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by Ron Ratkevich

During the Fall of 1996, amateur fossil collectors Richard Thompson and Gordon Nelson were exploring the rolling hills near Sonoita, Arizona looking for petrified logs. They knew the logs were there because geologists had recorded their existence in many scientific papers written about southeastern Arizona. These collectors also knew that fragments of dinosaur bone were also recorded, but only mentioned as footnotes in a few scientific reports. Chances were slim on finding anything significant, but, like gold hunters, the lure and the dream of "striking it rich" was always with them. This so happened to be the day that luck would be with them. In the cliff face of a remote canyon, jutting out of the hard sandstone of 100 million years ago, were fossilized bones too big to be from anything other than a dinosaur.
Understanding the importance of their find, they contacted paleontologist and archaeologist Ron Ratkevich who is an experienced dinosaur hunter formally with New York's American Museum of Natural History.
After several test excavations at the site and visits to major dinosaur museums to compare bones, the staff at the Arizona-Sonora Desert Museum determined that these fossils belonged to a brachiosaur, one of the largest kinds of dinosaurs ever to roam the earth. Much of the skeleton was eventually uncovered, and from the size of these bones it was clear that this dinosaur was about 55 feet long, and when alive it would have weighed as much as an entire herd of elephants. This is not particularly big for a brachiosaur, but still it was the largest dinosaur ever found in Arizona. It is also a new kind of dinosaur, and it was officially given the scientific name Sonorasaurus thompsoni to commemorate its discovery in the Sonoran Desert, and after its discoverer Richard Thompson.
Desert Museum Geologist Robert Scarborough is studying the geology of the rocks which contained the bones. It is likely the dinosaur died in a forested environment and was washed into a narrow river where floodwater and thousands of feet of sandstone eventually covered the carcass for millions of years until volcanic action uplifted the rocks and exposed the fossils. Geologists call these 100 million year old rocks the Turney Ranch, a period from the middle of the Cretaceous, the last age of the Mesozoic, or "Age of Reptiles".
There are some clues as to how this great reptile died. A tooth found near the bones belonging to a huge meat eater matched scars on the bones of Sonorasaurus. Was Sonorasaurus killed and eaten, or was it being scavenged on by opportunistic carnivores of the time? The answer may never be clear, but it is likely that it didn't die a natural death.
The Desert Museum has developed an extraordinary replication of the dinosaur site showing exactly how the bones were preserved in a wall of sandstone near Sonoita. Added to this bone display is a unique opportunity for kids to excavate replicas of bones and skulls. The Desert Museum continues to clean and preserve these fragile petrified bones so that future generations of scientists and the public can marvel at one of the most important finds of ancient life ever found in Pima County. Museum officials may eventually donated this one of a kind dinosaur fossil to the Mesa Southwest Museum which is planning to become a major natural history facility, and has adequate laboratory facilities which will assure the preservation of these extremely fragile fossil bones.


West of Tucson about 30 miles is the small town of Benson, a turn of the century cattle town, railroad junction and farming community. In the 30's the American Museum of Natural History ran field crews through the area looking for Pleistocene vertebrate fossils. Of course they were successful collecting many horse, camel, mastodon, turtle, Glypodon, and many other specimens. The low bluffs east and south of town produced many good specimens. To the west of the highway and again south of town, many Glypodon scoots and other bones have been collected. Further south about 5 miles passing through and south of St. David more Pleistocene deposits, University of Arizona has been collecting these localities for years and are still producing. As you travel further south towards Tombstone you will cross over low limestone ridges always check these out. In and around tombstone you will encounter the North Naco Formation Limestone, there you will find Mississipian invertebrate fauna, brachiopods coral, bryozoan etc. lots of good specimens. GOOD LUCK

Mammoth Bones

North of Benson

Along the San Pedro River which runs North and South through Benson Arizona is a tremendous Plio-Pleistocene deposit. North of the town of Benson, next to the San Pedro River, a dirt road runs north all the way up to San Manuel. Along this route one must be vigilant vertebrate fossils are hard to see, get out and walk the badlands, you might be surprised, again look and be prepared for the typical mammalian fauna.

Pleistocene Fossil Collecting

Tucson West

Avera Valley

Traveling west of Tucson on rt. 86 Ajo Way then on Kinney rd past TUCSON MINERAL AND GEM WORLD, Old Tucson and the Desert Museum approximately 8+ miles you can see Avera Valley this used to be a large shallow Pleistocene lake with a large diversified vertebrate fauna in the area. Brawley wash rubs north /south through this area and many specimens of turtle, camel, mastodon, horse etc have been found eroding from the banks of this wash. Most of the specimens found there have rather poor perversion as a result of exposure, food source etc. and other factors allowing deterioration of the bone prior to being covered. Remember this is Tucson, be prepared for all extreme climate conditions and critters that you could expect to encounter. Please remember to research all areas that you want to collect on. Remember most areas around Tucson are a mix of private, state, federal, blm, and state trust lands. Each has it's own set of rules regarding the legality to collect fossils

Southwest Paleontological Society and Mesa Southwest Museum, Mesa, AZ 85201 Copyright 1994



