Dinosaurs Without Bones (5 page)

Instead, you want to know more about how this theropod, ornithopod, sauropod, prosauropod, stegosaur, ankylosaur, or ceratopsian behaved. What was it doing in the place where it left its tracks? When did it arrive on the scene relative to other dinosaurs, insects, or worms living in the same area? Where did it go after it made the tracks? Could its body be nearby, or did it travel a long way before dying? Was it with any others of its species, or looking for love in all the wrong places? How long were these tracks there before they were buried and preserved for us to see them millions of years later? You want to know more. Much, much more.

To understand dinosaur behavior from their tracks, one must absolutely study sequences of tracks, or
trackways
. Knowing that most dinosaurs either got around quadrupedally or bipedally, trackways therefore can be expected to show right–left rear foot impressions or a combination of all four feet. However, a few dinosaurs mixed it up, switching from bipedal to quadrupedal and back again, just like how someone can go from walking upright while filled with pride to crawling on hands and knees begging for forgiveness to walking tall again. In a dinosaurian sense, though, a change from a four-legged to a two-legged gait meant that a dinosaur was
facultatively bipedal
(became bipedal when it wanted) and a normally two-legged dinosaur going on all fours was—you guessed it—
facultatively quadrupedal
. These changes in
which limbs touched the ground were likely related to dinosaurs altering their speed, foraging, or other such behavioral shifts necessitated by daily life.

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
= 0.25
g
^{–0.5
_s
1.67
_h
–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.