(144 - 65 Million Years Ago)
Two distinct geographical and environmental phases separated by millions of years are represented within the Cretaceous of Arizona. Geologists subdivided these into Early, Middle and Late Cretaceous and the fossils of each indicate quite different and remarkable histories. Worldwide, the Cretaceous Period was unique in many respects; It was a time of extensive shallow seaway development and massive chalk deposits in the Northern Hemisphere. The Greek word for chalk is Creta from which the period's name is derived. Cretaceous seas were filled with an amazing variety of live that is preserved today in many places as fossils; Cephalopods and oysters were extremely abundant and the marine lizards, the mosasaurs, made their first appearance to share space with the plesiosaurs and icthyosaurs.
The skies must have been filled with both tiny pterodactyls and giant fish-eating pterosaurs with 40 foot wingspreads (greater than the wingspread of many aircraft today!) Birds became more modern, but still retained ancestral teeth. Deciduous trees such as figs, cypress, ebony, poplar, willow and sequoia became widespread by Mid Cretaceous times and primitive marsupial and placental mammals flourished. the Cretaceous was the climax of the Age of Archosaurs, and reptiles in general continued as the dominant land vertebrates. The first snakes appeared, but turtles changed little since the Triassic, and most other reptile groups progressively modernized. Dinosaurs throughout the period were abundant and the Cretaceous saw the first appearance of horned ceratopsians, advanced ankylosaurs and "duck billed ornithopods". The end of the Cretaceous was truly cataclysmic, for many of the animals, including most of the characteristic reptiles and all of the Mesozoic's traditional dinosaurs vanished in a geological instant 65 million years ago.
 CRETACEOUS GEOLOGY
During the Cretaceous the Sevier Orogenic Mountain belt trended northwest to southeast along western Utah to the Mogollon Highlands of central and southwest Arizona. The southern highlands of Arizona, which were eroding into the Colorado Plateau, were worn away enough that deposition also occurred more locally. Sediments accumulated in the Colorado Plateau causing subsidence of the land and a rise of sea level in Arizona, reaching all the way to Flagstaff 90 million years ago. Later a marine regression pushed sedimentation sequences toward the eastern Cretaceous sea. Arizona's landforms were relatively stable during the Cretaceous until 75-80 million years ago, when the Laramide orogeny (birth of the ancestral Rockies) uplifted the southern half of Arizona. Sediments accumulated again in the Colorado Plateau basins between other uplifts.
The Late Cretaceous saw uplift of the Chihuahua trough and downwarping of basins during the early stages of the Laramide uplift, depositing the Late Cretaceous Fort Crittenden Formation within these basins in southern Arizona. Volcanism occurred in Latest Cretaceous across southern Arizona while northern Arizona subsided as part of the foreland basin from the Sevier Orogeny during the Jurassic. The Mogollon Highlands existed from Late Triassic all the way through the Cretaceous. The Cretaceous/Tertiary uplift resulting from the continuing Laramide Orogeny caused the Interior Cretaceous Seaway to regress from Arizona about 60 million years ago, and has remained dry ever since. The largest remaining deposits from the Late Cretaceous are primarily in the Black Mesa Basin in northern Arizona.
THE LOWER AND MIDDLE CRETACEOUS ENVIRONMENTS OF ARIZONA (144-97.5 Million Years Ago)
In southeastern Arizona the characteristic fossil bearing rocks are sediments likely deposited in the Chihuahua trough, a northwestern extension of the Gulf of Mexico and in nearby brackish lakes, lagoons, deltas and sand deserts. These sediments, which outcrop in the mountain areas of Cochise and Santa Cruz Counties, are called the Bisbee Group and are represented by various limestones, marine mudstones, sandstones or conglomerates. Early Cretaceous dinosaurs are rare in the Bisbee Group, but a growing number of specimens have been reported from southeastern Arizona. In the Shellenberger Canyon Formation, bones of an ornithopod dinosaur resembling Iguanodon or a large species of the related Tenontosaurus were found along with petrified logs similar to Brachyoxolon, belonging to an extinct but long-ranging family of conifers. Iguanodonts were medium size bipedal or semi-bipedal ornithopods which reached a length of about 25 feet. Characteristically these plant eaters had three hind toes, and five fingered hands with a specialized spike-like thumb and a prehensile "pinky". Likely, iguanodonts existed in upland, wooded environments and are found worldwide in Late Jurassic to Late Cretaceous times. The related tenontosaurs were also igunadonts of about 20 feet in length, but unlike Iguanodon, it did not have a spiked thumb, and its tail was held ridged by many rod-like tendons that ran the length of its tail. Tenontosaurus derives its name from the Greek word tenontos meaning sinew or tendon.
Newly discovered 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 scientists off camera, 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" Say paleontologists. 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.
SONORASAURUS THOMPSONI, A NEW CRETACEOUS BRACHIOSAUR FROM ARIZONA
In November of 1994, amateur fossil hunters Richard Thompson and Gordon Nelson were prospecting in the southwestern foothills of the Whetstone Mountains of southeastern Arizona. Following upslope a litter of petrified wood and bone fragments, he came across the exposed parts of a dinosaur skeleton. Owing to severe volcanic activity and rock deformation since the Cretaceous, any fossil bone from Mesozoic strata in southern Arizona is very rare indeed. The isolated discoveries of bone fragments and teeth of dinosaurs since the turn of the century in southern Arizona most often appeared as a brief mention or a footnote in the geological literature. Thompson, a student of such things, knew enough about the importance of vertebrate fossils to call inform the Desert Museum of his discovery. The discovery ultimately led to the excavation of a new species of dinosaur that surprised paleontologists around the world.
This new dinosaur discovery excavated by the Arizona-Sonora Desert Museum is from the middle-Cretaceous Turney Ranch Formation which dates about 97 million years ago.

