Western U.S. climate proves complex.

• Position: Jurassic Period

The climate 150,000,000 years ago of a large swath of the western U.S. was more complex than previously known, according to research from Southern Methodist University, Dallas, Texas. It has been held that the climate during the Jurassic Period was fairly dry in New Mexico, then gradually transitioned to a wetter climate northward to Montana but, based on new evidence, the theory of a gradual transition does not tell the whole story, maintains paleontologist Timothy S. Myers.

Geochemical analysis of ancient soils, called paleosols, reveal an unexpected and mysterious abrupt transition from dry to wet. He collected samples from the Morrison Formation, a vast rock unit that has been a major source of significant dinosaur discoveries for more than 100 years. The Morrison extends from New Mexico to Montana, sprawling across 13 states and Canada, formed from sediments deposited during the Jurassic.

"I don't have a good explanation," he admits. "Normally, when you see these dramatic differences in climate in areas that are close to one another, it's the result of a stark variation in topography but, in this case, there weren't any big topographic features like a mountain range that divided these two localities in the Jurassic."

Surprisingly, paleosols from the sample areas did not reveal marked differences until they were analyzed using geochemical weathering indices. "It's sobering to think that, by just looking at the paleosols...