Petrified Forest National Park
Triassic Treasure Trove
Article and images provided by Bill Parker, Petrified Forest National Park paleontologist.
The Chinle Formation (Arizona, Utah, and western New Mexico) and Dockum Group (eastern New Mexico and west Texas) are terrestrial rock formations deposited by a very large northwesterly flowing river system between approximately 228 and 205 million years ago. At this time these areas were situated along the western coast of the supercontinent Pangaea, about 5-10 degrees north of the equator. Through the long history of deposition of these formations, the climate changed from wetter conditions in older rocks to more arid conditions in younger deposits. It is still debated if this is because of extreme global climate change or more simply because of northerly plate tectonic movement of the area across latitudes. New research in the park and Colorado Plateau aims to answer some of these questions.
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A block of Desmatosuchus bones (vertebrae and armor) from the Chinle Formation of Arizona. NPS photo.
Both the Chinle and Dockum are highly fossiliferous, with the fossils reflecting the broad changes. Plant floras are dominated by conifer trees, with ferns and cycads in the understory. The animal faunas are dominated by a group of specialized reptiles called archosaurs ("ruling reptiles") which include present day crocodiles and birds, as well as their common ancestors including the dinosaurs. Interestingly during the Triassic, reptiles more closely related to crocodiles (called pseudosuchians) are very common and in North America dominate terrestrial faunas. These include animals such as the aetosaurs and the rauisuchids. The latter were larger carnivorous forms that have a very dinosaur-like body plan, but were not dinosaurs. Dinosaurs themselves were smaller and less diverse during the Triassic. They are extremely rare in North America, yet more common in other parts of Pangaea. Why this is, is still an important question for paleontologists. Dinosaurs do not become dominant in North America until the Jurassic Period following the mass extinction of the majority of the pseudosuchians at the end of the Triassic.
Aetosaurs were heavily armored, possibly omnivorous, reptiles that lived during the late part of the Triassic Period. Found worldwide, they are one of the most commonly recovered fossils from Triassic terrestrial rocks. Their armored carapaces, often replete with sharp spikes, and their commonality must have made them imposing members of their respective ecosystems. The largest aetosaur, Desmatosuchus spurensis—featured on the 2014 National Fossil Day artwork—reached lengths of close to 18 feet (6 meters), yet one of the smallest, Coahomasuchus kahleorum, barely reached 3 feet (1 meter) in length. They generally inhabited river floodplain areas where they fed on plants and, very probably, insects and smaller animals as well. The Chinle Formation of the American southwest where Desmatosuchus is found is also rich in plant fossils including pollen and spores, leaves, and wood. Insect fossil are much scarcer in these deposits, however, traces of their behavior including burrows in logs, are very common.
Petrified Forest National Park History and Paleontology
Today, Petrified Forest National Park's climate is very different from that of the Triassic; the park is located in a an arid grassland near the southern edge of the Colorado Plateau. Petrified wood was first documented in the region in 1851, and in 1895 it was first proposed that the region become a national park—21 years before the National Park Service was created! In 1906, President Theodore Roosevelt created Petrified Forest National Monument using the authority of the Antiquities Act (learn more about the Antiquities Act here [3 MB PDF]), stating: "...the mineralized remains of Mesozoic forests...are of the greatest scientific interest and value and it appears that the public good would be promoted by reserving these deposits of fossilized wood as a National Monument with as much land as may be necessary for the proper protection thereof." Congress redesignated the monument as Petrified Forest National Park in 1962 and greatly expanded the boundary of the park in 2004. In 2001, a comprehensive paleontological inventory was started to document historic fossil sites in the area. More than half of the 200 known fossil localities were re-documented, and 50 new sites were discovered! Among the discoveries were about a dozen skeletons of Revueltosaurus callenderii, a psuedosuchian archosaur that had only been known from teeth. New discoveries are made each field season in Petrified Forest National Park. You can read more about the geology of the park, and the paleontological research now underway here
Aetosaur is Greek for "eagle reptile" ("aetos" and "sauros"). The very first complete aetosaur discovered, Aetosaurus, was found in Germany and had a triangular skull with numerous openings that reminded the original describer of a large bird skull.
Desmatosuchus spurensis means "link crocodile from Spur". The first specimen of this species was collected from near Spur, Texas in 1918 and was named by University of Michigan paleontologist E. C. Case in 1920.
Coahomasuchus kahleorum means "Kahle's Coahoma crocodile" and was discovered by the Kahle Family near Coahoma, Texas in the 1980s.
More Great Places (and NFD partners!) to see Triassic Fossils and Exhibits
2014 Mesozoic Ecosystem Partner feature articles:
| January: Fossils of the 2014 National Fossil Day Artwork
| February: Petrified Forest National Park
| March: Garden Park Paleontology Society