The area was much warmer and wetter during Middle Cretaceous than it is now. Evidence of this is from the petrified wood fossils found in the Turney Ranch Formation, where the Sonorasaurus was found and from other Cretaceous age rock formations. The Turney Ranch formation is composed of well-exposed light pinkish gray and pale yellowish orange sandstones that typically crop out as ledges and ridges. The sandstones lenses are separated by pale red marl (lake-orgin limestone), siltstone, or calcareous (calcite-bearing) shale. The Turney Ranch formation is believed to a floodplain deposit. The sandstones are thought to be river or shallow marine channel deposits and the finer deposits are thought to be flood deposits. The paleo-environment of the formation is interpreted as being a fluvial (river) deposit and nearshore marine in an estuary setting. A present example of this environment would be the coast of North Carolina, where there are many rivers emptying into shallow marine bays. The fossilized wood found in the Turney Ranch formation indicates that there were forested areas around or near this nearshore marine environment.

The Skeleton is disarticulated, but some of the elements are anatomically positioned correctly relative to associated bones. Typical of many dinosaur finds, several bones recovered so far are complete or nearly so, but there is considerable breakage, displacement in the sandstone matrix and attrition seen on others. Tooth marks on several bones and the discovery of the tooth of the large meat-eater Acrocanthosaurus, is evidence of either scavenging or dismembering by large carnosaurs or even predatory activity. Even so, this new skeleton is by far the most complete dinosaur yet found in southern Arizona, and additional bones are expected as excavation continues.
Sonorasasurs bones at the Desert Museum
Quite surprisingly Sonorasaurus turned out to be a brachiosaur that survived the supposed extinction of the sauropod family brachiosauridae at the beginning of the Cretaceous. Among the brachiosaurs were some of the largest creatures to have walked the earth. Comparative measurements of the feet and limbs of Sonorasaurus indicate that this individual may have been a juvenile, a teenager as dinosaurs go, having fully adult size feet (metacarpals are 22 inches long!) with the rest of the body would still have been growing. Even though Sonorasaurus appears to have been young, a sub adult, it was a large dinosaur weighing approximately 35 tons and was nearly 55 feet in length. Its head was a towering 25 feet above the ground.
Significant remnants skin impressions and what was apparently carbonized flesh and muscle tissue was found adhering to bone on the skeleton of Sonorasaurus. Indeed, the entire rotted carcass could be seen as huge impression in the sandstone face, and amble sedimentary evidence showed us that it caused an enormous dam in the river channel as it lay there. This dinosaur was studied and classified by this author at Tucson's Arizona-Sonora Desert Museum where a replica of the skeleton in place has become part of a permanent interpretive exhibit. The full scientific report is available on the Internet.
The tooth gouges in the limbs of Sonorasaurus have been show to be those of Acranthosaurus. In addition, a large, serrated tooth identified as belonging to this large carnosaur was discovered in the sandstone in association with the sonorasaur bones on which it was scavenging. The three inch long tooth belonged to Acranthosaurus.
Acrocanthosaurus "High-Spined Lizard"