Ron Ratkevich


Arizona-Sonora Desert Museum Tucson, Arizona


Upper or Late Cretaceous non-marine faunas from southern Arizona are in general minimally represented in all survey collections of Arizona fossils. Generally, Late Cretaceous rocks exposed between Morenci and the Mexican border are non-fossiliferous continental clastics and volcanics. However, in sedimentary rocks which have not been metamorphosed or obscured by igneous intrusions, a significant but under-reported paleontological resource e~sts. The paleontology of the Late Cretaceous Fort Crittenden Formation is currently being investigated by the Arizona-Sonora Desert Museum. This freshwater shallow lake deposit is providing a good snapshot of a typical late Mesozoic environment. Fossil discoveries from the Fort Crittenden Formation exist in both public and private collections and are now being assembled at the Desert Museum for research and will be included in a future exhibit depicting dinosaurs and other Mesozoic life from the Sonoran Desert. UPPER CRETACEOUS ENVIRONMENT OF SOUTHERN ARIZONA Substantial geological uplift was occurring in southern Arizona during the Middle and Late Cretaceous. The 9,000-foot thick accumulation of the Lower Cretaceous deltaic Bisbee Group had long been exposed and deep erosional surfaces provided a series of freshwater and continental basins in which later sediments could accumulate and fossils could readily be preserved. During a relatively short period of geological time, many small lakes or freshwater lagoons existed inland from the more extensive Late Cretaceous seaway of western Arizona. Seventy-two million years ago, today's familiar mountains of southern Arizona did not exist, but existing Cretaceous mountain ranges provided the runoff and erosional sediments which eventually were preserved as conglomerates, sandstones, siltstones and mudstones. It was also a time of intermittent volcanic activity, but likely, the environment's volcanic episodes were separated by centuries, if not thousands of years. We find ample evidence of this late Cretaceous volcanic activity as layers of ash or breccias within the continental sedimentary rocks. Some of this volcanic activity was horrific. Caldera or crater formation (exemplified by what is now the 10-mile wide Tucson Mountain caldera remnant) likely represent volcanic activity many hundreds of times more violent than the recent eruption of Mt. St. Helen's. Slowly, lakes and lagoons and the Upper Cretaceous seaway itself began to drain. The retreat of waters was a result of the general tectonic uplift and isostatic rise of igneous magma plumes from below the region. Lush, wet lowland conditions so prevalent prior to Arizona's late Cretaceous orogeny were transforming into dryer highlands and dinosaurs and other Mesozoic life forms were forced to move on, likely following the declining shorelines and security of peripheral lagoons to the south. THE FORT CRITRENDEN FORMATION The Fort Crittenden Formation is primarily exposed in various canyons of the Santa Rita Mountains with marginal exposures in neighboring ranges, and has been measured to include five distinct units with a total thickness of 5,500 feet. Of these units within the Fort Crittenden, only one identified by Stoyanow and Miller as "Clam-Siltstone" contains abundant fossil remains. More recent determination, however, shows that significant vertebrate fossils exist in the upper reaches of this unit which is generally barren of other macrofossils. The clam-siltstone is a relatively thin 125-foot thick zone within an overall 550-foot thick shale member. Four distinct units exist within this shale member and it appears that only during the time of the clam-shale deposition were conditions right for the teeming of molluscan life forms as represented by its fossil record. The other three units, a conglomerate and two additional shale units, may have been deposited in periods of suffocating high turbidity and are sparsely fossiliferous, but do contain the majority of dinosaurs, other tetrapod and fish remains, species that would have been less inclined to be affected by suspended silts as were the filter-feeding mollusks and browsing gastropods. Date Established for the Fort Crittenden Formation Fossils in the Fort Crittenden Formation clearly indicate a late Cretaceous Santonian to Maestrichtian time, a time interval corroborated by potassium-argon radiometric analysis from an overlying welded tuff that provided a date of 72 million years. This deposition represents Late Cretaceous but not the latest Cretaceous, which would have a terminal date of 66 million years. Thus a period of seven million years remained in the Mesozoic after conditions ended for accumulation of Fort Crittenden sediments. Except for small remnants of latest Cretaceous sedimentary rocks, such as the upland conditions represented by the Latest Cretaceous Solero Formation, most of the remaining years of the Cretaceous in southern Arizona either saw little sediment accumulation or that which existed has been eroded away or is obscured or metamorphosed by area volcanics. FOSSILS The Desert Museum has made several survey trips to the Fort Crittenden Formation exposed in Adobe Canyon, on the eastern flanks of the Santa Rita Mountains. Surface collections resulted in more than 100 representative fossil specimens, with species recovered generally paralleling those listed in ............ ....... :;'' The large camosaur Albertosaurus (Gorgosaurus) ready to feed on its prey, a duck-billed hadrosaur. Fossils of both of these Late Cretaceous dinosaurs are found in the Fort Crittenden Formation of southeastern Arizona. (From: Ratkevich, Dinosaurs of the SouthwesO 78 existing literature about the Fort Crittenden. A number of the fossils found, however, may be additions to the fossil record, which was assembled primarily prior to 1971. Vertebrate fossils more recently discovered by amateur fossil collectors when made available to researchers should lengthen considerably the faunal list, and without a doubt when a systematic collection is completed by the Desert Museum, the environment that produced the Fort Crittenden will show it to be typically rich and varied in its diversity. Fossils from Adobe Canyon The following fossil species were collected from the shale member of the Fort Crittenden Formation which outcrops just east of the head of the road in Adobe Canyon below Pilar Tank by this author and by David Thayer, Curator of Geology at the Arizona-Sonora Desert Museum. The collections were made under the permit authority of the National Forest Service, Coronado National Forest. Invertebrates

