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FOSSIL COLLECTING
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PIMA COUNTY'S GRAND DINOSAUR
COURTESY OF DESERT TIMES / MONUMENT NEWS
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.
BENSON
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
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
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
PALEONTOLOGY OF THE LATE CRETACEOUS FORT CRITTENDEN
AND SALERO FORMATIONS OF SOUTHWESTERN ARIZONA
Ron Ratkevich
Paleontologist
Arizona-Sonora Desert Museum Tucson, Arizona
ABSTRACT
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]
Ostracods
Vertebrates
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]
Plants
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.
CONCLUSION
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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