Acrocanthosaurus (meaning "high-spine lizard" because of the spikes growing out of its spine) was a fierce predator that was roughly 30-40 feet (9-12 m) long and weighed about 5,000 pounds (2300 kg). It had a big head, with a 4.5 foot (1.4 m) long skull and 68 thin, sharp, serrated teeth. It had 17-inch (43 cm) spikes extending from its vertebrae, along the neck and tail, that may have formed a thick, fleshy sail on its back. It had powerful arms, and each hand had three fingers, equipped with long, sickle-like claws. Acrocanthosaurus generally lived during the early Cretaceous period, roughly 115-105 million years ago, in the tropics near sea level. Its first bones were found in Oklahoma, and is now known from Arizona to Texas.

DINOSAUR OF THE TUCSON MOUNTAINS
In the Tucson Mountains near the Arizona-Sonora Desert Museum, river and lake deposits of the Amole Formation contain dinosaur fossils as well as petrified logs, fish, invertebrates and plants. A trackway of a medium-size ambling dinosaur was recently identified by this author not far from the gates of the Desert Museum. Near Gates Pass, a beautiful geographical feature that represents the geographical center of the volcanic Tucson Mountain caldera, a fragment of sandstone that appears to have been caught up in the cooling magma of that Cretaceous volcanic eruption contained limb bones of a large iguanodont dinosaur. This find was excavated by the University of Arizona and was identified by paleontologist Everett Lindsay. Cretaceous sandstone of the Tucson Mountains represents little understood events in geologic history, and holds the keys to remarkable story of life in Arizona 100 million years ago. Continued exploration of these early to middle Cretaceous rocks may eventually produce more, and perhaps new dinosaurs from this critical period of vertebrate evolution.
As the Early and Middle Cretaceous period ended, a period of massive volcanic activity and topographic uplift began. As mountains slowly rose, the great Cretaceous Seaway in Arizona and wet lowland environments drained. This region has remained well above sea level ever since.
THE UPPER CRETACEOUS
By Late Cretaceous time, other seas had intruded into low-lying basins of Arizona. In particular the rich fossil beds of Black Mesa and the Mogollon Rim are the sediments of a shallow sea and associated environments that extended from the Gulf of Mexico to the Arctic Ocean during the Late Cretaceous. The remains of ammonites, oysters, sharks and land plants are abundantly preserved. Dinosaur bones have yet to be identified from any of northern Arizona's Late Cretaceous rocks, but considering the existence of the well preserved deciduous plant fossils, coal and near shore-living invertebrates, the likelihood of dinosaur discoveries in these rocks is promising.
Mesozoic Rocks Exposed at Black Mesa
The Cretaceous stratigraphy exposed at Black Mesa in northeastern Arizona records a transition from continental, to coastal, and then marine deposition in a series of sedimentary rock strata.The shallow Cretaceous Interior Seaway separated western North America from the eastern half of the continent. It spanned a thousand miles from western Utah to Iowa and three thousand miles from the Gulf of Mexico to the Canadian arctic. Exotic dinosaurs lived on the land and the sea teemed with marine life. Sharks, skates, rays, a wide variety of bony fish, and ancestral sea turtles swam the waters. Arthropods, oysters, and clams lived in the mud and soft sediment of the sea floor.
All of these animals were lower on the food chain than the dominant, predatory plesiosaurs that ruled the seas. Often weighing one ton or more with massive skulls and four-foot-long jaws full of piercing teeth, plesiosaurs had paddles instead of legs that propelled them through the murky waters in search of supper. It is the muddy, fossil rich sediments, now shales and sandstones, that formed much of Black Mesa.
These seas encroached upon northern and eastern Arizona several times during the Cretaceous Period (65-135 million years ago). Consequently Cretaceous marine fossils such as ammonites, snails, brachiopods, bivalves and coral are some typical invertebrate materials to be found here. Shark and ray teeth are common fossils of Black Mesa, and a single vertebra of the marine crocodile similar to Metriorhynchus is recorded at the Museum of Northern Arizona. Vertebrate remains of other marine reptiles such as mosasaurs, plesiosaurs, and pliosaurs have been discovered too (fig?) The fossil remains of Mosasaurus arizonicus have been discovered on the Black Mesa plateau of northeast Arizona. Mosasaurs, the largest lizards that ever lived, were enormous, fierce marine monitor lizards (related to the living Komodo Dragons of Indonesia) that could reach 35 feet in length. Swift propulsion through the water came from flexing of the powerful body and sculling with the tail. The pelvic bones were not fixed to the backbone, consequently the hind limbs could not have been involved much in the power of propulsion. The front limbs were much larger than the hind limbs and moved vertically up and down for steering. The limb structure of these marine reptiles clearly shows that they descended from land-living quadrupeds and their evolution apparently paralleled that of the mammalian whales. The jaws had large sharp conical teeth used to impale their prey. Fossil ammonites have been found with punctures in their shells that match the shape of mosasaur's teeth. If there were ever true sea serpents, these giant marine lizards would have truly fit the bill.
The Late Cretaceous pliosaur Trinacromerum is also from the Black Mesa region of Arizona. The short-necked but large-headed pliosaurs are less common than the small-headed but long-necked plesiosaurs such as the 40-foot Elasmosaurum. Trinacromerum was discovered in the Mancos Shale Formation, which seems to reflect a deposit from a sea level rise 90-94 million years ago. One of this marine reptile's forelimbs was 22 inches in length. Fossil evidence in the Mancos Shale reveals and extinction of many invertebrates about 91 million years ago, but the mobile marine reptiles were apparently unaffected by what ever conditions brought about this local extinction.
Sea turtles also lived in the local Cretaceous seas as well. The two and a half foot long turtle Desmatochelys lowi was found in 1983 in the Mancos Shale of Black Mesa. Turtles belonging to the anapsid group reptiles that are identified by a lack of openings in their skull for muscle attachment, and are the most primitive of the living reptile groups. Turtles were widespread in Jurassic and Cretaceous seas and additional turtles and other marine reptiles will no doubt be found in Arizona's late Cretaceous rocks as exploration continues.
LATE CRETACEOUS OF SOUTHERN ARIZONA

