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Triassic Dinosaurs and Other Animals
Brochure Triassic Dinosaurs and Other Animals at Petrified Forest National Park (NP) in Arizona. Published by the National Park Service (NPS).
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Petrified Forest National Park Service U.S. Department of the Interior Petrified Forest National Park Arizona Triassic Dinosaurs and Other Animals Fossils are clues to the past, allowing researchers to reconstruct ancient environments. During the Late Triassic, the climate was very different from that of today. Located near the equator, this region was humid and tropical, the landscape dominated by a huge river system. Giant reptiles and amphibians, early dinosaurs, fish, and many invertebrates lived among the dense vegetation and in the winding waterways. New fossils come to light as paleontologists continue to study the Triassic treasure trove of Petrified Forest National Park. Invertebrates Scattered throughout the sedimentary layers of the Chinle Formation are fossils of many types of invertebrates. Trace fossils including possible insect nests and beetle borings in the petrified logs. Thin slabs of shale have preserved more delicate animals such as shrimp, crayfish, and insects, including the wing of a cockroach! Horseshoe crabs Horseshoe crabs have been identified by their fossilized tracks (Kouphichnium arizonae), originally left in the soft sediments at the bottom of fresh water lakes and streams. These invertebrates probably ate worms, soft mollusks, plants, and dead fish. Clams Various freshwater bivalves have been found in the Chinle Formation, some species forming vast colonies in the muddy beds of the ancient lakes and rivers. Antediplodon thomasi is one of the clam fossils found in the park. Freshwater Fish The freshwater streams and rivers of the Triassic landscape were home to numerous species of fish, especially sharks and lungfish. Bony fish of the Late Triassic included Turseodus sp., Semionotus sp., and Hemicalypterus weii. Freshwater Sharks Lissodus humblei was a blunt-toothed shark about 6-9 inches (15-23 cm) long. The blunt teeth indicate it survived on clams and clam shrimp; it was probably a bottom dweller. “Xenacanthus” moorei was a 3 foot (1 m) long prong-toothed shark that fed on smaller fish, aquatic reptiles, and amphibians. Amphibians Metoposaurs (meh-toe-poe-sores) were giant amphibians. A common fossil animals found in the lower portion of the park is the large flat-headed amphibian Koskinonodon perfectus (see below), 10 feet (3 m) long and weighing up to half a ton. These animals were most likely voracious predators feeding on fish and smaller animals. With their flat heads and upward Coelacanth Living species of coelacanths (seal-a-kanths) are still found in the world today. The fossil species found in Petrified Forest National Park is Chinlea sorenseni. This large lobe-finned fish could reach up to 5 feet (1.5 m) long and weigh up to 150 pounds (68 kg). Its jaws were equipped with large, sharp teeth for catching and holding prey. Lungfish Arganodus dorotheae (see below) were heavy fish up to 3 feet (1 m) in length and weighing up to 70 pounds (32 kg). They had fascinating, combshaped teeth used as crushing plates for clams. Some lungfish living today are able to leave the water for periods of time and breathe air. directed eyes, Koskinonodon probably settled in the muddy bottom of ponds and ambushed prey from below. Koskinonodon rarely occurs in the northern section of the park, which contains sediments younger than the Blue Mesa and Rainbow Forest. Giant amphibians are represented in these layers by a smaller yet similar animal named Apachesaurus gregorii. Archosaurs Archosauriformes Archosauriformes are a specialized group of reptiles that includes birds and crocodiles. In the Triassic, archosauriformes were represented by aetosaurs, phytosaurs, rauisuchians, and dinosaurs. Phytosaurs Phytosaurs (fie-toe-sores) were crocodile-like reptiles, some species reaching lengths possibly exceeding 20 feet (6.1 meters). Distantly related, phytosaurs probably filled similar ecological niches as crocodiles, feeding mainly on fish and any other animals that came too near. Phytosaurs are the most common fossil animal found in the park, species including Smilosuchus gregorii (see below) and Machaeroprosopus pristinus. Aetosaurs Aetosaurs (a-ee-toe-sores) were 3-18 feet (1-6 m) long, herbivorous reptiles with broad flat bodies protected by plate-like scutes. Some species had large spikes on their sides or back that were possibly used for defense. Aetosaurs had short limbs and small skulls with a piglike snout for rooting in soil for plants and roots. Desmatosuchus spurensis (see below) and Calyptosuchus wellesi are two of the aetosaurs found in Petrified Forest National Park. Therapsids 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. Placerias hesternus (pla-seer-ee-us) was a dicynodont therapsid. This massive planteater was up to 9 feet (2.7 m) long and might have weighed as much as two tons. Placerias had a short neck, barrel-shaped body, small tail, Rauisuchians Rauisuchians (rau-i-su-key-ans) ranked as the top terrestrial predators of the Late Triassic, thanks to huge skulls armed with powerful biting jaws and 3 inch (7.6 cm) long serrated teeth. Species of rauisuchians found in the park include Postosuchus kirkpatricki (see below) and Poposaurus gracilis. Some rauisuchians could grow up to 20 feet (6 m) in length. Revueltosaurus Revuetosaurus callenderi is a small, about 3 feet (1 m) long herbivorous reptile that until recently was quite an enigma. Known only from distinctive leaf-shaped teeth it was thought to be a primitive plant eating dinosaur. Discovery of the skeleton of this animal in Petrified Forest in 2004 showed that it was not a dinosaur and instead more closely related to aetosaurs and rauisuchians. Dinosaurs Most visitors to the park are surprised to learn that dinosaurs are a relatively rare and minor component of the Triassic fauna preserved at the park. Separated from the other archosaurs by characters of the pelvis and ankle, Late Triassic dinosaurs were mainly small, bipedal carnivorous predators including Chindesaurus (see below) and Coelophysis. Coelophysis (sealo-fie-sis) was an early carnivorous dinosaur that probably walked on two legs. It was about 8 feet (2.4 m) long and could weigh 50 pounds (23 kg). and a beak-like skull with large tusk-like bones protruding from its upper jaw. The beak-like jaws helped them pull up and tear tough plants and roots. While Placerias is represented in the park by isolated elements, it is common near St. Johns, just southeast of the park, where large numbers of Placerias were found in a single quarry. Like pieces of a puzzle, fossils have long provided clues to the past. Paleontologists are reconstructing the Triassic ecosystem in Petrified Forest National Park by piecing together fossil records. The scientific and educational value of a fossil can only be interpreted when it is properly documented and studied. Help us by not disturbing any fossils you find during your visit. Report any discoveries to any uniformed employee. www.nps.gov/pefo EXPERIENCE YOUR AMERICA April 2013