You can do the math yourself: If the average T. Rex walked two or three miles per day, it would have left behind thousands of footprints. Multiply that number by T. Rex's multi-year life span, and you're in the neighborhood of millions. Of these tracks, the vast majority would quickly have been erased by rain, floods, or the subsequent footprints of other dinosaurs, but a tiny percentage would have baked and hardened in the sun, and an even tinier percentage would manage to survive down to the present day.
Because they’re so common--especially compared to completely fossilized skeletons--dinosaur footprints are an especially rich source of information about the size, posture, and everyday behavior of their creators. Many professional and amateur paleontologists devote themselves full-time to the study of these "trace fossils," or as they’re sometimes called, "ichnites" or "ichnofossils."
How Dino Tracks Are Formed
One of the odd things about dinosaur footprints is that they fossilize under much different conditions than dinosaurs themselves. The holy grail of paleontologists--a complete, fully articulated dinosaur skeleton--usually forms in sudden, catastrophic circumstances, such as when a dino is buried by a sandstorm or chased by a predator into a tar pit. Newly formed footprints, on the other hand, can be preserved only when they're left alone--by the elements and by other dinosaurs--and given a chance to harden.
The key to a dinosaur footprint surviving for 100 million years is that the impression has to be made in soft clay (say, along a lake, coastline or riverbed), and then baked dry by the sun. Assuming the footprint is "well-done" enough, it can then persist even after being buried under layers of sediment. What this means is that dinosaur footprints aren’t necessarily found only on the surface--they can be unearthed from deep beneath the ground, just like ordinary fossils.
Identifying the Culprit
Except in extraordinary circumstances, it's pretty much impossible to identify the specific genus or species of dinosaur that made a given footprint. What paleontologists can figure out is whether the dino was bipedal or quadrupedal (that is, whether it walked on two or four feet); what era it lived in (based on the age of the sediment where the footprint was found); and its approximate size (based on the size of the footprint).
As for the type of dinosaur that made the tracks, the suspects can at least be narrowed down. For example, bipedal footprints (which are more common than the quadrupedal kind) are produced either by meat-eating theropods or plant-eating ornithopods. A trained investigator can distinguish between two sets of prints--for example, theropod footprints tend to be longer and narrower than those of ornithopods--and hazard an educated guess.
At this point, you might ask: can't we identify the exact owner of a set of footprints by examining any fossil remains dug up nearby? Sadly, no: as stated above, footprints and fossils form in very different circumstances, so the odds of finding the skeleton of a Stegosaurus buried next to its own tracks are virtually zero.
Footprint Forensics
Paleontologists can only extract a limited amount of information from a single, isolated footprint; the real fun happens when the prints of one or more dinosaurs (of the same or different species) are found along extended tracks.
By analyzing the spacing of a single dinosaur’s footprints--both between the left and right feet and forward, in the direction of motion--researchers can make good guesses about the dino's posture and weight distribution (not a small consideration for larger, bulkier theropods). It may also be possible to determine if the dinosaur was running rather than walking, and if so, how fast--as well as whether or not it held its tail upright (since a droopy tail would have left a telltale "skid mark" behind the prints).
Dinosaur footprints are sometimes found in groups, which (if the tracks are similar in appearance) may indicate herd behavior. Numerous sets of footprints on a parallel course may be evidence of migration or the location of a now-vanished shoreline; these same sets of prints, arranged in a circular pattern, may be the remnants of an ancient dinner party (that is, the dinos responsible may have been feeding around a heap of carrion, or a tasty, long-gone tree).
More controversially, some paleontologists have taken the proximity of carnivore and herbivore footprints as evidence of ancient chases to the death. This may certainly have been the case, but it's also possible that the carnivore in question tromped along the same ground as the herbivore a few hours or days (and not a few seconds) later.
Dinosaur Tracks: Don’t Be Fooled
Because they're so common, dinosaur footprints were identified long before dinosaurs themselves--but since no one had yet conceived of the existence of giant reptiles, these track marks were attributed to giant, prehistoric birds! This is a good example of being right and wrong at the same time: it's now believed that birds evolved from dinosaurs, so it makes sense that some types of dinosaurs had bird-like footprints.
To show how quickly a half-baked idea can spread, in 1858, the naturalist Edward Hitchcock interpreted the latest footprint finds in Connecticut as meaning that herds of flightless, ostrich-like birds once roamed the plains of North America. Over the next few years, this picture was taken up by writers as diverse as Herman Melville (author of Moby Dick) and Henry Wadsworth Longfellow, who referenced "birds unknown, that have left us only their footprints" in one of his more obscure poems.
