One of the richest known troves of dinosaur fossils -- newly discovered in a cliffside in Nova Scotia -- has yielded footprints of the world's smallest known dinosaur, a creature no bigger than a sparrow.

The site may give paleontologists a close look at a wide-ranging extinction of dinosaurs that might have been triggered by the impact of an asteroid known to have hit 500 miles away. The findings could shed light on the theory that a different asteroid impact much later wiped out all the dinosaurs.

No bones from the littlest dinosaur have been found, but the cliff face is "absolutely splattered with bone," more than 100,000 fragments of many species recovered so far, according to Neil H. Shubin, a Harvard graduate student in biology who discovered the fossils along with Paul E. Olsen, a geologist at Columbia University's Lamont-Doherty Geological Observatory.

The fossils are being dug out of a sedimentary layer cake that ranges in age from 225 million years at the bottom to 175 million years at the top. Paleontologists hope to track dinosaur evolution in great detail up from the bottom and through the first mass extinction, which marks the end of the Triassic and the beginning of the Jurassic periods. Once many dinosaur species died out, much of the ecological slate was wiped clean and new dinosaur species evolved. Dinosaurs as a group lived on through a second mass extinction that ushered in the Cretaceous and and a third dinosaur era.

Then they died out utterly 65 million years ago in a mass extinction that many experts think was triggered by the collision of an asteroid.

A similar fate may have sealed the Triassic, Olsen and Shubin say. Not far from the fossil beds is a 90-mile-wide crater from an asteroid that is thought to have hit 214 million years ago. It may have caused the first mass extinction of dinosaurs by producing a fireball some 1,600 miles wide and triggering a sudden climate change.