The oldest rocks in the Tucson Mountains are over 570 million years old. These rocks are what is left of sediments laid down during a time when shallow, tropical seas covered most of Arizona, and the rocks are so old and were subjected to so much heat and pressure, the changed, or metamorphosed to altered rocks called gneiss and schist. During Paleozoic times, new seas flooded our region and more sedimentary rock was laid down. These remnants of the sea bottom, now limestone, is visible near Sus Picnic Area, Twin Hills, and along the western side of the Tucson Mountains. These early sea-bottom limestone deposites were subsequently covered by geologically later sediments and in the end covered with vast amounts of volcanics. Mostly, these early rocks are now under the entire mass of the Tucson mountain caldera rocks.
Here, during the age of dinosaurs, different episodes of volcanism contributed to the development of the Tucson Mountains. Volcanoes, active about 150 million years ago during the Jurassic Period laid down deposits of ash and dust. The red rocks see to the righ are derived from the erosion of tall Arizona mountains that have been long ago completely weathered away.
150 million years ago, southern Arizona began a period of intense volcanic activity. Ash from these eruptions are found in the Jurassic rocks seen at right.
Mid Jurassic Recreation Redbeds in Saguaro NP East
When combined with mud and sand, the volcanic-derived sediments carried into a low, swampy catch-basin, created the Recreation Redbeds that can be seen behind the Saguaro National Park Headquarters in the Tucson Mountains. The red sandstone and shale is colored by hematite, and in places a hiker can see impressions of raindrops, lizard tracks, plant fossils and water ripple marks in these rocks. In the redbeds seen above, the surfaces of some of the rocks which have bedding plains (the natural surface of the ground preserved and seen when you split the rocks open) have masses of fern and horsetail fossils, and large tracks of sprawling reptiles or amphibians. Sorry, a federal permit is needed to collect fossils here and besides the Park Rangers will see you from their back window.
After this volcanic activity, streams passed through the area depositing layers of sandstone and limestone, forming Amole Arkose. Amole is Spanish for "grinding stone"; arkose is a sandstone containing angular fragments. Few fossils are found in the arkose or redbeds, so little is known about these formations, but in some places in the Tucson Mountains, there is a considerable amount of petrified wood, and some dinosaur bones.
Shale in the Amole has lots of plant fossils
Ripples in the Tucson Mts. Cretaceous has dino tracks
At the end of the Cretaceous Period some 75 million years ago, volcanic activity covered the area that would be the Tucson Mountains with lava flows and ash. Much of the rock near Gates Pass is a result of a complex series of events called by geologists "The Tucson Mt. Chaos." The rocky deposits were formed by super-hot landslides coming from the Cretaceous crater. These deposits contain boulders and house-sized blocks of older diverse rock, such as Paleozoic limestone. Big hunks ("mega breccia") of Cretacous rock that floated in the red-hot maga like iceburgs have contained dinosaur bones.
Tucson Mountains Today. Dinosaur bones are found in blocks of Cretaceous strata blasted apart by the volcanic eruption.
Tucson 75 million years ago. Not healthy for dinosaurs OR their fossils. Toast!
The Tucson Mountains seen today were uplifted during a ten million year period, 15-5 million years ago called the Basin and Range orogeny. The scenery produced by all of this geologic activity is mostly a result of massive faulting and subsequent erosion. Because of this regional up and down movent of the earth's crust, long-buried sedimentary rocks became exposed and their fossil content accessible. In the Tucson Mountains, the Amole Sandstone seen on eroding fault lines provide us with petrified wood and other plant fossils, fresh-water mollusks, and occasionally dinosaur bones and dino footprints.


