BRENT H. BREITHAUPT
Geological Museum, University of Wyoming, Laramie, WY 82071
AbstractThe Late Jurassic dinosaur Allosaurus has been known for over 100 years. However, it was not until 1991, when "Big Al" was discovered, that one of the most fascinating paleontological mysteries began to unfold. The skeleton was found on public lands administered by the Bureau of Land Management near Shell, Wyoming in the Upper Jurassic Morrison Formation. Research on this specimen has provided exciting new information on Allosaurus, the dominant predator of the Late Jurassic of North America), as well as the environment in which it lived. "Big Al"(MOR 693) is a 95% complete, partially articulated, and pathologic (i.e., with broken, fractured, and infected bones) skeleton of a subadult Allosaurus fragilis. "Big Al" has gained international recognition, as the scenario of its tragically painful life, early death, and rapid burial has been told through interactive exhibits and state-of-the-art-television programs.
In addition, the intricate tale of this dinosaur's discovery adds intrigue to "Big Al's" story. A short distance to the south and slightly lower stratigraphically from the "Big Al" Quarry, the famous Howe Quarry was worked by crews from the American Museum of Natural History in the 1930s. Over the years, hundreds of people, including teams from the American Museum of Natural History, walked right over "Big Al," still buried in the rock. The discovery of this dinosaur exemplifies the fact that significant paleontological material remains to be discovered in the Rocky Mountain West. Interagency cooperation is essential to collect, study, interpret, exhibit, and preserve these specimens for future generations. The "Big Al" project serves as a model of such cooperation. As it involved scientists, students, volunteers, land managers, educators, media, local museums, and the general public, it is an excellent example of a "paleodetective" partnership.
The carnivorous dinosaur Allosaurus has been known since the late 1800s. This theropod was first documented in 1869, when Ferdinand Vandiveer Hayden was presented with a "petrified horse hoof" by locals in the Middle Park region of Colorado (Hayden, 1869). This specimen of a fragmented, caudal vertebra was identified by Joseph Leidy (1870) as belonging to the European dinosaur genus Poicilopleuron. Later Leidy (1873) suggested that this specimen may represent a new genus, Antrodemus. Several years later, remains of this type of theropod were found when additional caudal vertebrae, as well as a variety of other elements (i.e., dorsal vertebrae, phalanx, tooth, and humerus) and a separate, partial skeleton were uncovered in the Garden Park area of Colorado. This material (more diagnostic than the earlier finds) was described by Othniel Charles Marsh (1877) as representing a new taxon of theropod dinosaur, Allosaurus fragilis. Since the 1870s, thousands of Allosaurus specimens representing individuals of different ages have been recovered in exposures of the Morrison Formation in the Rocky Mountain West (Gilmore, 1920; Madsen, 1976; Bilbey, 1999).
Allosaurus fragilis meaning "fragile, different reptile" lived approximately 145-150 million years ago during the Late Jurassic Period, with a recorded geographic range in Wyoming, Montana, Colorado, New Mexico, South Dakota, Utah, Oklahoma and Portugal (Glut, 1997; Prez-Moreno and Chure, 1999). It is known to reach 12 meters in length, 4.5 meters in height, and may have weighed up to 2 tons as an adult. A. fragilis was one of the top predators of its time and is found primarily in the Morrison Formation. It lived on the lowland floodplains of the Western Interior in an environment similar to some of the large plains of Africa today (Russell, 1989) Allosaurus was the most common large carnivore of the Late Jurassic in North America. Strong forelimbs with sharp claws, powerful hind legs and recurved, dagger-like teeth are evidence that Allosaurus was a formidable predator. During the Late Jurassic, herds of plant-eating sauropods (such as Apatosaurus) were constantly on the move in search of food to satisfy their large appetites. Predators, such as Allosaurus, may have followed these herds, preying on the young and weak. Although it was much smaller than the sauropods, quite possibly Allosaurus hunted in packs to bring down larger prey. Like many meat eaters, it may also have been a scavenger.
