment. Add in little behavioral nuances such as head position (up, level, down, right, left?), turning to one side or another while moving, stopping to scratch its back, or bending down to nab a small mammal with its mouth. All of these factors culminate in what paleontologists consider as “track tectonics,” miniature landscapes wrought by a dinosaur’s foot pushing or twisting against a muddy or sandy medium.

  Another important factor in the formation of such structures around a track is the size and anatomy of the trackmaker’s feet. For instance, think of how a sauropod’s elephant-like foot made far different structures compared to the foot of a small, thin-toed theropod. This all means that tracks can be quite complicated; and to better understand them, they are the subject of much experimental work with real animals (more on that later) and three-dimensional computer simulations. In contrast, drawing a simple cartoon outline of a track would be like summarizing a Salvador Dali painting as “art,” a terrible misdeed that omits all of its colors, hues, gradations of tones, and themes. What a pity to miss all of those metaphorically melted watches.

  Before moving on to excitedly interpreting dinosaur behavior from their tracks, though, I should point out one minor disadvantage caused by how most tracks were preserved. Dinosaur tracks, just like modern ones, were probably weathered soon after they were made, placed under assault by the erosive effects of rain, wind, gravity, and other animals stepping on them. This means that dinosaur tracks may not reflect the original surface impacted by a dinosaur. Consequently, a common way for dinosaur tracks to have made it into the fossil record was as undertracks. That is, many of the tracks we see were actually transmitted below the surface where a dinosaur walked, trotted, or ran.

  This phenomenon is similar to how, when exerting pressure with a pen or pencil while writing or drawing on a sheet of paper, an image of the writing or drawing is also impressed on underlying pages. The decided preservational advantage of this phenomenon is that such tracks were already buried, protecting them from destruction. Hence, all paleontologists who study dinosaur tracks first assume they are looking at undertracks, and only modify these realistic expectations if confronted by the delightful details of skin.

  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, boo
king, 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.