The beginning of the new millennium marked the 200th anniversary of the discovery of two types of Jurassic dinosaur tracks in Massachusetts, in 1802. At that time, dinosaurs were unknown. They would not be named until 1841. The three-toed tracks, later named Grallator (meaning “stilt-walking bird”), were dubbed “the trail of Noah’s raven” and attributed to unknown ancient birds. We now attribute them to theropods — two-legged carnivorous dinosaurs that paleontologists consider the ancestors of birds. The larger four-toed tracks, later named Otozoum (meaning “giant animal”), were very enigmatic at the time but are now attributed to prosauropods — early relatives of the giant sauropods, more familiarly known as the long-necked, long-tailed “brontosaurs.”

This dinosaur print is one of thousands at the Twentymile Wash Dinosaur Tracksite, located at the Grand Staircase Escalante National Monument in Utah. Discovered in 1998, the site contains dinosaur footprints preserved in the upper part of the Entrada Formation. Image courtesy of Brent Breithaupt.

These early finds, like many footprint finds today, belonged to creatures previously unknown or were found before the discovery of fossilized skeletal remains of the trackmakers — representing but one unique contribution of dinosaur tracks to the dynamic study of the ancient world.

Paleoartist Greg Paul (see story, this issue) once suggested that dinosaur trackways are the nearest thing we have to dinosaur movies, emphasizing the dynamic nature of tracks made by living animals, in contrast to the death and decay represented by fossilized skeletons. Although much neglected until a generation ago, the burgeoning field of dinosaur and vertebrate tracking — ichnology — has made rapid strides in the wake of the dinosaur renaissance that has been taking place over the past few decades. With more track discoveries has come a greater understanding of how dinosaurs moved and lived, and the discoveries have also helped capture the imagination of the public.

Forging a trail

The dinosaur renaissance of the 1960s and 1970s reflected a new ecological-environmental awareness that infused paleontology and sedimentary geology with a more dynamic and holistic mindset (see story, page 18). Almost overnight, dinosaurs were transformed from defunct, extinct failures into athletic superheroes that had suppressed mammalian evolution for much of the Mesozoic. Debate over the posture, gait and speeds of dinosaurs peppered the pages of Nature and other prestigious journals, and, despite a scarcity of known tracksites, footprints from a few important sites figured prominently as concrete evidence supporting these “revolutionary” new interpretations.

David Attenborough (left) and a BBC crew films at the 160-million-year-old megatracksite in the Entrada Formation near Moab, Utah, made by a carnivorous dinosaur. Image courtesy of Martin Lockley; taken in 1987.

By the mid-1970s, paleontologists began using trackways of long-striding bipedal dinosaurs to estimate the maximum speeds they attained. A 110-million-year-old theropod trackway from Texas provided an estimated speed of about 43 kilometers per hour (26 miles per hour) — as fast as (or faster than) Olympic sprinters. Another theropod trackway, found in China in 2001, provides a close second, representing a silver-medal performance at an estimated 41 kilometers per hour. To date, almost all reported runners were theropods, suggesting high activity levels among these carnivorous predators, at least for the short distances over which trackway segments have been recorded. Narrow-gauge brontosaur trackways, more like those of elephants than wide straddling hippos, dispelled the archaic notion that the animals were cumbersome, primitive swamp-dwellers unable to support their weight on land.

Despite these discoveries, it was not until the 1980s that people reported many large, previously overlooked tracksites, and dinosaur ichnologists began publishing detailed maps of large sites with thousands of footprints comprising the trackways made by hundreds of different individuals representing a variety of identifiable groups. These important sites shifted the emphasis from individual behavior to ideas about social behavior, especially among gregarious sauropods and large ornithopods (duckbilled dinosaurs) that evidently sometimes traveled in large herds.

Such tracksites also proved useful for understanding the relationships between predators and prey, and for determining the preference that various dinosaurs and other vertebrate groups had for particular habitats. The rock type in which the tracks are found, or facies, helps reconstruct ancient environments and ecology.

