How did dinosaurs leave tracks in solid stone?

If you were standing at the bottom of a sheer rock face looking at a crisscrossing array of dinosaur tracks high above your head, it might seem as though you were witnessing the impossible. On top of being preserved in solid stone, the tracks run along a nearly vertical surface.

­But the formation o­f such tracks started with an ordinary walk on the beach. Millions of years ago, dinosaurs left their tracks in sediment. Typically, the soil was wet — part of a shoreline, a mudflat or even the bottom of a shallow sea. As the area dried, the tracks hardened. Eventually, another layer of sediment filled the prints, protecting them from erosion or damage. Very heavy dinosaurs could also leave underprints, stepping so forcefully that they compressed deeper layers of soil, essentially leaving their tracks protected underground.

Over millions of years, these layers of sediment hardened into sedimentary rock — the same type of rock that preserves dinosaurs’ fossilized bones. Then, over another long time span, erosion, weathering and geological forces gradually revealed the buried tracks. In some cases, these same forces have drastically changed the position of the land, moving it from flat to vertical.

For a track made tens of millions of years ago to survive until the present, several specific steps had to happen. The sediment the dinosaurs walked through needed to be just the right texture — not too soft and not too hard. Prints in very wet soil would collapse on themselves, and walking in hard soil didn’t make much of an impression. It also helped when the sediment that filled the tracks fell slowly and was a different type than the one on the ground. For example, falling ash drifting from a distant volcano would be a better preservative than a sudden mudslide burying an expanse of muddy tra­cks. And, of course, geological events must have combined in just such a way to make the tracks visible today.

Dinosaur tracks have been found on every continent of the world except Antarctica, but they’re still comparatively rare. Their rarity and longevity aren’t the most amazing things about them, though. Often, paleontologists can puzzle out what dinosaurs were doing when they made their tracks. Sometimes, the tracks reveal more information about dinosaur behavior than skeletal fossils can. Learn how on the next page.

Trackways and Trace Fossils
A fossilized track of a lower Jurassic theropod dinosaur found on a Navajo reservation in Arizona.
A fossilized track of a lower Jurassic theropod dinosaur found on a Navajo reservation in Arizona.
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.

Martin Lockley stands with a series of parallel dinosaur tracks along Purgaoire River, Colo.
Martin Lockley stands with a series of parallel dinosaur tracks along Purgaoire River, Colo.
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.

Track down more dinosaur information by following the links on the next page.

Related posts...