Late Cretaceous dinosaur fossils in southern Arizona are primarily found in the Fort Crittenden and related formations which are exposed along the eastern flanks of the Santa Rita Mountains, the Empire Mountains and in other mountain exposures that cross the boarder into Mexico's extensive Late Cretaceous Cabullona Basin.

Seventy two million years ago, a series of freshwater lakes, rivers and lagoons existed in southeastern Arizona, supporting typical Late Cretaceous habitats. bodies of water teemed with invertebrates, turtles and fish, while forests and swamps supported dinosaur populations on nearby higher ground. Intermittent volcanic eruptions punctuated this period and were often horrific in their magnitude. The Tucson Mountain caldera (the remnant which is now the Tucson Mountains), represents one of the many late Cretaceous eruptions which may have been hundreds or thousands of times more explosive than the recent Mount St. Helens volcanic event. Generally, however, the Late Cretaceous habitats of southern Arizona were stable with life continuing relatively unabated for very long periods of time.

DINOSAURS OF THE FORT CRITTENDEN FORMATION
Although no complete dinosaur skeletons have so far been recorded from the Fort Crittenden Formation, isolated bones and teeth clearly show that the Late Cretaceous environments of southern Arizona supported several species of dinosaurs. Late Cretaceous dinosaurs appear to belong to a regional community of dinosaurs, which differ from those further to the north. This difference can likely be attributed to climate. Although the general temperatures of the Cretaceous were warmer than they are today, regional differences are clearly evident from the fossil record, and evidence from fossil plants in particular show North America's climate to be cooler trending northward. A southern community has been defined, and is characterized by the presence of the titanosaurid Alamosaurus, a Diplodocus-like sauropod which likely migrated into Arizona, New Mexico, Texas and Utah from South America long after sauropods became extinct in all other regions of North America. Recent discoveries of a yet to be excavated partial Alamosaurus skeleton in the Fort Crittenden Formation appear to confirm this interpretation. Alamosaurus was not a particularly large sauropod, weighing in at only about 25 tons, but it was probably the largest dinosaur of its time. Tail vertebra of Alamosaurus found in the Fort Crittenden Formation are characteristically domed at the rear, and fit into cups at the front of the following vertebra. Even though a rare fragmentary fossil where ever it has so far been discovered, this unique skeletal feature exists in all Alamosaurs and is a handy tool for its identification. Tyrannosaurid teeth, conforming to the size and structure of Albertosaurus and Daspletosaurus are common. Both Albertosaurus, previously known as Gorgosaurus, and Daspletosaurus were relatives of Tyrannosaurus rex and each stood about 11 feet tall and 30 feet in length and weighing close to three tons, these very large predators were likely the largest of the meat-eaters that inhabited southern Arizona. The teeth of Albertosaurus found in the Fort Crittenden Formation are two to four inches long and fully serrated to produce a sharp cutting edge. Paleontologist Walter P. Coombs suggests that because Albertosaurus teeth are relatively thin in cross section and have the effective cutting edge, they were well suited for slicing meat off large carcasses. Tyrannosaurus, quite similar in general design to Albertosaurus had teeth which appear to have been better adapted to stabbing and swallowing in killer whale fashion. As with all carnosaurs and many herbivorous forms, worn teeth or teeth that are broken while chewing or grabbing are continuously pushed out of the jaw by new teeth growing out from underneath. Such shedding explains why teeth are often the most common dinosaur fossil found.