(all were found in massive numbers)

Protelliptio sp. [elongated clam]
Unio sp. [round clam]
Viviparus sp. [conically spired gastropod]
Plesielliptio~sp. [elongated clam]
Bellamya sp. [gastropod]
Eupera sp. [round clam]


Carnosaur teeth tentatively identified as the tyrannosaurid Gorgosaurus.

Ceratopsian tooth, indet.
Dinosaur bone, indet. [parts of small limbs, vertebra, etc. mostly left in place]
Lepidosteus sp. [garfish]
Plastomenus sp. [turtle]
Asperidetes [turtle]
Coprolites [common and often mixed in with and mistaken for concretions]


Petrified wood, indet.

A greater number of species is presently in the possession of amateur collectors who have not yet made the fossil specimens available for research. These include the distal articular end of a tyrannosaurid femur, a complete ceratopsian mandible, possible champsosaur vertebra, fish vertebra and teeth, dinosaur teeth, turtle bone, possible lizard remains and a possible small mosasaur tooth. Little attempt has been made to collect microfossils, but a large variety has been reported by various authors. The Desert Museum will systematically collect these in the future. Known micro- or small macro- fossils known to exist include fish teeth, tiny bones, fungus remains, a mosquito and a variety of shelled invertebrates. Mammals are not yet reported, but the Mesozoic environment and faunal content is nearly identical to those elsewhere which contain mammal fossils. This author believes that mammals will eventually be identified and most likely will be found during wet screening of sediments. If found, a mammal fauna will be of eminent importance to the Mesozoic paleontology of the Sonoran Desert region. POSSIBLE CRETACEOUS-TERTIARY BOUNDARY

While the Fort Crittenden Formation was deposited at least seven million years before the end of the Cretaceous, the stratigraphically higher Salero Formation does contain sedimentary units which may have been forming 66 million years ago (late, Late Cretaceous) and may contain important evidence of the catastrophic events which conceivably ended the Mesozoic. Drews reports that mildly metamorphosed fossils of pine needle like fossils and scraps of plants were found in the Salero Formation unit comprised of silty shale. Not only are these very late Cretaceous fossils important, but the potential existence of a K/T boundary clay layer in southern Arizona may shed additional light on the nature and geography of terminal events or the long distance effects of a possible K/T asteroid or comet impact that occurred at Chicxulub, Yucatan, Mexico. Possible Difficulty in Establishing K/T Boundry

The Salero is primarily of volcanic origin and any thin lens of clay may closely resemble other clastic rocks of igneous origin. Additionally, such clay may have been altered by contact metamorphosis and again its origin may be masked and could resemble some other form of rock such as the metasediment hornf els. Petrographic studies should clearly determine the origin and source of suspect deposits, and also determine if any samples contain traces of an iridium marker.


Fossils of the Fort Crittenden Formation of Adobe Canyon are abundant and often well preserved. However, many of the vertebrates are encased in a concrete-hard siltstone that weathers very slowly into very high angle slopes that are difficult and/or dangerous to access. Most fossils recovered are those that have weathered out, or are found in blocks that have broken loose and found their way lower on the talus slopes. Amateur collecting, even though done by enthusiastic people of good intention who are willing to share their finds with researchers, has created a problem in that all surface vertebrate material has been in effect "vacuumed up", leaving little or no bone scrap on the weathered slopes. The importance of such bone fragments, or float, is paramount in paleontological field work because they are often pieces of unexposed and more complete fossil material. Without this bone, it is often impossible to locate fossils obscured by talus or by having a narrow profile and indistinct color within the priginal matrix. It is the hope of this author that much of the material in private collections from Adobe Canyon can eventually be consolidated for study and interpretive display.

REFERENCES DREWS, H. 1971, Mesozoic Stratigraphy of the Santa Rita Mountains, Southeast of Tucson, Arizona, Geological Survey Professional Paper 658-C. Washington D.C. JENNEY, J.P. and REYNOLDS, S.J. (eds.) 1989, Geological Evolution of Arizona. Arizona Geological Society Digest 17, pp. 447-461 MILLER, H.W., JR. 1964, Cretaceous Dinosaur Remains from Southeastern Arizona, Journal of Paleontology, v. 38, no. 2, pp. 378- 384. NATIONS, J.D. and STUMP, E. 1981, Geology of Arizona, Kendal/Hunt Publishing Company, Dubuque, IA, pp. 133-135. STOYANOW, A. 1949, Lower Cretaceous Stratigraphy in Southeastern Arizona. The Geological Society of America Memoir 38. pp. 59-60 80



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