Pace Angulation 180 Animal

Straddle

FIGURE 14.1 Measurable parameters that can be derived from a well-preserved dinosaur track and trackway, assuming bipedalism. Note diagonal pattern to the trackway, which is typical for those made by dinosaurs.

Straddle

FIGURE 14.1 Measurable parameters that can be derived from a well-preserved dinosaur track and trackway, assuming bipedalism. Note diagonal pattern to the trackway, which is typical for those made by dinosaurs.

individuals of some quadrupedal dinosaurs. For example, stegosaurs show five digits on the manus but only three on the pes. The number of digits in the pes is unknown for some dinosaurs, such as pachycephalosaurs (Chapter 13), so their tracks are unrecognizable until skeletal data provide some basis of comparison.

A trackway is defined as a series of two or more successive tracks made by the same foot, which is ideal for making an important measurement, stride length. Stride length is the distance between each track made by the same foot in a trackway. Tracks made by appendages from successive opposite sides, such as right-left or left-right in a bipedal animal, can comprise a partial trackway, and the measurement gained from the distance between these impressions is the pace length. The straddle is the width between the tracks on each side of a trackway, measured directly as the distance between the outsides of tracks on the left and right sides of the trackway. Straddle is also approximated by pace angulation. This is an angle described by the pace of one side in comparison to the overall stride. For example, a pace angulation of 180° is a straight line, made as if the animal walked on a tightrope. This is typical of theropod trackways and is also very common in modern birds. In contrast, a pace angulation of 120° represents more of a sprawling movement and thus reflects a greater straddle. High pace angulations and correspondingly narrow straddles of most dinosaur trackways verify skeletal inferences that dinosaurs walked with their legs underneath their bodies in an upright posture (Chapter 1). A semi-sprawling posture has been interpreted for some cer-atopsians, but this is still subject to debate (Chapter 13). Regardless, nearly all dinosaur trackways show a diagonal pattern if a line is drawn from one pes to the next. Because almost all of these trackways indicate walking speeds (discussed later), they can be described as diagonal walkers. Dogs, cats, cattle, sheep, deer, and many other modern mammals leave such trackway patterns.

Trackway patterns for both bipedal and quadrupedal animals can be observed in modern environments and compared to patterns left by both bipedal and quadrupedal dinosaurs. As discussed in previous chapters, animals can have two modes of locomotion, obligate or facultative. Obligate means that they can walk only in a certain way, whereas facultative means they have the ability to walk a different way from normal if required. For example, seemingly all theropods were obligate bipeds (Chapter 9). In contrast, some ornithopods were facultative quadrupeds (Chapter 11), in which they walked bipedally most of the time but switched to a quadrupedal locomotion at other times. Bears that walk on their hind legs are thus demonstrating that they are facultative bipeds. Humans are obligate quadrupeds early in their development and learn to become obligate bipeds later in life.

Trackway patterns of quadrupedal animals are potentially more complicated than those of bipedal ones. These can vary considerably, depending on:

1 an animal's adaptations for its most efficient motion; or

2 changes that occurred in its behavior while it was making the trackway.

For example, a commonly encountered trackway pattern for a quadrupedal mammal is made by it walking and leaving a visible pes track just to the back of or overlapping a manus track, resulting in right-left pairs of foot impressions (Fig. 14.2A). This is the most common pattern observed in quadrupedal dinosaur trackways, seen for tracks of some ornithopods (Fig. 14.2B; Chapter 11), sauropods (Chapter 10), thyreophorans (Chapter 12), and ceratopsians (Chapter 13). However, the trackway pattern changes a great deal when an animal picks up its pace by trotting or

(B)

FIGURE 14.2 Manus-pes placement in typical walking pattern by quadrupedal animals. (A) Manus-pes pair from dog (Canis domesticus) trackway preserved in modern sidewalk, Emory University, Atlanta, Georgia. (B) Manus-pes pair from iguanodontian trackway in Lower Cretaceous bedding plane near Morrison, Colorado.

running. Trackways caused by a running quadruped typically have a considerably longer stride length in relation to the footprint size and greater spacing between the same-side manus-pes pairs. Likewise, when an animal otherwise makes movements that are more like those of animals that normally bound (hop), pace, or gallop, the resultant tracks will reflect these behaviors also.