A unique deposit, the lithologically variable Morrison Formation represents a 1.5 million square kilometer "sedimentary sheet" extending 1500 kilometers from New Mexico to Canada and 1000 kilometers from Idaho to Nebraska with major fossiliferous outcrops in Wyoming, Colorado, Utah, Montana, New Mexico, Arizona, and Oklahoma (Russell,1989). Research on the Morrison Formation, has significantly altered our view of the dramatic diversity of terrestrial life that has existed on our planet during the Late Jurassic Period.
The first major vertebrate paleontological sites in the Morrison Formation were discovered in 1877 in Wyoming and Colorado (Ostrom and McIntosh, 1966; Breithaupt, 1998). Since that time, tens of thousands of fossils and hundreds of skeletons (both partial and complete) have been recovered for institutions throughout the world. Over 125 years of extensive collecting has resulted in one of the world's best known prehistoric terrestrial faunas; ranging from algae, cycads, and bivalves to mammals, pterosaurs, and dinosaurs (Chure et al., 1998). Work within the Morrison Formation has added greatly to our understanding of the "life and times" of the Mesozoic Era, as well as to the development of current collecting and preparation techniques.
The Morrison Formation in the eastern Bighorn Basin of Wyoming consists of approximately 65 meters of fluvial, lacustrine, and aeolian sediments (Swierc and Johnson, 1996). This unit is underlain by the primarily marine, Jurassic Sundance Formation, and overlain by the terrestrial, Early Cretaceous Cloverly Formation. The Morrison Formation within the eastern part of the Bighorn Basin was deposited on a low-relief floodplain over the course of seven million years during Tithonian time (Swierc and Johnson, 1996). The "Big Al" skeleton was found stratigraphically slightly above the primary producing layer of the famous Howe Quarry, and is inferred to be approximately 145 million years old (Swierc and Johnson, 1996). However, stratigraphic work by Turner and Peterson (1999) suggests that the "Big Al" and Howe quarries may be roughly 152 million years old and Kimmeridgian in age.
In 1932, Barnum Brown was notified of the existence of large bones on the ranch of Mr. Barker Howe. Reconnaissance of the site revealed the promise of an extremely rich quarry in the Morrison Formation. Brown returned to the site outside of Shell, Wyoming in 1934 with a field crew from the American Museum of Natural History. Although only two sauropod skeletons were initially uncovered, by later that summer a veritable disarticulated herd had been discovered. Skeletal remains were crisscrossed and interlocked in a confusing, almost inextricable manner in a clay unit beneath a relatively thick sandstone layer (Bird, 1985; Breithaupt, 1997). For two months, bones were exposed, mapped and viewed by thousands of visitors from around the world. After being mapped and photographed, the specimens were painstakingly removed from the quarry. Over 30 metric tons of bones were collected from the Howe Quarry. Roughly six months after the quarry was opened, the last box of fossils (totaling 144) was loaded on a train bound for New York (Brown, 1935; Colbert, 1968).
The Howe Quarry produced over 4,000 Late Jurassic dinosaur remains. Measuring 14 x 20 meters, the quarry contained one of the densest concentrations of Jurassic dinosaur bones ever found. The assemblage of fossils represented at least 20 different animals. The quarry was dominated by the sauropods Barosaurus, Diplodocus, Apatosaurus, and Camarasaurus, although some remains of the ornithopod Camptosaurus were found. Only isolated theropod teeth attributable to Allosaurus were discovered in the quarry. Most remains were disarticulated, but a number of bones showed some degree of association. The Howe Quarry represents an accumulation of desiccated carcass parts that were washed into a small depression during a times of seasonal flooding (Breithaupt, 1997). In addition to bones and teeth, footprints, skin impressions, and gastroliths have also been found in the Howe Quarry (Brown, 1935; Ayer, 2000).
