The setting today. A Triassic Chinle Formation badlands
During the Triassic, the dinosaurs' direct ancestors, the early thecodonts and rhynchosaurs began to replace other land vertebrates in rather dramatic fashion worldwide, and nothosaurs and icthyosaurus filled the sea and fed on abundant early Mesozoic fish.

In the Triassic Period, the archosaurs began their reign worldwide. Pterosaurs had evolved flight, taking to the Triassic skies and filling a vast niche with leather-winged reptiles for all of the next 175 million years. By the end of the Triassic, the dinosaurs had dramatically taken over the land, becoming the top dogs in all terrestrial food chains.

(To the right are some Triassic Reptiles)

The Late Triassic environment was generally filled with relatively dull-witted amphibians and reptiles fighting for territory, food resources and mates. It is no small wonder that when the firs advance Thecodonts and dinosaurs first evolved they were able to out-compete other creatures and ultimately dominate the entire terrestrial world. Late Triassic dinosaurs and thecodonts had to compete with advanced mammal-like reptiles. These soon-to-be mammals even with their nearly upright in posture and probable degree of warm-bloodedness), were generally not match for the new dinosaurs which still had the advantages of superior agility, and likely the kind of general intellect researchers are discovering in some modern birds. Some early dinosaurs may have hunted in packs or prides and may even have cared for their young long after birth. Such socialization and cooperative instincts seem to have been a key factor in their extremely long success throughout the Mesozoic.

Coelophysis was one of the earliest sauirischian, predatory dinosaurs, an order of animals which possessed a characteristic lizard-like hip structures and clawed feet. Coelophysis, like all early saurischians, was an active biped with powerful bird-like hind limbs, and small forelimbs. This small dinosaur grew to only about six feet long as an adult, and half of that was tail. Its long mouth was filled with a battery of sharply curved teeth, many of which were serrated and designed to slash at flesh and hold on to struggling prey.  

A light, strongly constructed skull sat at the end of a long, slender neck that was characteristically S-shaped and provided an ability to make birdlike thrusts or very quick maneuvers to catch prey. Its long, muscular bird-like hind limbs and a long tail acting as a balance mechanism surely made it one of the swiftest land animals of its time. Coelophysis was a successful carnivore that fed on many of the small reptiles and amphibians in its highland environment, and its only defense against much larger predators such as the monstrous rauisuchids, was its superior agility, speed and perhaps bird-like intellect.
Plant eating sauropod ancestors called prosauropods browsed the late Triassic landscape of Chinle time. Primitive anchisaurids, similar to Plateosaurus, were dinosaurs of medium build reaching a length of up to 21 feet. Originally thought to be upright bipeds, new evidence shows that even though their forelimbs were two thirds as long as their hind limbs, anchisaurs were primarily quadrupeds rising on two legs when feeding on high vegetation or perhaps in combat. Defensively, this browser was equipped with a huge, sharp claw on each of its front feet, that may have also been effective for gathering leaves or digging for roots and tubers. Like the later sauropods, anchisaurs had relatively small heads and had to spend most of their existence browsing in order to consume enough food to sustain their great bulk.
Plateosaurus, a pro-sauropod
Chindesaurus, another late Triassic ancestor to the later long necks, was a small, quadrupedal plant eater weighing about 200 pounds. Little is known about his early prosauropod because only parts of its skeleton were uncovered in the Chinle badlands of the Petrified Forest near Holbrook, Arizona. Paleontologists suspect that it had a long neck and tail, a small hand, and may have been ancestral to the larger but still primitive sauropod Plateosaurus.
REVUELTOSAURS (ruh-VWEL-toe-sore-us)


Revueltosaurus was one of several animals from the Late Triassic that were known are only from their teeth, and was thought to be an ancestor of the plant-eating ornithischian dinosaurs like Stegosaurus and Triceratops, which roamed the world millions of years later in the Jurassic and Cretaceous periods. The fact is that this animal first thought to be a dinosaur, Revueltosaurus callenderi, is instead a crocodile ancestor. Because the teeth look like those we know from herbivorous ornithischians, people assigned them to the dinosaurs Revueltosaurus is a relatively recent discovery, having been named only in 1989 from teeth found in Revuelto Creek, New Mexico.