So far, only one trackway made by a quadrupedal dinosaur has indicated running, a Late Cretaceous ankylosaur trackway from Bolivia that was described recently. On the other hand, at least eight running theropod trackways have been documented. This evidence supplements anatomical data to support the hypothesis that bipedal dinosaurs, in general, were capable of moving faster than quadrupedal dinosaurs. No dinosaur trackways found so far are interpreted as demonstrating bounding, pacing, or galloping. Based on current trackway evidence, nearly all dinosaurs just walked, which is the most common locomotion mode for modern animals, too.

One of the more mysterious-looking trackways formed by a quadrupedal animal occurs when the animal places its same-side pes into the preceding manus print, which can give the appearance that the animal was walking bipedally. This placement, called "direct register" by some modern trackers, is observable in some trackways made by modern feral cats and foxes. One of the ways to detect direct-registered tracks is to look for a pes impression within the manus print, which can be visible if the manus print is larger. However, if the pes is larger than the manus, it can obscure or obliterate the manus impression that immediately preceded it. Some quadrupedal dinosaurs did have differently-sized feet, and in most cases the rear feet were larger, although brachiosaurids had larger feet in the front than the rear (Chapter 12). Direct register has been documented in some sauropod trackways, although many of their trackways also show distinct manus and pes impressions (Chapter 10).

Some trackers or hunters refer to a well-worn, unvegetated path through a field or woodland area made by repeated trackways of deer and other mammals as a trail. However, this term is also applied to surface traces left by the movement of legless animals such as worms, snails, or snakes. Trails in modern environments are typically caused by only one or two species of animals. Thus, if clear tracks are preserved, the trailmaker usually can be correlated with its trail. In this sense, no dinosaur trail has ever been described, which may be a function of their low preservation potential or lack of distinctiveness. For this reason, our discussion of dinosaur trackways will concern their numerous well-preserved tracks and trackways.

tHunting and tracking animals has probably been a part of the human experience for the past 100,000 years. 4s a result, the usefulness of tracks as a source of data about animal behavior was probably tested long before the earliest known language was invented, and certainly predates modern scientific methods.

Stories Told by a Single Footprint

Evidence that ancient peoples were aware of animal tracks can be inferred from one of the oldest known art forms, macaroni (not to be confused with pasta). This art form consists of finger tracings in clay, made on the walls of caves in Spain and France during the Late Paleolithic, about 30,000 years ago. Some of these tracings include line drawings of animals, and the association of these representations with many hand and footprints are interpreted as imitations of animal tracks. Likewise, the indigenous peoples ("aborigines") of Australia made rock art in northern, central, and southern Australia that prominently featured identifiable animal tracks. The earliest form of this art style is panaramittee, which also dates from about 30,000 years ago.

The San ("bushmen"), who comprise the modern indigenous peoples of the Kalahari region of southern Africa, still use tracking for their hunting. Their methods are thoroughly scientific, as tracks are observed and questions are asked immediately about them:

■ What animals made these?

■ Which directions were they moving?

■ How fast were they moving, and where do they vary their speeds?

■ How many animals were in the group?

■ How many of the animals were adults and juveniles?

■ Which animals were male and which were female?

■ Were any of them sick or injured?

