Painted Desert: Cold Winds and Buried Dinosaurs

Many experts consider the sprawl of colorful high-elevation desert in the northeast corner of Arizona to be a part of the higher, colder Great Basin Desert to the north. The Painted Desert is far from the moist storms of either the distant Pacific or the Gulf of California. The deluge of summer storms that make the Sonoran Desert one of the richest in the world do not as reliably reach the Painted Desert. Its higher elevation makes the region subject to winter snows and frost, which makes it ecologically more like the Great Basin than either the Mojave or Sonoran. The Painted Desert is made of an eroded, multicolored rock layer called the Chinle Formation, laid down in a swampy, inland area some 225 million years ago during the Late Triassic period. The layers and the treasure trove of fossils they contain are best seen at the Petrified Forest National Monument.

With only a scattering of bushes that can stand the combination of low rainfall, hot summers, and chilly winters, the landscape of the Painted Desert looks like a scene from another planet. Minerals have tinted the sedimentary layers red, brown, purple, and sometimes green. The soft, crumbling, rounded landscape seems hollowed and sculpted by wind and rain, a weary, ancient landscape redeemed by its lurid colors. In the Petrified Forest National Monument, this already surreal landscape is pushed toward fantasy by a profusion of fossils, including a whole forest of trees that have been turned to brightly colored stone. Trees buried in saturated mud were cut off from oxygen, which means that bacteria could not consume them. Instead, the trees were buried under a deepening weight of sediment. Time and pressure allowed mineral-laden water to seep into the matrix of wood, eventually fossilizing it with a mingling of brightly colored minerals and jewel-like crystals.

This same process also preserved the bones of many of the creatures that lived more than 200 million years ago, at the dawn of the age of dinosaurs. The fossil experts working at the Petrified Forest find one or two new species every year, casting new light on old theories about many creatures. For instance, one recent discovery of a small relative of modern crocodiles casts doubt on existing theories of how dinosaurs developed.

Previous discoveries of the fossilized teeth of this ancient crocodile, Revueltosaurus callenderi, led experts to believe that it was the ancestor of the plant-eating dinosaurs such as stegosaurus and triceratops, two of the best-known examples of the dinosaurs that dominated the planet’s living things until some 65 million years ago. At that point, most experts believe the impact of a giant meteorite caused a dust cloud that caused the extinction of the great majority of living species, including all the dinosaurs. The recent find in the Petrified Forest demonstrated that this alligatorlike swamp-dwelling hunter was not related to the dinosaurs after all. Instead, the bones suggest this supposed dinosaur that lived more than 200 million years ago was a relative of modern crocodiles. Revueltosaurus was about four feet long, with a rounded skull and legs less slung to the side than modern crocodiles. Twin strips of armor plating down its back protected it. The find proves that the dominant plant-eating dinosaurs like the triceratops may have emerged about 25 million years later than the experts thought. So they did not arise at the same time as the meat-eating ancestors of the tyrannosaurus, which modifies the scientific understanding of the dinosaurs’ family tree. The discovery demonstrates the importance of deserts in reconstructing the history of the Earth and its inhabitants. The barren, easily eroded desert soils easily yield up their secrets, which explains why most of the fossils that have enabled scientists to reconstruct human evolution, mass extinctions, and the shifts of evolution have been found in desert regions like the Petrified Forest in the Painted Desert.