Revueltosaurus, a wanna-be dinosaur that wasn't
New discoveries of more complete specimens show that Revueltosaurus was not of a plant-eating dinosaur, but of a herbivorous or perhaps omnivorous crocodilian ancestor living a mostly terrestrial life in the uplands of the Late Triassic. It may have been one source of meat for the developing theropods around the world, just as tenontosaurs were in the middle Cretaceous; Reptilian cattle.
Teeth fragments and vertebra were recognized as belonging to the genus Revueltosaurus, a small, two-legged (bipedal), fast archosaur . This occurrence is in the upper unit of the Petrified Forest Member in the Chinle Formation and is of late Triassic Age.
Other dinosaur remains have been found the Chinle Formation but are yet to be adequately described by paleontologists. Isolated or fragmentary bones of other flesh-eating ceratosaurs and herbivorous ornithischians may prove to be entirely new forms of dinosaurs and it is likely that those forms described in this book represent only a small portion of the Late Triassic dinosaurs that lived in Arizona about 225 Million years ago.

One of the earliest scientific records of fossil life from the beginning of the Mesozoic was that of large animal tracks. From 1938-1946, Frank Peabody and the University of California at Berkeley made collections from the Early Triassic (235-245 million years old) Moenkopi Formation in the valley of the Little Colorado River between Cameron and Holbrook, Arizona. Peabody discovered that these red sandstone deposits were rich in the fossil trackways of Chirotherium, and animal known only by its trackways, since no bones of Chirotherium have ever been discovered. Oddly, skeletal remains of the Triassic's abundant amphibians are common in Moenkopi sediments, but their trackways are not, a puzzle that might be explained by differences in habitats not clearly observed in the Moenkopi's geology.

Nobody's yet found the bones of Chirotherium, but his fingerpint is everywhere.
Remarkably, the Chirotherium's hind footprints left in the now hardened mud of an ancient tidal flat is similar to that of a reversed human hand. Surprisingly, much information can be gleaned from tracks. At least eight different types of Chirotherium tracks have been identified so far. Fortunately, even without having any skeletal parts preserved, the well-preserved Chirotherium's trackways revealed that the bottom of their feet to have granular or polygonal scales and to have strong claws. These clues tell us that the tracks were not made by one of the huge scaleless amphibian of the time.
The largest tracks were made by the hind feet of Chirotherium rex and measures about 33 centimeters (13 inches) in length. This trend towards very large, almost hippo-size, obligated these later chirotheres to walk flat on their feet (plantigrade) as do humans. At the same time the front feet developed from a possible grasping hand in the smaller primitive chirotheres to a hoof-like organ in the succeeding larger animals.
Researchers have evidently found tail marks along with some of the earlier and more primitive Chirotherium (C. diabloensis). The tail marks were not found running in line with the tracks but off to the sides in an undulating pattern more typical of its earlier ancestry and found in today's lizards. We still don't know exactly what these strange creatures of Arizona's past looked like, but they were dominant land animals for millions of years during the early Triassic Period of Arizona.
Other vertebrates, like the large, broad-headed capitosaur amphibians ( were still abundant in Arizona during this time. In addition, small lizard-like footprints are more abundant in the Moenkopi than any other kind of track. Worldwide the fossil record reveals the mammal-like reptiles as the dominant animal group of the Early and Middle Triassic, though the evidence in Arizona is scanty thus far. A tantalizing 1992 discovery near Holbrook, Arizona, reveal vertebrate tracks from the Early Triassic Moenkopi Formation which are reputed to be the very first known mammal-like reptile tracks from North America. Many more exciting discoveries are certain to be forthcoming as more researchers and amateurs begin to sleuth for the history of life in Arizona, but dinosaurs were still 15 million years into the future.
PLACERIAS; Reptilian Hippos
Placerias is found only in Arizona. Placerias hesternus remains were first discovered in the Lower Chinle Formation in 1904 near Cameron, Arizona. Thirty-nine individuals of the species Placerias gigas were found near St.Johns, Arizona during the 1930's.

