Vertebrates of the Late Triassic
Petrified Forest National Park is one of the premier exposures of Late Triassic terrestrial sediments in the world. Famous for its enormous reserves of fossil wood, the park also preserves numerous fossils of the Triassic vertebrate fauna.
The brightly colored mudstones and sandstones of Petrified Forest National Park were deposited by a complex system of streams. These streams were home to numerous fish including the freshwater sharks Xenacanthus and Lissodus as well as various types of bony fish. A common fossil found in the park are toothplates of the lungfish Arganodus. The presence of lungfish fossils and preserved burrows attributed to these animals suggests that the Chinle climate (the climate which allowed the deposition of the rock formation extensive through the region, the Chinle Formation) was monsoonal in nature with periods of extreme rainfall separated by long periods of dryness. Arganodus was probably similar to its modern relatives and aestivated in burrows during the drier times awaiting the monsoonal rains.
One of the most common animals from the park is the large, flat-headed amphibian Koskinonodon (= Buettneria). Belonging to a group of temnospondyl amphibians called the metoposaurs, these animals were most likely voracious predators feeding on fish and smaller animals. With their flat heads, sharp, conical teeth, and upward directed eyes, Koskinonodon probably settled in the muddy bottom of ponds and ambushed prey from below.
Koskinonodon is rare in the Painted Desert area of the park which contains younger sediments than the Blue Mesa and Rainbow Forest areas. There it co-occurs with a smaller metoposaur named Apachesaurus. Apachesaurus probably had a somewhat similar lifestyle as Koskinonodon, however by the end of Chinle times both of these animals and the metoposaurs were extinct.
Archosaurs are a specialized group of reptiles that include birds and crocodiles. Triassic groups including aetosaurs, phytosaurs, and rauisuchians are included in this group as are the dinosaurs.
Phytosaurs had elongate, broad snouts very similar to modern crocodiles. Distantly related to crocodiles, phytosaurs probably filled similar ecological niches feeding mainly on fish also on any other animals who approached too closely. Phytosaur fossils are the most common animal fossils found in the park and are characterized by the genera Leptosuchus and Pseudopalatus.
Aetosaurs were large, heavily armored, herbivorous archosaurs, and a common element of the Triassic fauna. Five genera of aetosaurs are found in the park including Desmatosuchus, Stagonolepis, Heliocanthus, Typothorax, and Paratypothorax. Forms such as Desmatosuchus possessed highly developed armor complete with large shoulder spikes. The large spikes were most likely for defense against predators such as rauisuchians and the phytosaurs, however they may have also been used for sexual display.
Rauisuchians were quadrupedal archosaurs with large heads full of serrated teeth. The largest of the rauisuchians, Postosuchus, reached lengths of up to 20 feet and was the top carnivore in the food chain. A smaller form, Chatterjeea, also occurs in the park but its remains are rare and it is not well-known.
Most visitors to Petrified Forest National Park are surprised to learn that dinosaurs are a relatively rare and minor component of the preserved Triassic fauna. Separated from the other archosaurs by characters of the pelvis and ankle, late Triassic dinosaurs were mainly small, bipedal, carnivorous predators such as Coelophysis. Coelophysis is especially well known from Ghost Ranch, New Mexico where a single quarry contains almost 10,000 specimens! Unfortunately, with the exception of a partial skeleton of Coelophysis, most dinosaur remains found in the park to date have been isolated bones. Ornithischian (herbivorous) dinosaurs are not known from the park.
"Gertie" (Chindesaurus) is probably the park's most famous fossil receiving much press when it was discovered. While considered by some scientists to represent a true dinosaur, Chindesaurus is actually a dinosaur ancestor. With both true dinosaurs and these earlier forms present in the park, Petrified Forest preserves the "dawn" of the dinosaurs.
Therapsids were large reptiles that possessed many mammalian characters including a "cheek" bone, enlarged canine teeth, pelvis, and a specialized attachment of the skull to the spine. As such, they have been known as "mammal-like" reptiles, although this term has recently fallen into disfavor in scientific circles. Placerias was a large dicynodont therapsid known from isolated elements in the park but common elsewhere in Arizona, especially near St. Johns, just southeast of the park, where large numbers of Placerias were found in a single quarry.
Other groups of vertebrates have been recovered from the park, although as minor elements. These include the crocodile precursor Hesperosuchus, the enigmatic reptile Trilophosaurus, and the horned-toad looking Procolophonid reptiles. Unfortunately complete specimens of these animals are lacking from the park. The flying reptiles, pterosaurs, are conspicuously absent. However, this may be because the small, hollow bones of these animals are not commonly preserved as fossils.
As paleontological research continues in the park, new and better specimens are sure to be discovered filling in gaps in our knowledge of the vertebrate faunas of the Late Triassic.