"BIG AL" QUARRY
|Figure 1. "Big Al" after it was uncovered by Swiss fossil collector.|
In 1990, Siber + Siber, Ltd. (a commercial fossil collecting company from Aathal, Switzerland) reopened the Howe Quarry with plans to uncover more dinosaur bones. The original Howe Quarry was expanded and additional material discovered (Ayer, 2000). However, it was not long until the bones of this fossiliferous lens in the Morrison Formation "played out." The Swiss collectors decided to expand their operations laterally with hopes of finding the Howe Quarry bone layer elsewhere. In 1991, they brought in a backhoe to dig test pits along the slope to the north. A new quarry was soon found and a small bonebed exposed. Test digs continued. Approximately 300 meters north of the Howe Quarry, the backhoe hit sauropod bones at about the same level as the Howe Quarry. As excavation of these bones ensued in August, the collectors encountered some theropod vertebrae slightly above the sauropod remains. As they collected this material, they realized that they had stumbled upon many bones of a carnivorous dinosaur. Theropod dinosaurs are relatively rare and very few theropod remains were found in the Howe Quarry. In addition, careful excavation revealed not a random accumulation of disarticulated bones (as had been found in the Howe Quarry), but a single, nearly complete, articulated skeleton. Following the vertebrae to the head-end of the specimen revealed the glint of black enamel, indicating a tooth, which eventually led to an entire tooth row. Later, the jaws and the entire skull were found. Continued work uncovered other parts of the skeleton (Figure 1). The specimen appeared to be about 6 meters long and preliminary identifications hinted that it might be the rare Jurassic dinosaur Ceratosaurus.
As excavation continued on the specimen in September of 1991, the lead collector, Kirby Siber, traveled to the Denver Rock, Mineral, and Fossil Show and is rumored to have proudly described his new discovery. While Kirby was in Denver, a Bureau of Land Management (BLM) crew was flying over the area en route to the nearby forests to check for fires. As they flew over the Howe Quarry they recognized that one of the dirt roads in the area had been dramatically modified. As these road improvements on public lands had not been sanctioned, BLM officials inspected the road. This road had been modified to bring in the heavy equipment used by the Swiss crews for excavating bones at the Howe Quarry and the test pits to the north. The road led investigators directly to a tarp-covered quarry, where the partially uncovered dinosaur lay. Fortunately, few of the fossilized bones had been removed from the ground. It was clear to those visiting the site that the specimen under the tarp was a dinosaur and an interesting one at that. Checking the land records indicated that this quarry and the small bonebed to the south were on BLM land. The dividing line between private land and that administered by the BLM ran between the Howe Quarry and the new theropod skeleton. Unfortunately for the Swiss team, which had permission to collect on private land at the Howe Quarry, they did not have a permit to collect the fossils on BLM land (Figure 2).
|Figure 2. Billings Gazette newspaper story about the "Big Al" discovery - the highlighting the land status issue (Milstein, 1991).|
Because vertebrate fossils found on public lands belong to the people of the United States and are managed by government agencies in the best interest of the public, vertebrate fossils (which are public resources) can not be privatized nor collected and sold. Fossils collected by commercial dealers often end up in private collections and are not studied scientifically. As a result, these unique, nonrenewable, educational and scientific resources are sometimes lost to science and the public forever.