The ox-sized Placerias was well equipped for foraging the ground for food such as roots and rhizomes, for it possessed a pair of tusks extending down from the upper jaw along side the beak. Microscopic examination of the striated wear-marks on the lower third of these tusks indicate a life of digging into abrasive soils hunting for food. The presence of polished surfaces overlying the wear marks show a switch in food types which may reflect seasonal changes and availability of foliage.

Placerias had a large head and powerful jaws, with its mouth terminating into a turtle-like cropping beak sheathed in horn. The jaws were functionally toothless; however, the animal could grind coarse vegetation between the grooved and ridged palatal surfaces inside the jaws.
Phytosaurs were crocodile-like reptiles found in the same strata as Placerias, indicating that they too were common and gives additional evidence for a Late Triassic time frame. Dermal scutes (skin armor) of the 15-foot Aetosaurs Typothorax and Desmatosuchus were also found in the St. Johns quarry. Desmatosuchus, which evolved earlier than Typothorax, had much thicker scutes and possessed remarkably long shoulder spines for additional protection against animals such as the Phytosaurs, similar to the late armored Ankylosaur dinosaurs. Altogether over 3000 bones were discovered in the quarry excavation, 1600 of the belonging to Placerias!
The floor of the excavation slopes down into a shallow depression and an analysis of the quarry fossils and depositional environment suggest that it was not a year-round marsh but instead a soil that was seasonally flooded. Some of the bones appeared pressed or trampled into the muddy swamp bottom, while some scutes (bony armor) were turned upright as if disturbed by scavengers. Coprolites (fossil animal feces) are found in the quarry in large concentrations.
Various small primitive sprawling reptiles and amphibians are also represented in the Chinle Formation and indicate that a diverse vertebrate fauna existed alongside the relative giants of the time. Among the creatures dwelling in lakes and ponds, insects, fresh water sharks, lung fish, lobefin coelocanths, bony teliosts fish, horseshoe crabs and clams played out the drama of everyday life in a tropical jungle environment. The late Triassic of Arizona was remarkably alive and today amid the beauty of the sculpted Chinle badlands 225 million years later, a fossil hunter need not walk far to discover petrified remains weathering from the red clay and imagine a time when dinosaurs first walked the earth.
Other tetrapod (four-legged) fossils that are more frequently found in the Chinle usually represent animals that lived in or near water. Acting somewhat like huge, six-foot long salamanders the bottom dwelling amphibian Metoposaurus fraasi is one of the most common tetrapods of the Chinle Formation. Metoposaurs, belonging to a group of amphibians called labyrinthodonts, characteristically have wide flat heads with short, wide flat bodies and stubby tails. Metoposaur eyes looked directly upward and had small limbs which were likely inadequate for effective movement across dry land. It is easy to imagine this predator lying on a muddy pond bottom, waiting for its next meal to swim over or to wade precariously far from the shore.

Reptiles are also common in the Chinle Formation and because of a generally wet lowland environment, it is not surprising that most of them were adapted to a watery environment. Phytosaurs like Rutiodon were large and aggressive archosaurian reptiles that inhabited rivers and lakes.

Looking and presumably acting much like a long-snouted crocodile, the phytosaurs were aggressive predators that preyed on anything that came within reach of their massive jaws. Their menu likely included fish, amphibians and even dinosaurs. Growing to nearly 30 feet in length, the phytosaurs were at the top of the lowland pecking order and most likely the adults had few if any other predators to fear.
On land, Late Triassic predators were common and dangerous. Largest of these thecodont reptiles were the rauisuchids, a group of voracious quadrupeds, that, like dinosaurs, carried their bodies high off the ground on vertically directed limbs. This erect posture provides good evidence that speed and a long chase might have been part of the rauisuchid's hunting strategy and the general body plan might remind us of a cross between a Komodo dragon and a leopard. The vertical posture in the group suggests a relationship to the dinosaurs, but this structure may have developed independently from either dinosaur orders. Rauisuchids grew to be between 9 and 18 feet in length and had two interlocking rows of bony armor plates that ran down their backs, and a single row of plates and below their tails.

