During the closing years of the Jurassic Period and the beginning of the following Cretaceous, northern Arizona was going through a time of erosion and non-deposition. Little of the rock, and its record of life remain to fill museum coffers with fossil treasures. In the millennia to follow, in the later part of what is known of as the Lower Cretaceous, two great seas began converging on Arizona from the northern Arctic region and from the south (joining in Upper Cretaceous times to form the Rocky Mountain Geosyncline Seaway). Rocks of this general age in northern Arizona are known of as the Dakota group.

Characteristic sandstone and shale outcrops of the Cretaceous Dakota Formation
Not always sediments of seas or waterways, the Dakota Formation is often interbedded with land deposits, telling geologists that Cretaceous seas were shallow and subject to great fluxes, advancing one century, and retreating the next, but never loosing all of the ground they had gained. A fossil hunter in northern Arizona can work an outcrop and find shark teeth and mollusks in one layer, and leaves from a forest in the next.
fossils of the
Marine mollusks are common fossils found in sandstones capping some mesas.  Localities include Steamboat Canyon, 15 miles west-northwest of Ganado, were shells of Ostrea and Corbula, Cardium, Turritella and Dentalium are abundant. Other Dakota sandstones also outcrop south-southwest of Morenci, where specimens of Mactra, Fusiformis, Corbula, Cardium, Cyrena, and Turitella are common. To be a successful fossil hunter, learn to recognize the "look" of your target formation. Too many hours are lost prospecting in the wrong places.
Seas spread across Arizona during the Late Cretaceous times, the climate slowly became mild and ideal for the development of many varieties of plant life. Non-marine sandstones in northern Arizona preserve abundant plant fossils, most of which are genera now restricted to warm temperate or subtropical regions of the earth.
Leaves of deciduous trees, those found in the Dakota and Mesa Verde Formation in Arizona, are oak, walnuts, maples, birches, beeches, buli trees, sweet gum, breadfruit and ebony, are relatively common fossils which make fascinating additions to any fossil collection.
It's one thing to dig a hole in a sandstone or shale outcrop and find fossil leaves but getting them out of the ground in good shape is a trick and takes a bit of practice. . It's the most difficult part of leaf excavation. Because fossil leaves often occupy a horizontal plane (anywhere from a paper-thin layer to meters thick), getting them out in good shape to them requires clearing the overburden. This can be done with several tools: pickaxes, your flat-ended rock hammer, and shovels. In other words, you've got to start with housekeeping, and nobody gets any really good plant fossils if they don't have a clean quarry surface, best an area about three meters square.
From that point, you can use a hammer and flat chisel to split the bedded rocks along the surface planes that will become obvious once you start learning the nature of the rock you're dealing with. One in a hundred leaf fossils, especially deciduous tree-leaves, will be complete enough to knock your sock off when you first see it. But persistence will pay off and you'll come home with some museum-quality specimens to do final preparation and place in your collection.



Outcrops of the Mancos Shale can be thousands of feet thick, having accumulated in near-shore and deeper sea environments over millions of years. And these Cretaceous rocks can hold a treasure trove of fossils, both invertebrates such as ammonites, oysters, and sea urchins, and vertebrates and bones of marine reptiles.