Research in these and other new areas has gathered increasing momentum. Computer models, for example, now help us visualize the biomechanics of sauropods — showing variations in posture, locomotion and weight distribution — by modeling the anatomy of the track-making animals. The rapid pace of discovery has also established the basic morphology of tracks of other previously unknown dinosaur groups such as the horned dinosaurs (ceratopsians) and the two-toed sickle-clawed raptors (dromaeosaurids).

The dinosaur tracks renaissance has also accompanied revolutions in the study of pterosaur and bird tracks. Debate over the distinction between crocodile and pterosaur tracks, and whether the latter walked on two legs or four legs, raged in the mid-1990s, even making the pages of Time magazine. The verdict: Pterosaurs were quadrupedal and their tracks were common worldwide, in some cases forming extensive track assemblages, or “ichnofacies.”

Likewise, bird tracks were once thought rare in the Mesozoic Era, 240 million to 65 million years ago, with only three sites known prior to 1980. Now sites are widely known, especially in Asia, and prove an early origin for shorebird-like species dating back to the middle Mesozoic, 145 million years ago, if not earlier. Mammal-like reptile tracks and associated spider and scorpion trails are also abundant in the Permian through the middle of the Mesozoic. Such track distribution patterns suggest a distinctive desert dune-field paleoecology, dominated by small vertebrates including mouse- and squirrel-sized protomammals and pigeon- or crow-sized dinosaurs.

Such track diversity reminds us that many of these track-rich formations contain few if any fossils. Even in cases where an appreciable fossil record exists, it usually lacks most of the small vertebrates and invertebrates.

The track record thus fills major gaps in the fossil record in many regions. For example, in the Jurassic of the western United States, only the well-known Morrison Formation has a significant record of vertebrate body fossils, including many brontosaurs. Most other well-known formations, including the Wingate, Kayenta, Navajo, Entrada and Summerville, are almost completely devoid of vertebrate remains, except for a few sites and isolated finds, celebrated for their very rarity.

By contrast, each of these formations has yielded dozens, even hundreds, of tracksites that consistently produce characteristic and consistent patterns of ancient ecology in formations previously dismissed as “barren.” For this reason, the vertebrate fossil record of many classic national monuments, parks and recreation areas, from Arches and Monument Valley to Capitol Reef, Glen Canyon and Zion, is often based almost exclusively on the track record. Abundant tracks are recorded in many other outcrop-rich protected park areas elsewhere around the world, especially in semi-arid and mountainous or coastal sections, such as northeastern Spain, Portugal, northern China and South Korea.

Further down the trail

By studying the trackways not only for clues about how dinosaurs lived but also for clues about where they lived, ichnologists have vastly increased understanding of the ancient planet. Combining particular track types and facies, they have discovered what type of environments certain dinosaur species favored. For example, brontosaur tracks are typically found in limestone that represents tropical coastal plain systems, whereas the tracks of large ornithopod dinosaurs such as Iguanodon, and the shield- and spike-bearing ankylosaurs are much more typical of temperate, higher latitude, sand-, mud- and coal-dominated coastal systems.

The Jurassic Museum of Asturias in Spain is built in the shape of an ornithopod footprint and contains one of the largest collections of Jurassic dinosaur tracks in the world. The La Griega beach along the Coast of Asturias is in the background, and the rocks at the high-tide mark have Jurassic dinosaur footprints. Image courtesy of Jose Carlos Garcia Ramos.

One of the most striking conceptual shifts has surrounded the recognition of megatracksites or “dinosaur freeways” — regionally extensive track-bearing units, sometimes confined to a single surface. These freeways, ranging from less than 10,000 to more than 100,000 square kilometers, provide data on the type and distribution of thousands of trackmakers. They also provide insight into changing coastal dynamics and appear to be related to the buildup of sediment on coastal plains as sea level rose.

At least a dozen such megatracksites have been reported since the first were recorded in the late 1980s, and several have been placed in proper stratigraphic context. Trampling, or “dinoturbation,” has also been recognized as a widespread phenomenon that has a huge impact on the substrate. Some formations have hundreds of track-bearing layers indicating that reworking by trampling or “plowing” by vertebrate feet can affect much of the entire rock volume in some formations.

