Who Was That Dinosaur?
Part of the Dinosaurs: Ancient Fossils, New Discoveries exhibition.
In detective stories, footprints often identify a suspect. For dinosaur experts, life is more complicated. Prints record skin, flesh and tendon as well as bone. But fossil dinosaurs are only bone, so prints often don't resemble what we have of the printmaker. As a rule, then, scientists can't usually assign a dinosaur species to a particular track.
Yet footprints are rich with information. Track experts have learned that these fossil traces can reveal how big an animal was, whether it was moving slowly or quickly--and sometimes, what it liked to eat.
Size and Feet
Scientists can estimate the height of some dinosaurs from their footprints: Multiplying print length by four gives a rough estimate of leg length. But this ratio isn't exact. It probably differed a lot among species and among different ages in the same species. In animals alive today, juveniles have feet that are big relative to their body size--as anyone who's ever noticed a puppy's paws will know!
A Rule of... Foot?
Scientists estimate the relative speed of a dinosaur from the distance between the prints of the right and left feet--that is, a step. In modern animals, and--presumably--in dinosaurs, a step length four or more times the footprint length means the animal was running. The dinosaurs at Davenport, with step lengths three times print length, were walking slowly.
But tracks of running dinosaurs are rare; the few that exist were made by theropods. Does that mean other types rarely ran? Or that when they did run, they avoided the muddy ground that preserves prints but might have slowed them down?
Dinosaur Detectives
Surprisingly, footprints are clues to an animal's diet. A three-toed, sharp-clawed print means the trackmaker was a meat eater--a theropod. A three-toed print with rounded toes probably belonged to an ornithopod dinosaur, a plant eater. And pairs of unequal-sized prints were the work of another plant eater--the long-necked, long-tailed dinosaurs called sauropods.
Preserving the Past
What makes a footprint last? Our prints on a beach disappear with the next high tide. On forest trails, tracks vanish with a sudden shower. How, then, can some dinosaur footprints have lasted for many tens of millions of years? One answer lies in the surface on which the animal walked or ran. Was it dry or wet? Was it rich with minerals that form natural cements? How heavy was the trackmaker, and what happened after it passed by? Was the site quickly buried by a flood or a volcanic eruption?
Time is important, too. The age of non-avian dinosaurs lasted more than 170 million years, during which billions of animals took countless steps. Even if prints were preserved only rarely, that's enough to leave an impressive trail of evidence.
Making Tracks
In experiments, geologists have found that a galloping horse can leave impressions of its hooves 25 centimeters (10 inches) beneath the surface of a sandy beach. Because such so-called "ghost prints" are never even on the surface, they're instantly protected. Perhaps many dinosaur prints were made this way.
Modern Footprints
The damp clay soil around a seasonal pool retains the prints of many animal species. If prints were baked hard by the sun, then buried by some natural event--a flood or a volcano--they, like dinosaur prints, might one day be trace fossils.
Climbing the Walls
One hundred million years ago, the sediments in which dinosaurs left their footprints were safely horizontal. Over long periods, though, powerful forces may upend geological layers, so tracks can appear vertically on cliffs.