How do we know where to look for dinosaur fossils?

First, dinosaur fossils are found almost exclusively in sedimentary rocks, in other words, rocks that form when sand, silt, mud, and organic material settle out of water or air to form layers that are then compacted into rock. So in looking for dinosaur fossils, one must find outcrops of sedimentary rocks.

Second, no dinosaurs are known to have lived in the ocean, where most sedimentary rocks form. Instead, one must look in sedimentary rocks that were deposited on the continents, primarily by rivers and streams or in lakes into which the streams emptied. Occasionally, dinosaur fossils are also found in sedimentary rocks representing ancient, desert sand dunes and their associated habitats.

Third, the sedimentary rocks must have been formed or deposited during the Mesozoic Era, the geologic time period when nonavian dinosaurs lived. Finally, it's best to search in regions where little vegetation covers the surface of the ground, so that any fossil fragments weathering out of the sedimentary rock layers can be more easily seen. These regions of barren ridges and ravines are often called badlands.

In order to find appropriate, Mesozoic, sedimentary rock layers, paleontologists often use geologic maps, which document the kinds of rock layers of different geologic ages that are exposed on the surface. Once appropriate rock layers are found, the search for dinosaur fossils can begin with a reasonable hope of finding the kinds of dinosaurs one is searching for. But commonly, other kinds of fossils are serendipitously discovered during the search.