You can do the dinosaur footprint math yourself: If the average Tyrannosaurus Rex walked two or three miles per day, it would have left behind thousands of footprints. Multiply that number by T. Rex's multi-decade life span, and you're well into the millions. Of these footprints, the vast majority would 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 have managed to survive down to the present day. (See a gallery of dinosaur footprint pictures.)
Because they’re so common--especially compared to complete, articulated dinosaur 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." (Other examples of trace fossils are coprolites--fossilized dinosaur poop to you and me.)
How Dinosaur Footprints Fossilize
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, including imprints of soft tissues--usually forms in sudden, catastrophic circumstances, such as when a Parasaurolophus is buried by a sandstorm, drowned in a flash flood, or chased by a predator into a tar pit. Newly formed footprints, on the other hand, can only hope to be preserved when they're left alone--by the elements and by other dinosaurs--and given a chance to harden.
The necessary condition for dinosaur footprints to survive 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 footprints are "well-done" enough, they can then persist even after being buried under successive layers of sediment. What this means is that dinosaur footprints aren’t necessarily found only on the surface--they can be recovered from deep beneath the ground, just like ordinary fossils.
What Dinosaurs Made the Footprints?
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 fairly easily is whether the dinosaur was bipedal or quadrupedal (that is, whether it walked on two or four feet); what geological period it lived in (based on the age of the sediment where the footprint is found); and its approximate size and weight (based on the size and depth 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) could only have been produced by meat-eating theropods (a category that includes raptors, tyrannosaurs and dino-birds) 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 unearthed nearby? Sadly, no: as stated above, footprints and fossils are preserved under very different circumstances, so the odds of finding an intact Stegosaurus skeleton buried next to its own footprints are virtually zero.
Dinosaur Footprint Forensics
Paleontologists can only extract a limited amount of information from a single, isolated dinosaur footprint; the real fun starts 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 dinosaur's posture and weight distribution (not a small consideration when it comes to larger, bulkier theropods like the huge Giganotosaurus). It may also be possible to determine whether 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 footprints).
Dinosaur footprints are sometimes found in groups, which (if the tracks are similar in appearance) counts as evidence of herding behavior. Numerous sets of footprints on a parallel course may be a sign of mass migration or the location of a now-vanished shoreline; these same sets of prints, arranged in a circular pattern, can represent the traces of an ancient dinner party (that is, the dinosaurs responsible were digging into a heap of carrion or a tasty, long-gone tree).
More controversially, some paleontologists have interpreted the proximity of carnivorous and herbivorous dinosaur footprints as evidence of ancient chases to the death. This may certainly have been the case, in some instances, but it's also possible that the Allosaurus in question tromped along the same patch of ground as the Diplodocus a few hours, a few days, or even a few years later.
Dinosaur Footprints - Don’t Be Fooled
Because they're so common, dinosaur footprints were identified long before anyone had even conceived of the existence of dinosaurs--so these track marks were attributed to giant prehistoric birds! This is a good example of how it's possible to be 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 evidence that herds of flightless, ostrich-like birds once roamed the plains of North America. Over the next few years, this image 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.