Hypotheses are then formed to explain the data. For example, the tracks may indicate that six animals, two adult females, one adult male, and three juveniles, moved slowly while grazing on some vegetation just a few hours ago, then they simultaneously turned to look at the source of a sound, were frightened by a predator, and ran for cover in the nearest grove of trees. The hypotheses are then tested for their veracity and possibly falsified, but if they are continually falsified, the hunters and their families starve. San hunters thus have good incentive to make careful observations and modify their hypotheses in the light of new data, and their methodology has more immediacy for these hunters than it would for, say, a tenured scientist at a university. Native American tribes, such as the Apache, and the aforementioned native Australians are among the indigenous peoples who have been also renowned historically for their tracking skills.

The old Chinese saying, "The longest journey begins with a single step," attributed to Chinese philosopher Lao Tzu (about 600 bce), also can be applied to the description and interpretation of a trackway. Any single track can be examined in detail and is a potential storehouse of information about the animal that made it. This is not just for identifying the trackmaker and associated measurements, but also for interpreting the behavioral dynamics of the animal. For example, the micro-topographic changes imparted to a substrate by an animal's foot are among the qualitative data from a single track that can be used to infer detailed interpretations of behavior. Because changes in foot movement involve applying pressure against the substrate around, inside, and underneath a track, as well as releasing that pressure, the resultant deformations can be called pressure-release structures. By describing such features, a track can reveal the general direction of the animal's movement and indicate whether that animal stopped and looked in a certain direction, made an abrupt change in direction, moved backward, or was carrying something in front of it or on its back (Fig. 14.3). These same criteria, related to such variations in behavior, can then be applied as models for dinosaur tracks (Figs. 9.13B and 14.4).

One of the most important principles for understanding the morphology of a single track is pressure, or stress. The force applied per unit area associated with a track depends on the mass of an animal in combination with gravitational force (at 9.8 m/s2: Chapter 1), and the area of the foot making contact with the substrate. For one example, take a standing, vertically oriented bipedal animal, such as a human, with equal distribution of its weight on each foot. This human causes stress on the substrate below each foot almost equal to half of their weight divided by the area of the foot. Let us assume a rectangular shape for a man's foot (28 cm long, 9 cm wide, in typical running shoes) and a current weight of 66 kg. The stress applied per foot as a bipedal animal (sf, herein called foot stress) is

FIGURE 14.3 Different pressure-release structures caused by different behaviors, transmitted by the right foot of the same person walking in firm sand. (A) Moving straight forward. (B) Making an abrupt right turn.

FIGURE 14.4 Different pressure-release features caused by displacement of sediment from movement of a dinosaur. (A) Large theropod track in sandstone, Late Jurassic, Utah. (B) Sauropod track in sandstone, Late Jurassic, Utah. Theropod was moving straight forward, whereas sauropod had began to make an abrupt right turn.

FIGURE 14.4 Different pressure-release features caused by displacement of sediment from movement of a dinosaur. (A) Large theropod track in sandstone, Late Jurassic, Utah. (B) Sauropod track in sandstone, Late Jurassic, Utah. Theropod was moving straight forward, whereas sauropod had began to make an abrupt right turn.

This number may seem high, but realize that it is the amount of stress the man would cause if his weight were distributed equally over a square meter. Perhaps a more meaningful measurement is stress as applied to a square centimeter. Using 1m2 = 1.0 x 104 cm2 makes the foot-stress about 1.3 N/cm2. Because the area of the shoe imprint is actually more oval than rectangular, this represents less area. As a result, a correction can be accomplished by multiplying the value by 0.8, which is an approximate conversion factor for an inscribed oval within a rectangle; this changes the value to 1.6 N/cm2. This corrected value increases even more once shoes are removed. This is because the surface area of the foot in contact with the substrate decreases but the weight does not (other than subtracting the weight of the shoes, of course).