Teeth and bones of hadrosaurs and other ornithopods are also found in the Fort Crittenden Formation. Hadrosaurs, or "duck billed" dinosaurs were generally large ornithischians, up to fourty feet in length and weighing between 2 and 12 metric tons that typically had long, flat snouts which reminded early researchers of a duck's bill. Hadrosaurs are known to be bipedal browsers. Using the hundreds of teeth which lined its jaws, it could process the vast amounts of woody plant material found in upland environments. Hadrosaurs were likely equally at home near waterways, and could use their powerful flat tails to maneuver in lakes or rivers. Their tails were also muscular enough that a well placed swipe across a large, menacing, predator's limb could easily have been crippling, and, in nature, usually fatal.
Ceratopsians, the horned dinosaurs, are also known from the Fort Crittenden Formation. Although not yet scientifically described, ceratopsians from the Fort Crittenden are known from a few isolated bones and one multi-element dinosaur site, and may represent Torosaurus or Pentaceratops. Ranging from 12 to 25 feet in length and up to 19 metric tons in weight. Ceratopsians were quadrupedal herbivores, which likely existed in herds, and like other dinosaurs, are thought to have exhibited complex social behaviors such as parental care. The skull of all ceratopsians are quite remarkable, often exhibiting massive neck shields, characteristic horns, and powerful parrot-like beaks with which they cropped off leaves and stems of pine, cycads and other upland plants. Teeth and bony scutes of the armored ankylosaurs are found too, buy such fossils so far have remained rare. Evidence of small dinosaurs too have been found in the Fort Crittenden Formation. The well preserved vertebrae of an ostrich-like ornithomimid dinosaur were recently found, and in microfossil laden sediments processed through window screen, teeth and bones of the small raptors Sauronitholestes and Ricardoestecia are found among the remains of small crocodilians, fish, amphibians, and mammals.

The American Flag Dinosaur Bed

Streams and rivers that coursed their way across a pine-wooded landscape during the Late Cretaceous Period of southern Arizona deposited conglomerate, sandstone, and finer grained rocks what are together called the American Flag Formation. In these rocks, which outcrop on the northeast side of the Catalina Mountains in Pima County are found some intriguing, if only fragmental, dinosaurs. These rocks may represent an environment that existed at the same time as that which is today represented by the Fort Crittendon Formation, with its dinosaurs, in Adobe Canyon in the Santa Rita Mountains to the Southeast of Tucson. In one layer of fine, black shale in this formation, there is an abundance of pine needles and cones, and probably represents the fossil remains of these trees that live along the shore of a quiet, Mesozoic lake.

Even though the evidence is often fragmentary, my recent exploration is showing that Late Cretaceous dinosaurs were a common and diverse lot in Arizona. Arizona's Mesozoic geology is vast and the rock strata often exposed in very remote reaches of the state. In these areas, eroding from seldom seen cliffs and badlands, petrified bones await.

