In Arizona, there are thousands of square miles of excellent exposures of gently dipping, redbeds of Lower Triassic age. The underside of almost any sandstone ledge may have a cast impression of an old stream channel or mudflat, some of which are traversed by trackways of vertebrates long extinct.

Triassic age rocks of northern Arizona, the Moenkopi, Shinarump, and Chinle Formation, are predominantly redbeds, deposited along margins of shallow Triassic seas of northern Arizona and in seas of the following Jurassic period. Streams and rivers flowed from highlands in central Arizona and deposited as distinct muds and sands which now form the spectacular Chinle deposits of the Painted Desert and the Petrified Forest.

The Triassic-age Moenkopi sandstones and shales are thought to have been deposited by stream action or by lake deposition in a semi-arid, or possibly sub-tropical region (which might remind one of a tropical desert island). A typical characteristic of the Moenkopi redbeds is the great number of ripple marks, channel deposits and mudcracks - evidence of frequent, intermittent flows of water.
Preserved mudflat surfaces within the Moenkopi often show small crescent-shaped impressions like those produced by streams flowing onto recent, soft mudflats; often, such a surface will be found covered with tiny raindrop impressions. It was an ideal situation for the preservation of footprints, but rarely do these muds preserve bones of the actual beasts or evidence to indicate the diversity and abundance of the plant life we know existed during the Triassic period of northern Arizona.  It is virtually impossible to determine exactly what kinds of reptiles produced the trackways seen in the Moenkopi Redbeds; paleontologists have rarely been able to match the skeletal foot of a fossil to any specific reptile track. Eight different species of the single reptile genera Chirontherium (the "Hand Animal"), have so far been identified, but in reality each of these forms may represent entirely different reptiles.
In Arizona,
the Triassic Moenkopi

redbeds attain a thickness of more than 4,000 feet, with each of the above formations containing its own distinctive fossil assemblages. The Moenkopi is composed of reddish-brown sandstone, reaching a thickness of about 1,000 feet. This lower Triassic formation is the sediment of grading streams which sluggishly meandered over wide, low, alluvial plains. Channel sands are common, with rare bone and teeth fragments of reptiles, and on certain bedding planes (with mudcracks) are found abundant tracks of reptiles and large amphibians.
Chirotherium, tracks of the "hand beast"
Good examples of Moenkopi footprints can be seen in the Museum of Northern Arizona, at the University of Arizona, and at the Naturalist's Workshop at the Grand Canyon National Park's south rim. Also, three well-preserved footprints of Chirotherium are mounted in the walls of the hotel lobby at Cameron, Arizona.
Plants of Chinle, other than petrified wood of Araucanioxylon anizonicum, are not conspicuous. In an area which contains such an enormous quantity of petrified wood, and which consists largely of well-stratified, fine-grained shales and sandstones, it would be reasonable to suppose that there would exist beds in which foliage, fruits and flowers would be preserved, but only in the Chinle in the Painted Desert, and near Blue Forest, have fossil leaves been discovered in any appreciable numbers. In these areas, the fossiliferous zones are comprised of grey, purplish, and reddish bentonitic (clayey) shales in regular, nearly horizontal beds of what is termed in early reports as Newspaper Rock Sandstone (I assume because of the thin, fine bedding of the rocks). Here, directly beneath the Newspaper Rock Sandstone, numerous, well-preserved fossil leaves occur. (see FOSSIL COLLECTING SITES ). Ripples, sun cracks, and fossil reptile tracks can be found.
Although these fossil plants are less noticeable than the brilliantly colored woods of the Chinle, plant fossils preserved within the fine clays of this formation are important evidence of their Triassic environment. More than forty different forms of fossil plants have so far been collected from within the Petrified Forest's boundaries: cones, seeds, leaf impressions, ferns, cycads, and most abundantly the horsetail rushes, Cordities, some of which grew thirty to forty feet in height, and ferns which undoubtedly provided a heavy ground cover beneath which the beginnings of the Mesozoic reptilian dynasty was evolving.

