Exposed at Petrified Forest is one of the most continuous sections of Triassic-aged rocks anywhere in the world. These rocks were deposited by enormous rivers between 208 and 228 million years ago and include an incredible diversity of fossils. The park is even named after some of those fossils! While Petrified Forest contains abundant and colorful fossil plant deposits, much of the what has made the park's paleontology resources famous worldwide is its myriad fossilized reptiles, some of which still have living descendants!
Before learning about the fossil reptile groups found in the park, it's important to understand how paleontologists talk about and depict evolutionary relationships. You can think about how certain groups of animals are related to one another using evolutionary trees, or cladograms. Cladograms show lineages of organisms through time; the closer two lineages are on the cladogram, the more closely two lineages are evolutionarily related to one another.
For example, dogs and cats are more closely related to one another than either of them is to elephants. Dogs and cats share a recent common evolutionary ancestor that lived in the Eocene, 55 million years ago. We cal the group to which dogs and cats belong "Carnivora." You'd have to go far back in time to find the common ancestor between Carnivorans and elephants. Carnivorans and elephants are all members of Mammalia, a group of animals to which humans also belong.
Learn about the many animals that lived in Petrified Forest over 200 million years ago.