Also, this value represents a mean for the given surface area. Consider that the weight of a standing person is not distributed evenly over the entire area of the foot but is mostly on the heel (metatarsal), which is directly underneath the main part of the body weight. Because most dinosaurs had their metatarsals elevated above the ground during life, the stress imparted by their feet was mainly on the phalanges. A few dinosaur tracks show metatarsal or "heel" impressions, but these are uncommon. The weight of a bipedal dinosaur was distributed over two feet, whereas a quadrupedal dinosaur had it divided by four feet that were probably unequally sized. These variations require estimation of the areas for the manus and pes. The areas for all four feet then should be added to calculate a modified general formula for cumulative stress (sc) caused by an animal on an underlying substrate, which is identical to Equation 3.8 (Chapter 3):

For dinosaurs, a value for this stress is calculated by measuring the area of a dinosaur's tracks. These data are then used in combination with a weight estimate for the probable tracemaker. However, this will be only a rough estimate of the foot stress and probably represents a maximum value. After all, the weight distribution of any given dinosaur was also over more of a horizontal plane than a human. Humans and penguins are the only bipedal animals known to have their head, spine, and metatarsals aligned in a more-or-less vertical plane, perpendicular to the ground surface.

Any movement of an animal also causes stress, especially where it typically pushes off the substrate, thus generating force applied to an area of that substrate. This happens whether the animal was moving forward, backward, laterally, or jumping upward. Evidence for this stress can be shown by visible zones of deformation in the substrate, which in the right substrate, such as a firm mud or sand, will preserve the effects of the movement as pressure-release structures. Normally the movement of an animal is straight forward but this cannot be assumed. Walking backwards, backing up, or moving laterally can happen for various reasons.

An individual track and all of its information can lead to a hypothesis about the track itself. Of course, the best way to test this hypothesis is to study successive tracks (if preserved) and look for corroborating or contradicting evidence. If the next track provides different information, then new questions can emerge about why the two tracks differ:

■ Did the animal change its behavior from one track to the next?

■ Did the behavior stay the same but the substrate change, so that a successive track was preserved differently from the preceding one?

■ Did the animal distort or otherwise damage its own track as it pulled its foot out of the substrate?

■ Did an animal of the same species follow another, causing overlapping tracks?

■ Were the tracks modified by physical processes (weathering), which obscured their original forms?

All of these questions, and the hypotheses attached to them, begin with observations made on a single track. Looking for such signs is an excellent exercise in observation that can carry over into everyday life (Chapter 2), prompted by the seeking of subtle clues about what may have happened in the recent or not-so-recent past.

Finally, a single well-preserved track with skin or toe-pad impressions can provide details about the soft-part anatomy of an appendage. A dinosaur track that shows skin impressions is still a trace fossil, although it does directly reflect a body part. A detailed body impression made by a tracemaker in association with a trace fossil is a bioglyph. Skin impressions not associated with tracks or those made by an already-dead dinosaur have occasionally been called trace fossils. Nevertheless, only in cases where the dinosaur was still alive and indicating behavior can this designation be made. In addition to skin impressions, toe-pad impressions are also commonly preserved in dinosaur footprints, which help to determine (along with skeletal data) what dinosaur feet looked like (Chapter 5). The toe pads often correspond with phalanges, so footprints with well-preserved toe pads can be compared to known phalangeal formulas of dinosaur groups, which helps in identifying which dinosaurs made which tracks.

What about not-so-well-preserved dinosaur tracks, ones that do not show individual toes, let alone skin impressions? In fact, vague depressions in Mesozoic rocks can often provoke arguments about whether they represent tracks. Even so, measurements still can be taken of a possible track's dimensions and its overall size and shape compared to known dinosaur tracks. A geometric outline of a possible track can be categorized through a compression shape and is a useful guide for trackers who do not have "perfect" substrates for preserving detail, such as firm mud or sand. For example, with modern animals, felines leave round compression shapes, whereas canines leave oval compression shapes, all within well-documented size ranges. In dinosaurs, the compression shapes varied considerably, especially as the Mesozoic progressed, but those for major groups of dinosaurs are distinguishable from one another through length-to-width ratios and overall outlines. Through using such methods, several examples of formerly disregarded "potholes" or "bulges" in strata turned out to be dinosaur tracks.