The Aetosaurs (Eat-e-o-sors) were large (10-15 feet long) armored archosaurs which were the only known herbivorous thecodonts. Nearly covered with dermal armor, the bizarre looking Desmatosuchus was distinguished by its large recurved shoulder spines and is found in the Chinle Formation or northern Arizona along with phytosaurs, metoposaurs, rauisuchians, and other aetosaurs.

Desmatosuchus, another large quadruped archosaur, was more than 12 feet long. Unlike the phytosaurs, this reptile was very heavily armored with horn-like spikes that extended over its limbs and thick and massive bony plates that thoroughly protected its back.
Desmatosuchus was well adapted to upland habitats, having evolved from predatory thecodont ancestors into a peaceful browsing plant eater. Its massive armor was ample protection against larger and more powerful contemporary predators that shared the same environment. Desmatosuchus attained a nearly upright, vertical limb posture common in other thecodonts and dinosaurs. Other aetosaurs found in Arizona were Calyptosuchus, Typothorax and Paratypothorax. Paleontologists think that the aetosaurs roamed the higher, drier upland areas. The resembled the much lager and geologically later ankylosaurian dinosaurs since both groups of animals were plant-eaters protected by extensive armor, but the two groups are not closely related.
The armor of Desmatosuchus was clearly needed
in the Triassic Chinle habitats
In 1947 a primitive pseudosuchian was found from the Middle Triassic Moenkopi Formation (Holbrook Member), named Arizonasaurus. The first specimen of Arizonasaurus was a maxilla, a fragment of an upper jaw. From fragmentary remains-the hipbone elements-it is difficult to identify the animal, but it could be an ancestor to the advanced rauisuchian Postosuchus. A new 50% complete skeleton and skull more recently discovered, named Arizonasaurus babbitti (Named after former Arizona Governor Bruce Babbit) from the Middle Triassic Moenkopi Formation turns out to be a "poposaurid pretending to be Dimetrodon!" Spines on the reptile's back resemble the fameous sail-back Dimetrodon of the earlier Permian Period, pictured in many prehistoric animal books. As an earliest Middle Triassic rauisuchian, it shows that the remarkable fossils is a true missing link...a link where the crocodiles and birds emerged from a common ancestor, the Pseudosuchia-Ornithosuchia split.


The thecodont Postosuchus was a large, graceful, carnivorous bipedal rauisuchian from the Late Triassic. This 14-foot carnivore had a narrow but robust skull on the end of a relatively short neck. The large skull had many fenestrations (openings) and sinuses to lighten it, yet it remained strong in structure. The teeth were large and serrated, with some flexibility in the palate and lower jaw that allowed the animal to swallow large chunks of flesh. Postosuchus would have looked somewhat like a miniature Tyrannosaurus rex and appears to have walked bipedally most of the time, although it could go down on all fours (become quadrupedal) for drinking or feeding. The large eyes of this rauisuchian faced forward and thereby positioned for stereoscopic capability, making it a formidable animal able to judge distances. Prominent skull ridges protruded the eyes which may have protected them from the glare of sunlight.
Inside the nasal area was a large chamber which may have housed an olfactory sensor, increasing the sense of smell. Sharp claws and flexible wrists allowed the front limbs to be used for gripping and manipulating prey. Postosuchus had a fully erect upright stance as advanced as dinosaurs, birds, and mammals, but it achieved it in a different fashion. Whereas most upright walking animals have an inturned femur head bent at a right angle inserted into the relatively vertical hip socket, Postosuchus had a straight femur that inserted into a cupped hip socket which was near horizontal. There is evidence from a Texas fossil quarry that Postosuchus hunted in packs. The association of 10 young, one sub-adult, and one adult skeleton indicates some kind of group activity. This fearsome carnivore probably lived in the uplands along with Desmatosuchus, Typothorax, and small dinosaurs, at the same time that phytosaurs and metoposaurs were living in the "lowlands" or wetlands nearer the rivers. It seemed to have been the largest land predator of its time and was at the top of the food chain along with the crocodile-like phytosaurs. Postosuchus is found in redbeds which are part of the spectacular redbed sequences seen all across Arizona, New Mexico, and Texas. The presence of a red rock sequence usually indicates that the paleoenvironment was a relatively dry upland. As mentioned earlier , the red coloration is caused by the iron component in the rocks "rusting" (oxidized to hematite) and turning a red color from exposure to the oxygen in the atmosphere. It is thought that the climate consisted of a combination of a wet and distinct dry season, now it is thought that many red-beds may represent the after effects of enormous meteorite impacts somewhere on the globe that caused massive continent-wide drying effects and causing normally black forms of iron to be exposed to the air and oxidize red..



