Glen Canyon Transition Tim
The transition from the Upper Triassic to the following Jurassic period was a time of great biological changes. This period, called in Arizona the Glen Canyon time, existed some 160 million years ago. It was a time when the descendants of Coelophysis grew larger and more advanced, in preparation, so-to-speak, to later dominate the earth in the Age of the Reptiles.
The Glen Canyon Group (Jurassic) Wingate Formation

The Wingate Formation is about 300 ft. thick in the area of Canyonlands, and lies conformably( geology for no interruption of deposition) atop the Chinle Formation. The Wingate, like most of the Glen Canyon Group, is composed of typical a aeolian (wind-blown) deposits, including fossil dunes and small playa or oasis deposits. The most common track type in the Wingate are small theropod Grallator tracks, typically 15-20cm long. These are about "twice the size of the diminutive, yet abundant, Grallator tracks found in the upper part of the Chinle Group." Based on a formula which relates foot length to hip height, these Grallator track makers were probably about 3-4 ft tall at the hips. Near the top of the Wingate Formation, larger theropod tracks, Eubrontes (up to 35-40cm long) are known. Eubrontes tracks are about twice the size of Grallator.



The great desert dune conditions of Arizona
now the Navajo Sandstone

Kayenta Formation

The Kayenta is a relatively thin layer (about 200-600 ft.) of weakly inclined river sandstone, sandwiched between two massive windblown, dune sandstones, the Kayenta and the Navajo. The Kayenta preserves both dinosaur fossils (such as Dilophosaurus) and footprints, as well as the fossils of the mammal-like reptile tritylodont.
Track of Dilophosaurus
at Tuba City
one mean reptile
Dilophosaurus was the largest flesh-eater of the early Jurassic. This dinosaur was made famous by its role the movie Jurassic Park, but that was Hollywood, and little about this species was portrayed accurately.
Dilophosaurus gets its name from the two thin crests of bone on the top of its head. These were probably used as a display for courtship purposes (scientists don't believe it had a frill on its neck like the movie version).
Dilophosaurs displayed at Flagstaff's
Museum of Northern Arizona
Dilophosaurus fossils are found in the U.S. and China. As an early predatory dinosaur, Dilophosaurus did not have forward facing eyes to give it stereo vision which normally is associated with good predators. Rather, this dinosaur probably used scent to a large measure as part its hunting technique. It had long and slender, rear-curving teeth in long jaws and strong front arms which would have been effective in grabbing prey. It was a fast runner, estimated to have a top speed of about 30-mph. It also had a long tail that could have been used as a whip in a fight, a deadly weapon by itself. Footprints attributed to Dilophosaurus appear in groups, so it may have hunted in small packs. It shares the same overall body configuration as its ancestor the, the much smaller Coelophysis from the earlier part of the Triassic Period.
Vertebrate footprints attributed to therapod dinosaurs are dominant by Kayenta time. Some of the Kayenta dinosaur tracks are very small and bird-like, such as Anomoepus, with relatively slender, widely-splayed digits, and "pigeon-toed," inwardly rotated tracks. Most scientists attribute Anomoepus to ornithopod theropods rather than early birds because Anomoepus tracks found in late-Triassic/early-Jurassic strata of the Connecticut Valley "occasionally show five-toed front footprints, as well as heel (metatarsal) and pelvic impressions.
Rocks of Glen Canyon time are designated the Wingate and Moenave, Kayenta and Navajo Sandstone Formations. Still dominant in the early Wingate zone are the phytosaurs, but as one searches the formation overlying the Wingate, the Moenave Formation exposed north and east of Cameron, the bones of a new reptile appear. No longer are there abundant primitive phytosaurs, but rather they have been replaced by the earliest known of the true crocodiles, Protosuchus. Protosuchus must have been better adapted in some way than the phytosaurs for phytosaurs became extinct by the end of Glen Canyon times; it was only the crocodiles debut.
Kayentatherium profile
A crocodile smile has nothing on this guy
Dinosaurs in northern Arizona were common animals during Glen Canyon times. Most evidence of this great reptilian population is in the form of footprints, such as those seen near Kayenta and Tuba City, which belong to bipedal, carnivorous forms such as Coelophysis and the more advanced Segimosaurs, Navahopis and probably dilophosaur-descendents

Fossil bone in massive, wind-derived sandstones such as the Navajo Sandstone can be rare for a number of reasons. Partly, the rarity exists because sand dunes when being formed are continually re-cycled.

Track of the therapod dinosaur Navahopus
Ancient and mighty sand dunes of the Navajo Sandstone
Sand dunes are very dynamic, forming one day, covering bones, only to be dismantled during a subsequent windstorm and re-formed somewhere else. This is very hard on bones that become exposed that easy disintegrate when exposed to air and the forces of weather.
San Rafael Time-The Great Unknown in Arizona

For millions of years following the Glen Canyon times, geographical conditions in northern Arizona became unfavorable for the preservation of most forms of life. Paleontologists can only make educated guesses as to what evolutionary changes were slowly taking place. Scientists know well the Triassic fossil record, with its mostly-sprawling reptilian population and primitive dinosaurs, and they know of the mighty creatures which followed San Rafael Time - change in all aspects of life must have been monumental and geologically rapid for it became the premier of the true age of what most have come to expect of dinosaurs: The Middle and Late Jurassic.

Fossils in Glen Canyon rocks in Arizona are an evolutionary unknown as conditions were generally unfavorable for the preservation of its vertebrate fauna. In time, and with enough eyes prospecting the vast exposures like those seen at Zion above, and similar ones in Arizona, a rich dinosaur fauna will be uncovered.
As professional and avocational paleontologists comb Glen Canyon rocks in northern Arizona, it is just a matter of time before the elusive dinosaurs are revealed. Bone hunting is like this and what makes this kind of paleontogy exciting. You've got to be where something died, and at the very moment in time when the forces of nature, erosion, allows you to be the one who is there to see it's bones for the first time.



The Morrison Formation in Arizona consists of siltstones, shales and conglomerates which probably represent the deposits of slow, sluggish streams flowing eastward into lower lands in New Mexico. Shifting sands and gravel bars of the time account for the multi-colored appearance of this formation today; greenish-grey to white and pink. As in neighboring New Mexico, dinosaur fossils in Arizona's Morrison Formation are not as abundant as they are in states to the northeast such as Utah, Colorado and Wyoming. During deposition of the upper part of the Morrison Formation, a large stream complex in the Colorado Plateau region (Westwater Canyon and Fiftymile Members) gave way to a large shallow saline, alkaline lake called Lake T’oo’dichi."
Jurassic Morrison Formation is exposed below Hundreds feet of Cretaceous sea deposits
This lake covered parts of northwestern New Mexico, northeastern Arizona, southeastern Utah, and southwestern Colorado during deposition of much of the Brushy Basin Member. Although much shallower, Lake T’oo’dichi’ had about the same areal extent as Lake Michigan.
In outcrops of the Morrison Formation underlying the coal and carbon-rich marine sediments of Black Mesa's Cretaceous sediments, newly discovered bone deposits not unlike those in other states have been found, but have yet to be studied or collected in any way that has been reported. But the bones do exist and will likely eventually show that a classic Jurassic dinosaur population existed, and included many of the superstars of that middle Mesozoic time. Go to T-Rat's Dinosaur pages for more details.