Track Taphonomy

The preservation of a dinosaur track in a substrate, so that it is visibly identifiable as a track more than 65 million years after it was formed, can be complicated. As with body fossils, tracks are susceptible to weathering, erosion, and erasure by other organisms. These factors are especially pronounced on the surface where the animal was moving, and nearly all tracks formed on an exposed surface quickly disappear through natural processes. Consequently, the same circumstances conducive to preserving a dinosaur body fossil apply to their tracks as well. They need to have been buried quickly. In this scenario, most dinosaur tracks had a distinct advantage, as most tracks were probably "buried" (under the surface) as soon as the animal made them.

An impression made by the appendage of an animal on a substrate below the surface where the animal was moving is an undertrack. Heavy animals, such as adult sauropods, did not necessarily make undertracks. For example, small horseshoe crabs, not much larger than the average cockroach, also left visible under-tracks in the geologic record. The only requirement for the formation of an undertrack is that the impressions made on one layer of sediment by the weight and movement of the animal deformed an underlying layer. This is the case no matter how thick or thin the upper layer may have been. An indistinct or otherwise incomplete outline of a track is one clue that it may be an undertrack, but evidence for undertrack preservation also is provided by tracks that show cross-cutting by invertebrate burrows. The track had to have been already buried in unlithified sediment to be later modified by the burrowers.

Factors that affect the preservation of undertracks in sediments are:

1 the amount of water in the sediment, which affects its relative firmness;

2 grain size; and

3 cohesiveness.

The latter factor typically depends on the amount of clay minerals in the sediment, which can help sand grains to stick together. In general, fine-grained sediments, such as clay and silt, with only enough water between grains to make the sediments cohesive, are the best for preserving detailed tracks.

Coarse-grained sediments preserve little more than the compression shape of the track. Sediments with high or low amounts of water either preserve no impressions or leave impressions that tend to collapse inward. Local variations in substrate conditions, where one patch of sediment is more moist and loose than an adjacent patch, will cause different impressions by the same animal, literally from one step to the next. Similarly, a trackway where an animal walked over a substrate that was either well packed or cemented might leave no visible impression at all, which results in "missing" tracks if these areas bracket softer substrates (Fig. 14.5). Missing tracks in dinosaur trackways have been interpreted incorrectly, such as some being used as evidence for swimming dinosaurs, incredible stride lengths, or amazing leaps.

FIGURE 14.5 Differences in tracks as a function of substrate firmness, illustrated by a juvenile human female, weighing about 30 kg, making a trackway on a beach where tracks "disappear" in the middle because of firmer sand in that area.

The vast majority of dinosaur tracks probably formed through a wide variety of behaviors that imparted different stresses to the sediment, which were then preserved as undertracks in a variety of sediments with different grain sizes and initial water contents. The tracks therefore did not preserve many of the numerous details revealed by fresh surface tracks made by modern animals, such as all of the pressure-release structures. In some cases, the changes in movement by a trackmaker were transmitted through the layers underneath the track surface and were recorded by pressure-release structures (see Fig. 9.13B). Nonetheless, such features should not always be expected in fossil tracks.

The track preservation discussed so far only has concerned molds (negative images) of dinosaur feet, but many dinosaur tracks are also preserved as natural casts (positive images). For casts, the original footprints were filled with sediment that differed in grain size from the original substrate, such as a sand cast of a footprint made in an underlying muddy surface. Three scenarios have been proposed for how casts of dinosaur tracks were formed:

1 tracks on a muddy surface were filled with sand that later lithified;

2 tracks were impressed first as undertracks on a muddy surface, which were later exhumed and filled with sand that lithified; and

3 the casts may be undertracks, where the pressure of a dinosaur foot caused sand to squeeze into an underlying muddy layer.

In the last case, the sand formed an outline of the dinosaur foot that later lithified.