The names given to geological time periods are obviously artificial, but their beginnings and ends are ordinarily clearly seen the record of the rocks. Where fossils are found at these boundaries, significant and often dramatic changes in ancient habitats are preserved. Mass extinctions often help define and separate one age from another and it is at these biological demarcations that paleontologists have felt confident enough to give distinctive names. In some cases, transitions do not always signify a catastrophe to the environment nor are they always readily observable. Lifeforms can rapidly evolve and aspects of the biosphere that may not be preserved in the geological record were subject to quick change. So an obvious observable change in the fossil record may exist, and the biological reasons for that may remain ethereal and beyond our present understand. All we know is that the drama of life and death was being played out in the past, as it is today, and the rocks of the Glen Canyon Group hold some truly wonderful stories of changes that were occurring during the early part of the Mesozoic Era.
The sequence of predominantly red sandstones and mudstone known of as the Glen Canyon Group is such a transition. Even though hundreds (thousands?) of feet thick, these sediments represent a relatively short span of geological time that bridged the changing environments between the Triassic and Jurassic periods. Except for the strange unaccountable desert dune formation, which began and ended the Glen Canyon time, swamplands and river systems continued unabated as they had for millions of years. Some vertebrates, like the phytosaurs, flourished for a while in the lowland swamps until apparently outcompeted by the better-adapted proto-crocodiles. Coelophysis, the little dinosaur of the Chinle time, became extinct, but other dinosaurs in Glen Canyon time took its place. Some of the new dinosaurs began a trend towards gigantic forms and their numbers and diversity dramatically increased and became a biological revolution all over the earth's land surface. Likely, pre-Archaeopteryx-like birds or "proto-birds" and pterosaurs filled the air and roosted in the jungle trees or took off from high crater rims of the many momentarily dormant volcanoes. Fish and lizards were abundant in Glen Canyon time, but the abundant monstrous amphibians of the earlier Triassic were gone, having also lost their battle of competition to the advanced reptiles. Perhaps most noticeable in the fossil record there is an absence of the colossal fossil trees which collected in great numbers to jam the earlier Triassic waterways. These changes from the latest Triassic to the lower Jurassic came slowly, but they took place on the same stage, in the same wet lowlands that was Arizona for more than 45 million years.
The new dinosaurs began a trend towards gigantic forms and their numbers and diversity dramatically increased and became a biological revolution all over the earth's land surface. Likely, pre-Archaeopteryx-like birds or "proto-birds" and pterosaurs filled the air and roosted in the jungle trees or took off from high crater rims of the many momentarily dormant volcanoes. Fish and lizards were abundant in Glen Canyon time, but the abundant monstrous amphibians of the earlier Triassic were gone, having also lost their battle of competition to the advanced reptiles. Perhaps most noticeable in the fossil record there is an absence of the colossal fossil trees which collected in great numbers to jam the earlier Triassic waterways. These changes from the latest Triassic to the lower Jurassic came slowly, but they took place on the same stage, in the same wet lowlands that was Arizona for more than 45 million years.