Tracks are dynamic evidence, the record in stone of a moment that passed into history as long as 200 million years ago. Perhaps more than any other kind of fossil, footprints evoke the walking, running, living and breathing animals of the past.

Two hundred years ago, farm boy Pliny Moody was plowing a Massachusetts field. Uncovering some mysterious fossil tracks, the 14-year-old showed them to family and friends. Others soon began collecting the strange relics.

Over the space of a few minutes, on a single day 100 million years ago, about two dozen long-necked plant-eating dinosaurs crossed a mudflat. The smaller animals in the group trailed after the bigger ones. A large meat-eating dinosaur visited later.

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.

In general, the bigger the animal, the less its footprint resembles the bony structure of its foot. The reason? Large animals need a lot of built-in foot padding to absorb the impact of their great weight.

Sometimes tracks are readable... and sometimes they're not. The texture of the ground surface and the number of animals that passed by are among the things affecting our ability to interpret what was going on when the tracks were made.

This is the only known footprint of the huge meat-eater Tyrannosaurus rex. Its one-of-a-kind status may be a clue to the ecology of these enormous dinosaurs.