Experimental evidence for the taphonomy of tracks, including how tracks are preserved in modern environments, is currently minimal. Nevertheless, track preservation is a worthy field for further study, considering that understanding it helps to evaluate whether:

1 fossil tracks observed on a rock surface are contemporaneous; or

2 they consist of a mixture of tracks from different levels and thus different times.

The most obvious way to tell which dinosaur made which tracks is to compare their feet with described tracks.

Classification of Dinosaur Tracks

Although the skeletal components of a dinosaur appendage show its general overall form, the bones normally do not include the fleshy parts (Chapter 6). As a result, their skeletal feet do not indicate their total width and length, let alone the parameters for individual digits. This situation can be illustrated by comparing a human skeletal foot to a track made by the same human. A track can provide a minimum size and other parameters for the original track-making appendage. Moreover, the geometry of a reconstructed skeletal foot can also give a minimum size to expect for a track made by that animal. Conversely, if a dinosaur walked through a soft substrate, it might have sunk deeply enough to make metatarsal impressions, resulting in a much larger track than that made simply by phalanges and tarsals.

Potentially many dinosaur species are still undiscovered, which only adds to the difficulty of matching a dinosaur trackmaker with its tracks. The tracks of these unknown dinosaurs may have already been found, but relatively few members of their original populations were preserved (Chapter 7). Thus, any comparison of track data, which are more abundant in some areas than skeletal data, to only known species of dinosaurs will likely result in mismatches. Yet another problem is distinguishing the tracks of juvenile dinosaurs of one species from those of small adults from another (but morphologically similar) adult species.

Body Impressions Sauropod
FIGURE 14.6 Typical track morphologies interpreted for theropods, ornithopods, prosauropods, sauropods, ankylosaurs, stegosaurs, and ceratopsians.

With those caveats in mind, several of the criteria applied to tracks can be used for successful correlation of dinosaurian tracemakers with fossil tracks. Criteria for individual tracks are number of toes, track size (including width-to-length ratios), and presence or absence of claws. Study of trackways will require the preceding information plus the number of feet used in locomotion. Using these characteristics as a guide, dinosaur tracks and trackways can be broadly allied to theropods, prosauropods, sauropods, ornithopods, ankylosaurs, stegosaurs, and ceratopsians (Fig. 14.6). Without careful descriptions of the tracks, they are too easily attributed to the wrong dinosaur tracemakers. For example, both theropod and ornithopod tracks have a tridactyl (three-toed) pattern, and they overlap in their size ranges. However, examination of width-to-length ratios and other specific descriptive criteria can help prevent such mistakes.

Some track types are more common than others. Theropod tracks are exceedingly common in some strata throughout the entire geologic range of dinosaurs, but ankylosaur and stegosaur tracks (Chapter 14) are rarely reported from any strata. A hypothesis for the disparity in track abundance between theropods and most other dinosaurs is that theropods, as active predators or scavengers, moved about a great deal more in their ecosystems in search of food than herbivorous dinosaurs (Chapter 9). However, occasional discoveries of horizons trampled by numerous herbivorous dinosaurs, such as sauropods and ornithopods, provide notable exceptions to such generalities (Chapters 10 and 11).

Paleontologists face a minor dilemma with dinosaur tracks compared to skeletal remains. This is because, in the vast majority of cases, trace fossils cannot be given names according to the exact species of animals that made them. This is especially true for instances where the tracemaker is otherwise unknown. One of the main advantages of the Linnaean binomial nomenclature (Chapters 1 and 5) was that it improved communication of information about particular organisms, modern or fossil, by providing standardized species names. A similar methodology is applied to dinosaur tracks, in which they are given names according to their distinctive forms, called ichnotaxa (= "trace names," plural for ichnotaxon). With an ichnotaxonomic nomenclature, dinosaur paleontologists can communicate more effectively about tracks by giving them ichnogenus and ichnospecies names, such as Grallator and Megalosauripsus brinoensis. Even though some paleontologists have attempted to connect certain dinosaur tracks with dinosaur genera, such as Megalosauripsus with the theropod Megalosaurus, the tracemaker should not be confused with the trace name. In ichnology, naming an ichnotaxon on the basis of its supposed tracemaker genus or species is a dangerous practice, because it mixes description with interpretation and is liable to lead to a false interpretation. For example, paleontologists in the nineteenth century originally interpreted some trace fossils made by invertebrates as body fossils, such as algae. Some of their ichnogenus names still reflect those mistaken body-fossil affinities.