As interesting as such large-scale megatracksite phenomena are for geologists in general, perhaps the most revolutionary recent application of dinosaur tracks has been in the area of paleogeography, especially in the Mediterranean. The discovery of dinosaur tracksites in areas thought to be open marine basins or isolated Bahama-like platforms has prompted some authors to call for a major reevaluation or rewriting of the geodynamics of parts of the region during the Mesozoic, 240 million to 65 million years ago. Either dinosaurs somehow crossed deep seaways between widely separated landmasses, or our reconstructions of ancient Mediterranean geography are simply wrong.

Maintaining the trail

The most striking aspect of the dinosaur tracking revolution has been the discovery of vast numbers of sites that require in situ protection for scientific study and public education (see sidebar). Unlike bone sites that are frequently destroyed or buried as excavated skeletons are removed to museums, most dinosaur tracksites, some larger than football fields, have to be preserved in place. As already noted, these sites number in the hundreds in many nations, and add up to thousands on the continental scale. For example, about 250 recorded sites are in Colorado, and more than 100 are in the small province of La Rioja, Spain. Equally high densities are typical of many other areas, including Utah and the southern coastal region of Korea.

In some cases, large-scale mining operations have exposed large tracksites, often in rugged mountainous terrain in the high Andes, Rockies, Alps or Pyrenees. Such sites pose special logistical, conservation and political problems. First, they are dangerous places in which to work, requiring specialized skills and equipment (expert mountaineers and helicopters). Second, they are often in imminent danger of complete collapse or highly accelerated rates of erosion. Third, their long-term conservation may be prohibitively expensive and thus administratively and politically problematic.

Technology, however, can help address these challenges. A 3-D landscape imagery technique known as photogrammetry can be employed at all scales, from single footprints, to large sites of several acres and beyond, to the scale of megatracksites. While contour maps of individual tracks are visually pleasing and can even be used to print out 3-D replicas, photogrammetry may have much greater potential for accurate mapping of large sites, especially those where access is difficult and complex topography defies mapping by traditional compass, tape and grid methods.

The abundance of dinosaur tracksites has generated significant research collections, interpretive centers and museums. For example, the Dinosaur Tracks Museum at the University of Colorado in Denver, the St. George Dinosaur Discovery site in southwestern Utah and the Jurassic Museum of Asturias in Spain (built in the shape of a giant footprint), have all sprung up in the last few years. All contain thousands of footprint specimens and strong links to local interpretive trails and displays, such as Dinosaur Ridge in Colorado. Such museums and associated interpretive sites and trails each receive hundreds of thousands of visitors annually on the merits of tracks alone, often without the added attraction of large dinosaur skeletons and models.

End of the trail

The rise in the scientific study of dinosaur footprints epitomizes the dynamic field of paleontology, as it covers almost half of the entire track record of vertebrate life on land — creating an evolutionary path through time. From the oldest trails of invertebrate and vertebrate animals (respectively about 450 and 400 million years old), we are led through the middle era (Mesozoic) age of dinosaurs, pterosaurs and early birds, to the age of mammals (65 million years ago to the present) and our own hominid ancestors, the latter represented by the footprints of 3.5-million-year-old “Tanzanian” Australopithecus — now carefully buried for protection and study by future generations. These eternal trails in the sands of time are highly evocative of our ancestry and among the spectacular legacies left us by the evolving geological landscape.

Henry David Thoreau once stated: “If I were to make a study of the tracks of animals and represent them by plates, I should conclude with the tracks of man.” This statement seems prescient in the light of recent finds of purportedly 40,000-year-old hominid tracks from Mexico that suggest colonization of the Americas more than twice as early as previously thought (see story, this issue). Again, we see footprints opening a new field — hominid tracking — of special interest to our own origins and prehistoric wanderings. Such rewriting of the tracks record has progressively recast our view of the ancient landscape and physically integrated it into our present cultural landscape.

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