PLANTS OF THE CRETACEOUS DINOSAUR'S HABITATS

Environmental reconstruction would be impossible if it weren't for the abundance of plants that are preserved in the fossil record. In fine-grained shale, the impressions or carbonized remains of leaves and fragile woody tissue can be preserved showing cell structure in remarkable detail. Petrified wood is often locally abundant and adds much to our knowledge about the forests and wooded habitats in which dinosaurs prowled.

In the Early Cretaceous, cycads, conifers and ferns were roughly equal in abundance. By Mid-Cretaceous times, a new group of plants had evolved. These were the angiosperms or flowering plants, such as the water lily, English ivy, and laurel, which made up 25% of the total flora. Angiosperms became the dominant type of plant life during the Late Cretaceous signifying another major evolutionary floral changeover (fig--) Confers were second in abundance and included cypress, juniper, pine and sequoia. Late Cretaceous angiosperms consisted of maple, figs, dogwood, walnut, magnolias, tulip trees, sycamore, and laurel. Cycads were reduced in numbers until they now remain only within certain tropical areas of the world. Surprisingly we would likely have felt quite at home in the Late Cretaceous...if it weren't for the dinosaurs.

THE DEMISE OF THE DINOSAURS Researchers in Arizona are searching for clues in the mystery of the mass dinosaur extinction that occurred at the end of the Cretaceous Period. Such crime scene evidence may be contained in a wafer thin layer of rock that separates the very last moments of the Mesozoic from the beginning of the Age of Mammals, the Cenozoic. This rock layer, a seemingly insignificant geological snapshot, may reflect a monstrous astronomical event that took place 65 million years ago in what is now the Caribbean Ocean. The layer contains the remains not of dead animals but rather the altered clay residue of glassy silicates that were melted and ejected from meteoric impact near the shore of a tropical continent. Evident suggests that it was a huge meteorite that hurled itself against our planet at 45,000 miles per hour. It was capable of vaporizing much of an ocean and enough of the crust to form a crater 100 miles in diameter. The resulting energy release was equal to 1000 nuclear weapons exploding on every square mile of the earth's surface. Enough force to create mile high sea waves, to spawn massive amounts of noxious gasses and to ignite the biosphere's plants into global fire storms that may have roasted much of the Earth's surface-bound terrestrial life. The biological domino effect would have been enormous, and mass death quickly spread from pole to pole.

  Fallout material, the micro-meteoric dust of ejected earthcrust, would have eventually settled nearly everywhere, and certainly would have formed a thin blanket of dust, glass tektites and ash over all over the bit of earth now called Arizona. Indeed this boundary clay layer is found in many regions of the earth; the thickets layers are located on Caribbean islands. A huge underwater crater near the Mayan town of Chixchulub on the Yucatan Peninsula of Mexico is a suspected impact site.