Reptiles dominated the Triassic vertebrate scene, and rapidly squeezed out the preceding primitive amphibians from most environmental niches. Phytosaurs, such as Rutiodon , resembling the living gavial crocodiles in form and habit, were common in Arizona. Coelophysis, one of the first dinosaur forms to have evolved, probably fed on insects and smaller reptiles which also inhabited the low, swampy lands of northern Arizona.

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 and Poposaurus gracilis. Some rauisuchians could grow up to 20 feet (6 m) in length.
Many other reptiles are known to have existed in Arizona during the Triassic period such as the large dinosaurs Segimosaurus and Dilophosaurus,
MAMMAL-LIKE REPTILES called therapsids are also found in the Triassic Chinle Formation. 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. A mass death assemblage of the mammal-like reptile Placerias gigas, were found at the base of the Chinle Formation near St. Johns, southeast of the Petrified Forest in eastern Arizona. There, nearly 1,700 bones were collected from the deposit, representing at least thirty-nine individual reptiles. The animals appear to have been dismembered by some larger meat eater before burial.
Phytosaurs (fie-toe-sores) were crocodile-like reptiles, some species reaching lengths up to 40 feet (12 m). Nostrils located strategically on top of the head just in front of the eyes allowed it to lurk in the water. Bony plates protected its body and jaws filled with sharp teeth made it a fearsome predator. Phytosaur bones and teeth are probably the most commonly found vertebrate fossils in the Chinle Formation.
Skull of phytosaur from Triassic Chinle Formation
Living in and near the water, phytosaurs, like Rutiodon seen above, had diverse diet of fish, amphibians, and reptiles. Several other species of phytosaurs have been found in Petrified Forest National Park, including Leptosuchus (Smilosuchus) gregorii and Pseudopalatus pristinus.
Aetosaurs (a-ee-toe-sores) were 10-15 foot (3-4.5 m) long 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 pig-like snout for rooting in soil for plants and roots. Desmatosuchus haplocerus or Stagonolepis wellesi are two of the aetosaurs found in Petrified Forest National Park.

Aetosaurs (a-ee-toe-sores) were 10-15 foot (3-4.5 m) long reptiles with broad flat bodies protected by plate-like scutes.

Some species of aetosaurs had large spikes on their sides or back that were possibly used for defense. Aetosaurs had short limbs and small skulls with a pig-like snout for rooting in soil for plants and roots. Desmatosuchus haplocerus or Stagonolepis wellesi are two of the aetosaurs found in Petrified Forest National Park.
Metoposaurs (meh-toe-poe-sores) were giant amphibians that grew to be 10 feet (3 m) long and weighed up to half a ton. They used their cavernous mouths to sieve water for small fish using sharp teeth to trap them. Like most amphibians, metoposaurs had lungs instead of gills and possibly detected vibrations in the water for hunting. Short weak legs indicate they spent most of their time in the water where they may have waited on the muddy bottom for prey. Two species have been found at Petrified Forest, Buettneria perfecta and Apachesaurus gregorii.
The Metoposaur's skulls were broad and very flat
?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, the Petrified Forest preserves the “dawn" of the dinosaurs and will be a continuing source of information on the beginnings and evolution of the dinosaurs.

Petrified Forest National Park


Contrary to the popular conception of the great Triassic-age Petrified Forest near Holbrook, Arizona, fossil trees were not inundated in place by some great catastrophe. Rather, the masses of Araucharian pines,
Araucanioxylon anizonicum, represent the remnants of a colossal log jam, or series of seasonal log jams preserved in the remnants of some mighty river which flowed from heavily forested highlands (now eroded to low mesa levels) carrying broken, battered and limbless logs great distances to be covered rapidly (one of the requisites for fossilization) by the accompanying silts and clays in a wide Mississippi-river-like delta.
NOTE: Private collecting of wood or other fossils is not permitted within the boundaries of the Petrified Forest, but on private land adjoining the Park, permission can be obtained to collect. Trees of the Petrified Forest are most closely related to the large Araucharian pines of the Southern Hemisphere and are somewhat more distantly related to the popular ornamental Norfolk Island Pine.

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.


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 in the kinds of Mesozoic sediments in northern Arizona. That said, I'm willing to bet that the discovery of pterosaurs by you or a fellow fossil hunter in Arizona is just a matter of time . 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.