How did dinosaurs leave tracks in solid stone?

A dinosaur's skeleton can tell you a lot about what it looked like, as well as a little bit about how it ate and moved. But if you really want to learn about how a dinosaur lived, you'll need to turn to trace fossils. Trace fossils, also known as ichnofossils, preserve the evidence of how life forms existed in their environment. In terms of dinosaurs, they include things like tracks, nests, eggs, burrows and coprolites -- fossilized dung.

Trackways, or collections of two or more consecutive tracks, are rich sources of information for paleontologists. For example, most of the trackways discovered so far don't include the snakelike impressions that would come from a dragging tail. This means the dinosaurs making the tracks walked with their tails held up off the ground. The lack of drag marks also means dinosaurs held their bodies up over their legs, like horses, rather with their legs splayed out like alligators.

Researchers can also calculate the speed the dinosaur moved by comparing the length of the tracks to the distance between them. In general, if the length of the stride is more than four times the length of the foot itself, the dinosaur was running, not walking. Researchers have also used computer models to analyze tracks and estimate the height of the dinosaur's hip [source: Henderson].

But there's more to be learned from trackways than how a single dinosaur species walked or ran. A collection of tracks can reveal information about social behavior. This isn't necessarily true of a trackway full of lots of overlapping tracks, such as the Davenport Ranch tracksite in what is now Texas. This area contains the prints of 23 different sauropods -- four-legged, herbivorous dinosaurs [source: American Museum of Natural History]. While some researchers believe smaller dinosaurs followed along in larger dinosaurs' tracks, it's hard to tell exactly how much time passed between the formation of each set of prints.

However, collections of parallel tracks moving in the same direction, such as the ones shown above, suggest that some dinosaurs may have traveled in groups. A single set of tracks that covers lots of area without much overlap suggests that the dinosaur may have systematically looked for food.

There's one important piece of information that often can't come from trackways -- what kind of dinosaur the tracks belonged to. Very big dinosaurs had lots of muscle and padding on their feet, so their preserved footprints don't look much like the bones scientists have for identification. And since bones fossilize best when buried quickly -- and prints fossilize best when buried gradually -- it's rare for researchers to find preserved tracks next to the skeleton of the dinosaur that made them.

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