In the Tucson Mountains near the Arizona-Sonora Desert Museum, river and lake deposits of the Amole Formation seen at the right contain dinosaur fossils as well as petrified logs, fish, invertebrates and plants. I recently found the trackway of a medium-size ambling dinosaur 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 isn't Montana or Wyoming, states which have an abundance of dinosaurs, so each find in the Cretaceous rocks of the Tucson Mountains are important pieces the puzzle that fossil hunters love: What was life like when the rocks were deposited?
Nick's Peak. Lots of Cretaceous petrified wood, but far more Jumping Cholla cactus that can make this spot in the Tucson Mountains a nightmare to visit
Cretaceous sandstone of the Tucson Mountains represents little understood events in geologic history, and holds the keys to remarkable story of life in Arizona 75-100 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.
I found horn cores and vertebra of ceratopsian dinos in the Cretaceous shales of Adobe Canyon
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
Looking at the
Cretaceous Fort Crittendent Formation
Knife-like tooth of Albertosaurus.
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 forty 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.
A duck bill's hoof
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.
Rows of teeth in the hadrosaur's jaws were great for slicing and dicing tough plants of the Cretaceous uplands envornoments of southern Arizona.
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, but 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.
Multituberculate premolar X5
Multituberculate molar X5
Evidence of primitive mammals small dinosaurs too have been found in the Fort Crittenden Formation. I found quite a number of tiny, perfect teeth by collecting ant hills. Ants are hard workers and do the digging for me.
Cretaceous mamals are found in the Fort Crittenden Formation
The American Flag Formation exposed on the north side of the Santa Catalina Mountains near Oracle were an unexpected bonanza for paleontologists who wanted to understand the fossil history of the Mid Cretaceous Period. 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 as wall as an abundance of fossil plants and animals typical of a sub-tropical conifer forest. Fossil pine needles and cones are particularly abundant in some zones. 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, recent exploration by myself from the Arizona-Sonora Desert Museum 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.
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 Iguanadonts 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.
Cool thumb spikes,eh?
Iguanadon in the flesh
I found the teeth and bite marks of Acrocanthosaurus on my
brachiosaur Sonorasasurus in the Whetstone Mountains

Sonorasaurus thompsoni

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 front limb of Sonorasaurs exhibited at the Desert Museum
Limb bones turned out to be hind toes. Yikes!
Sonorasaurus was a mini-brachiosaur compared to this Jurassic form, but still big
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

Seen in the photograph to the right, what appears to be a jack-straw pile of bones actually had some anatomical order to them. Looking at the fossil from a taphonic perspective, how this animal died and what happened to it as the carcass lay rotting in the sun, the mystery of why the skeleton seemed so dissoveled. It was lunch for one or more gigantic acrocanthosaurs, a sort of sail-back allosasur of the Mid Cretaceous. The body of Sonorasaurus was ripped apart, with chunks the size of a full-grown man being swallowed, meant, bone and more meat. Shed teeth of the acrocanthosaurs were the "smoking gun" as noted paleontologist Dr. Bob Baaker stated. In addition, some of the bones showed clear, scars and scrapes as a result of the feeding frenzy. Think about it, what good carnasaur could resist tons of meat getting really stinkey as it cooked in the equatorial sunshine of the time. Not shown in the exhibit was the significant amount of carbonized flesh that we were not permitted to collect in order to be politically correct I guess. Too bad.

That's my son Nick who helped dig up the bones of Sonorasaurus. The exhibit at the Arizona-Sonora Desert Museum is a pretty faithful depiction of how some of the bones lay in the tilted Cretaceous rocks of the Whetstone Mountains.
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.
Sonorasaurus' 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 exploration continues by the Mesa Southwest Museum after what was termed "Valley of the Dinosaurs" near Sonoita was abanddoned by the Arizona-Sonora Desert Museum.
Lunch time in the Mid Cretaceous of Southern Arizona. With our brachiosaur,
the Acro had his way.
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 Paleontologist Ron Ratkevich (that's me!) at Tucson's Arizona-Sonora Desert Museum where a replica of the bones as they were originally seen in the Turney Ranch sandstone has become part of a permanent interpretive exhibit. The full scientific report is available on the Internet. (link)