Arizona’s Petrified Forest Is Changing What We Know About the Dawn of the Dinosaurs

Discoveries of new fossil sites that preserve small-bodied animals in particular have upended our prior understanding of the area, says Ben Kligman, a postdoctoral fellow at the National Museum of Natural History in Washington, D.C., who does fieldwork in the park. “Maybe we were actually missing most of the diversity of all of the animals that were living in these ecosystems,” Kligman says.

Petrified Forest, declared a national monument in 1906 and a national park in 1962, covers more than 220,000 acres in northeastern Arizona and contains exposures of a unit of rock called the Chinle Formation. The sediments that make up the Chinle were deposited between 228 and 208 million years ago, when Arizona was close to the equator. The environment at the time was much different than it is today, Marsh says, as it was a landscape of floodplains, ponds, and meandering rivers.

Specimens of iconic Triassic early reptiles such as crocodile-like phytosaurs and armored aetosaurs are associated with the Petrified Forest. Some of the earliest dinosaurs, such as Chindesaurus bryansmalli and Coelophysis bauri, have been unearthed there too. Fossils of these animals and others found in the Chinle Formation are often found within layers of rock deemed the coprolite layers—beds identified by high concentrations of coprolites, or fossilized poop, Kligman says. These layers “had been looked at in quarries going back to the early 1900s, but what people were excavating from them were the large bones,” he says. The tinier bones around them were often ignored, or deemed to be fish scales.

This coprolite layer is exposed at a locality called Thunderstorm Ridge, which has been key to new discoveries of small-bodied animals. Discovered about a decade ago, paleontologists noticed this site while prospecting rock outcrops on land that was added to the park in 2014.

Kligman remembers seeing coprolites weathering out of a hill and swinging his pickaxe to get past the rock on the surface. The first chunk of fresh rock that he pulled out had a “four-inch-long jaw of a reptile sticking out of it,” he says. It ended up being a jaw of a new species of an herbivorous lizard-like reptile that has not been given a name yet, but it made clear that this would be an important fossil site.

Now, Marsh says, they have around 7,500 specimens in the Petrified Forest collection just from Thunderstorm Ridge alone, although the fossils they are finding are so small this only takes up a small portion of a storage cabinet. It took 4,500 pounds of rock to produce this many specimens though, he adds. To find these fossils that are the size of a grain of rice, rock is hauled from the field site to the lab, screen washed with water to melt away the matrix, and carefully picked through. This painstaking work has led to recently described finds such as an early mammal relative called Kataigidodon venetus that was the size of a chipmunk, and a tiny lizard named Microzemiotes sonselaensis that has a grooves in its teeth—not dissimilar from a Gila monster—indicating it might have been venomous.