Severe global cooling would have followed the last of the firestorms as dust particles and sulfur compounds both blocked sunlight and reflect much of the remaining solar radiance back into space for years to follow. Dinosaurs and other large creatures that were only marginally warm blooded would have succumbed rapidly, and other dependent on the rapidly dwindling food resources would soon follow into extinction. All terrestrial animals 55 pounds or more in weight disappeared! Small dinosaurs that were perhaps fully warm blooded or insulated with feather-like scales may have indeed survived. Early mammals were furry, warm blooded and small enough to also be protected against the terrible, but temporary surface conditions in protected microhabitats. Food, in the form of the dead and dying, a vast quantity of large animal carcasses, would have existed for a very long time for both newly adapted avian dinosaurs, the already evolved birds and the mammals. Eventually the earth recovered from the ashes and a new world, and new habitats without the classic dinosaurs was born.
Even with the evidence for a horrific extraterrestrial event 65 million years ago, not every geologist and paleontologist agree that the Late Cretaceous mass extinction was the result of this monstrous meteorite impact. Mass extinctions could have occurred for other reasons, such a diseases being disbursed over newly formed land bridges, global warming or cooling, massive volcanic pollution, competition by the more adaptable (and arguably smarter) primitive mammals, or other yet to be determined reasons not preserved in the geologic record. All we know for sure is that no excavation of fossils has shown that one single species of dinosaur crossed the Cretaceous-Tertiary boundary, at least none that couldn't fly.
WHY DINOSAURS ARE SO RARE IN ARIZONA
The fossil record shows us that the landscapes of Mesozoic Arizona were abundantly fertile and alive. More than four million species may have existed on earth at any particular time during this "age of reptiles"; plants, birds, reptiles, fish, amphibians, mammals and a myriad of creatures from amoebae to 150 foot long ultrasaurs. Most of these environments, in their balance of carnivores, herbivores, providers and decomposers, might resemble many mid-latitude ecosystems in today's world. The vertebrate components of such a terrestrial community were mostly reptiles and archosaurs, and their's was a diversity and number that rivaled the primitive mammals of the later Cenozoic. Forests, jungles, swamps, coastal reaches and deserts all cradled ecosystems that eventually became part of the geology. With the staggering variety of life forms, accounting for an unimaginable number of individuals, it seems difficult at first to account for the 95% of species that surely existed but have not yet been discovered as fossils--the cumulative biomass of dead leaves, seeds, wood, shell, bone, teeth, feces traces and tracks seemingly should be everywhere abundant in the sedimentary rocks. But this is not the rule.
Our understanding of how animals decompose and are preserved, the study called taphonomy, is very selective and logical. Very few organisms, under most circumstances, will ever become fossils. A worm or butterfly is too delicate to leave an imprint or carbon residue in coarse sandstone. An oak leaf having fallen to the forest floor will quickly crumble and be reduced to powder. Birds, because of their habits and fragile construction are quickly destroyed. It is rally surprising that we have any fossil record at all.
Despite such odds, all of these kinds of life can and do become fossils when specific conditions are met; a quick and undisturbed burial that is protected from the effects of oxygen and severe biological decomposition. The rapid accumulation of sediments over an organism protects it from being scavenged, moved or eroded by the elements. Oxygen and sunlight chemically break down organic compounds and microorganisms will rapidly consume even the toughest tissue..
Dinosaurs, even the species with massive skeletal frameworks are not exceptions to this rule. Even where the are known to occur, they are very rare as fossils in Arizona. Another very big lifeform living today can give us a hint as to why dinosaurs are so rare. Very few if any giant redwoods, one of the largest organisms on earth, will become a petrified tree after it dies within the confines of a forest. Quite quickly a tree's tissue becomes the forest compost that provides energy for new forest growth to develop. In fact the likelihood that any of the redwoods in a forest of millions of redwoods will be preserved as future fossils is very remote. Barring a nearby volcanic eruption or massive flood, it may be true that future paleontologist will never know that a particular redwood forest existed at all. Most dinosaurs, nearly 100% of them, likewise were dismembered and consumed by scavengers, decomposed by bacteria and bone elements rapidly disintegrated o the ground surface. The chances of fossilization were infinitesimally small, yet thousands of dinosaur fossils have been found and there is likely many times that number still in the rocks. Seemingly contradictory statements, that fact is that dinosaur fossils are abundant yet still very rare finds.
Erosion, the effects of wind and water that produces the sculpted quality of sedimentary rocks has taken its toll on the fossil record. All fossils skeletons disintegrate rapidly when the protective sediments that tightly fond them within the strata are stripped away. In softer shales, a bone or even an entire skeleton might be exposed by one rainstorm and virtually obliterated by another. The process of erosion can be very slow, and the thousands of bones still entombed in shale or sandstone formations may never be revealed during the course of a human lifetime or even the over many centuries. After a fossil has been collected from the surface of these hard, cemented sediments, no clues whatsoever remain to indicate that more fossils lay deeper...even a few inches deeper.
One of the most often heard comments in a paleontology laboratory are "I'd never be able to recognize these fossils in the ground". Often bone is difficult to distinguish from the surrounding matrix, and an entire bone, that looks like a bone, is rarely come upon sitting on the surface. Paleontologists train themselves to infer what lies below the ground from the smallest fragments that are exposed on the surface. So a canyon frequented by hunters and hikers may be a paleontologist's gold mine, but important clues may appear only as desert gravel to the untrained eye.
Because dinosaur bones and many of the associated fossils that help reconstruct and ancient habitat are rare or difficult to find, each becomes a precious part of a very grand history. To lose a fossil bone because of neglect or poor custodial care, or to vandalism is unforgivable; to hoard them as art our curiosities is unconscionable. So pity the paleontologist when they cry foul if a dinosaur is auctioned off for a swimming pool decoration or its agatized parts are cut into semi-precious gemstones. Too few of them exist to be sacrificed to greed or vanity.