Exposures are extensive in parts of northern Arizona and this formation covers parts of serval states in the Four Corners area of Arizona, New Mexico and Colorado.
Following Dakota times, seas encroached further into Arizona bringing with them heavy accumulations of dark sediments. Conditions in these waterways were very favorable for the preservation of life forms in Arizona. The Mancos shale is perhaps the most fossiliferous of all Cretaceous rocks in Arizona. This formation occurs extensively in the Black Mesa region. Good fossil-bearing outcrops occur in the following areas: 1) in the mesa front twenty miles northwest of Ganado; 2) near Jadito Springs seven miles south of Keams Canyon; 3) at Shongopovi and Orarabi on the Hopi Reservation; 4) at Padilla Mesa, Howell Mesa and in Cheichinbito Canyon Mesa twenty miles southeast of Kayenta. In all these areas, many beds of fossil invertebrates and other marine-life occur at various horizons.
Marine vertebrates are also found in the Mancos shale. Bones of a giant carnivorous fish Portheus, shark and skate teeth and vertebra of bony fish can often be found . To property collect, transport and mount a treasure like the fish fossil at right requiters years of experience and dedication.
Fossils are found weathering from the crumbly exposures of the carbon-rich dark shale. Teeth are most easily seen because of the shiny reflective enamel surfaces.
A few Cretaceous Shark Teeth Species
Ptycodus (a ray)
Bones of the Plesiosaurs, mighty, sea-going lizards, have been found in the Cretaceous rocks north of Black Mesa. Plesiosaurs of Black Mesa in Arizona grew the size of small whales, as much 40 feet long. They were fully adapted for life in the water. Having a land-reptile ancestry, their limbs evolved long narrow flippers and developing tails and sturdy, deep bodies. Instead of using their flippers as great oars, their structure suggests that they probably used them more like wings, and 'flew' through the water in much the same way as penguins and turtles. They also had a dense series of belly ribs that made the body more rigid and also protected the belly when the animal may have left the water to lay its eggs in beach sands, much like their close cousins the turtles.
Plesiosaur Trinacromerum seen at the Museum of Northern Arizona
Those found at Black Mesa, members of the biological group Plisouridea, had short necks compared to other plesiosaurs which were equipped with long, snake-like necks and large heads, were able to swallow larger prey such as sharks, large squid and even large ammonites. An example of a Cretaceous plesiosaur from Black Mesa, Trinacromerum, is seen in an exhibit at the Museum of Northern Arizona in Flagstaff.
Such vertebrate finds are extremely important to the science of vertebrate paleontology and if found by an amateur or student collector, he discovery should be reported to professional museum collectors who are familiar with proper collecting techniques. Bones, especially in the fragile Mancos shales, are very delicate and usually cannot be simply picked up and taken home. Proper collecting techniques must first be learned, including the use of Paleobond and plaster jackets which protect fossil bones during transportation.
Cretaceous marine fossil species (below) that are found in Northern Arizona Rocks
Ostrea, Congesta, Ostrea lugubris, Gryphea newberryi, Exogyra laeuiuscula, Exogyra columbella, Mactra emmonsi, Inoceramus labiatus, Prinotropis and Camponectes. One of the most sought after of the Mancos shale invertebrates is the ammonite Prionotropis woolgari.
Cycads, in groves looking superficially much like pineapple orchards, were the most distinctive of the non-flowering plants, the gymnosperms. (NOTE: Various cycads during this period did develop flowers, and may well represent transitional forms between gymnosperms and the flowering plants.) Leaves of cycads are common in Arizona, the woody trunks are rare.
Fossil cycad trunk
Fossil cycad leaves
Fossils of evergreens, more abundant during earlier times of the Mesozoic, are also found in northern Arizona's Upper Cretaceous rocks. Cycads, too, in groves looking superficially much like pineapple orchards, were the most distinctive of the non-flowering plants, the gymnosperms. (NOTE: Various cycads during this period did develop flowers, and may well represent transitional forms between gymnosperms and the flowering plants.)
The Late Cretaceous was also a time of vast, lowland forests in Arizona, and enough dense plant growth for the region to allow the accumulation of huge, and now commercial, coal deposits. While coal itself is void of any recognizable fossils of interest to a casual fossil hunter, the sandstone and shales sandwiched between (see photo to far right) often contains a treasure trove of well preserved plants and sometimes animals. There are really a lot of Late Cretaceous rocks in northeastern Arizona and the potential for finding a remarkable number and variety of fossils, both marine and landforms is limitless.
Peabody Coal Mine at head of Coal Mine Canyon. And some complain about avocational collectors
Coal Mine Canyon south of Tuba City
Newly discovered dinosaurs from a locality called Fossil Gap along the Arizona-New Mexico border, were a 1-ton, potbellied vegetarian and a fierce, two-legged predator have surfaced from a 30-million-year gap in the dinosaur fossil record. And they are true-blooded Americans, chiseled from rock that, aside from one area southeast of Tucson, is one of the few sediments that has yielded fossils from the middle of the Cretaceous period, which spanned from 146 million to 65 million years ago. The overall lack of mid-Cretaceous fossils has made it difficult for scientists to chart the evolutionary origin of specialized horns, bills, claws, and other anatomic innovations apparent in the more abundant fossil record beginning 75 million years ago.
The herbivore, now named Nothronycus, is, say paleontologists from the Mesa Southwest Museum," is just plain bizarre." They look at the creature as a feather-coated biped with a tiny head and long neck. Each neck vertebrae is remarkably bigger than its skull. Large claws extend from its forelimbs, possibly for defense or ripping up vegetation. "It walked like Godzilla with this big gut." Nothronycus (12-foot-tall) towers over similar species uncovered in the early Cretaceous, and the dinosaur probably links early and late Cretaceous species of the therizinosaur group. Another yet-unnamed type of Coelurosaur, has been called "the coyote of the Cretaceous." Like coyotes, it surely consumed a wide variety of smaller reptilian and mammal preys. As a zoological group, the Coelurosaurs-which include duck-billed dinosaurs, Tyrannosaurus and the oviraptors, were given a variety of special features, including beaks and horns, but his one appears to lack adornments. Paleontologists speculate that modern-day birds arose from Coelurosaur roots, an idea bolstered by recent finds in China of early Cretaceous fossils of what were apparently feathered Coelurosaurs. The sediments from this Arizona-New Mexico dinosaur bed will probably continue to yield some amazing and rare surprises.