Another problem with naming ichnotaxa for dinosaur tracks is that the conditions for track preservation, such as substrate type and behavior of the trackmaker, were so variable that a single trackway made by the same dinosaur could yield multiple ichnogenera. So tracks made by modern animals in a variety of substrates, reflecting myriad different behaviors and preservation modes, can provide models of comparison for testing the validity of ichnotaxa. Additionally, many of the same morphologically distinctive dinosaur tracks have been given different names by different authors. Similar synonymies have happened with biological species names based on skeletal data (Chapter 5). Finally, when track size is used as a reason for splitting a morphologically similar ichnotaxon into multiple new ichnotaxa, then the possibility that the track sizes are simply from juvenile and adult dinosaurs of the same species cannot be discounted. This diversity of ichnogenus names is one reason why "track diversity" may be a misleading indicator of actual biological diversity of dinosaurs, and such interpretations should be scrutinized carefully. The best ways to avoid the preceding problems are:

1 to make detailed descriptions of individual tracks;

2 only name a track if it is part of a well-preserved trackway;

3 review valid ichnogenera given to dinosaur tracks in previous studies; and

4 make sure that a potentially new ichnogenus of dinosaur track has a geological context to it, including a stratigraphic position and geographic location.

Unfortunately, the nomenclature of fossil vertebrate tracks, especially for dinosaurs, remains one of the most unwieldy subdisciplines in ichnology because such advice has not been followed in the past, although many of these mistakes happened in the nineteenth century.

Most dinosaur paleontologists keep in mind the vagaries and potential pitfalls of naming tracks, and few want to wade through the confusing scientific literature on dinosaur track names. Consequently, they will simply use dinosaur clade names for tracks made by members of those clades. Thus, "theropod tracks," "sauropod tracks," and so on, are the most commonly encountered descriptors for reported dinosaur tracks, despite the fact that they combine description with interpretation. Nevertheless, use of the criteria for identification is normally sufficient for correlating descriptions of dinosaur tracks with at least broad taxonomic categories. More specific designations (such as "allosaurid tracks" or "hadrosaurid tracks") can be reasonably made later in consideration of known skeletal data supporting the presence of such dinosaur taxa in the same-age strata. Nonetheless, one of the exciting facets of dinosaur track discoveries is that they may represent tracks from previously unknown dinosaurs. Such discoveries can cause much spirited discussion and debate among dinosaur paleontologists, as they attempt to resolve the apparent discrepancy between the body and trace fossil records of dinosaurs.

Individual Dinosaur Behaviors Indicated by Tracks

The best method for describing individual dinosaur trackways, also applicable to multiple trackways on the same bedding plane, is to construct a track map. These maps are similar to those used to look at distributions of skeletal components at a site (Chapter 3). The map should be a scale representation of the trackway as seen from above where the dinosaur was walking, and should include a legend, scale, and direction indicator (Chapter 4). These maps provide important information for interpreting a trackway in terms of its completeness. Moreover, they help as a tool for visualizing the spatial relations of the dinosaurian tracemaker in the context of its original environment. Qualitative information can also be derived from a mapped trackway, relating to whether a dinosaur lived in a particular environment. For example, was the dinosaur well adapted to that habitat, and did it reflect its paleoecologic relationship to that environment (Chapter 10)? More specific idiosyncrasies can also be detected, such as where an individual dinosaur:

■ Had trouble walking through a difficult-to-traverse substrate.

■ Abruptly changed its direction of movement.

■ Had a noticeable limp, probably related to an injury.

■ Was following another dinosaur, perhaps as a predator.

■ Was avoiding another dinosaur, perhaps as prey.

Trackways made by individual dinosaurs can provide much qualitative and quantitative data, which can then be used to tell a great deal about how these dinosaurs were behaving.

All of the preceding examples have been hypothesized for dinosaurs on the basis of trackway data.

Quantitative methods applied to dinosaur footprint data have illuminated the behaviors of individual dinosaurs. Probably the best-known application is with regard to calculating how quickly dinosaurs moved. The speeds of individual dinosaurs have been a point of curiosity for paleontologists almost for as long as dinosaurs have been studied, but a technique for estimating speed, based on a combination of skeletal and footprint data, was only developed a little over 25 years ago. Based on empirical data from living animals, this mathematical application uses stride length and footprint size in dinosaur trackways, together with the leg length (hip height) measured from dinosaur skeletons.

An easily made observation is that, compared to a slow walk, people's feet become spaced farther apart when they walk quickly or run. Also, when a shorter person runs alongside a taller person, the shorter one must take more steps in the same distance despite their equal speed. The discrepancy in the mean leg lengths of adult men versus adult women also translates to differences in mean stride length; calculations from representative samples of men and women show mean values of 1.46 m and 1.28 m, respectively.

These observations of stride lengths in association with leg lengths can be expressed for the land-dwelling animals walking on the Earth's surface, through a general relation called relative stride length (sr):

where sl is stride length and ll is leg length. Relative stride length is a dimension-less number, as the units cancel out (because length is in both the numerator and denominator). Thus, it has no units of measurement associated with it.

Stride length is measured directly through a trackway, such as the one illustrated in Fig. 14.1. However, measuring leg length is a potential problem because the size of the dinosaur that made the trackway cannot be directly observed. Nevertheless, the leg length for a dinosaur can be estimated by using another dimensionless number, 5.0, as a constant in the following equation:

where f is footprint length. Paleontologists have calculated ratios of leg height versus foot length for dinosaurs by measuring the leg length from the ground surface to the acetabulum in dinosaur skeletons, as well as the total skeletal foot length. The ratio is derived simply as

Some paleontologists have calculated ratios close to 4.0, but others use 4.5 to 5.5. These variations depend on the dinosaur groups used or their sizes (e.g., small theropods = 4.5, large theropods = 5.5). Consequently, a compromise figure of 5.0 is used here for the sake of illustration.

For example, given a Jurassic theropod track 45 cm in length, the leg length could have been:

Using different constants of 4.0 and 5.5 would have derived leg lengths with a range of 180 to 248 cm, respectively. Either way, this theropod probably had a leg length that was taller than some professional basketball players, and most people probably could have walked between its legs without stooping. Consequently, footprint length can be used as a quick method for visualizing the size of a dinosaurian tracemaker.

If the same theropod had a stride length of 307 cm, then its relative stride length was sr = 307 cm/225 cm = 1.4

If the theropod was moving faster, then the value would have correspondingly increased to greater than 1.4 as a function of stride length. So this is a relative method of working out that one theropod was moving faster than another theropod of the same size.

Although relative stride length is a good way to "equalize" dinosaur trackway data, it is only as valuable as, say, relative age dating, as compared with absolute age dating (Chapter 3). What paleontologists would definitely like to know is how fast a dinosaur was moving in some absolute measurement, such as meters per second, which can be translated to kilometers per hour. Scientists who have studied the movements of tetrapods on land have noted that increased body size results in larger animals moving faster than smaller animals, even if their relative stride lengths are identical. This means that a small theropod with a relative stride length h = ft (5.0)

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