The BLM called in investigators to study the possibilities of a potential criminal case (i.e., illegal collecting), surveyors to verify the land status, and paleontologists to examine the specimen. Paleontologists from Montana State University's Museum of the Rockies (MOR), the University of Wyoming (UW) Geological Museum, and the Royal Tyrrell Museum in Canada convened at the site to examine the specimen. Although there had been some preliminary speculation that the specimen might represent the theropod Ceratosaurus, it was quickly concluded that the fossil was the better known genus Allosaurus. Unlike most previously discovered Allosaurus specimens, this one was very well preserved, nearly complete, and in partial articulation. Although some carnosaurs have been found which are over 60 percent complete, allosaurids are usually represented by less complete specimens. This specimen appeared to be a subadult, which added to its importance. The Swiss collectors were asked to stop any further activity on the BLM lands and counseled to work strictly on private lands. Although investigators verified unpermitted collecting activity on BLM lands, the Swiss collectors were not charged with any violation. They were, however, warned to be more careful of the land status in any areas of future work. Surprisingly, the landowners in the area were also unaware of the correct boundary lines until this discovery was investigated.
|Figure 3. Overview of the "Big Al" Quarry looking to the east.|
The next problem, was what to do with a partially exposed specimen of a very important fossil. As the dinosaur was found on federal land, regulations required it to be collected by a qualified paleontological team and put into a recognized federal repository for vertebrate fossils. Although many paleontological museums in the country would be acceptable and the National Museum in Washington, D.C. was contacted, it was decided that the specimen should stay in the State of Wyoming and go to the UW Geological Museum. Unfortunately, the author was the sole staff member of that museum at that time and the museum lacked the finances, equipment, and people to undertake such a project. Fortunately, the well-staffed and fully-equipped Museum of Rockies agreed to work with the UW Geological Museum and the BLM to excavate and prepare the specimen with plans to provide a display cast for the UW museum in Laramie (Breithaupt, 1996). After the fossil discovery, the BLM's primary concern was to protect the bones and to make certain that Wyoming citizens would have a chance to see "Big Al." The cooperative effort between BLM and the museums fulfilled both of these objectives.
As the BLM considered this find a very important resource for them to protect and preserve, they provided some of the financial support for the project. Shortly after this decision was made, crews arrived at the site to remove the specimen from the ground. It was September and the specimen was already partially exposed (Figure 3). Therefore it was necessary to get the material out of the ground as soon as possible, prior to the first snows of winter. In essence, the project became a salvage operation and crews needed to work quickly and efficiently. An experienced group of students and staff from the Museum of Rockies worked diligently to remove the bones from the ground, while the author worked to photograph and map the bones as they were exposed (Figure 4). The excavation team used small hand tools (hammers, chisels, awls and brushes) to expose as much of the skeleton as possible. All bones were given field identification numbers and their locations at the site mapped. Roughly eight days after the paleontological crews began working at the site, the specimen was exposed, treated with preservative, "plaster-jacketed" (i.e., bones encased in plaster-soaked burlap wraps, making a hard, protective cast), and removed from the ground. "Big Al" was taken to Bozeman for detailed preparation.
|Figures 4a - 4d. Museum of the Rockies and University of Wyoming Geological Museum team working at the "Big Al" Quarry.||Photos 4a, b, and d by MOR photographer Bruce Selyem.|
|4a, Allison Gentry and the author locating bones on the quarry map.||4c, Museum of the Rockies crew working in the "Big Al" Quarry.|
|4b, MSU student Greg Erickson excavating vertebrae.||4d, Excavation of "Big Al" by Allison Gentry and MSU student Yoshi Tomida.|
|Figure 5. The author describing the "Big Al" excavation to school children in 1991. Photo by MOR photographer Bruce Selyem.|
As the work progressed, crews undertook an unusual approach to dinosaur excavation; the media and public were actively encouraged to visit the site. With the help of the BLM and public affairs people at the Museum of the Rockies, newspaper and television reporters visited the site. Schools in the Bighorn Basin were closed for part of a day to allow field trips to the "Big Al" Quarry. Over 4,000 students visited the site during the course of the excavation. In addition, BLM officials from around the country, including the Director from Washington, D.C. visited the site. As the stories appeared in newspapers and on CNN around the world, people flocked to the quarry, to see the excavation of the most complete dinosaur skeleton ever found in Wyoming. Thousands of people visited the "Big Al" Quarry and field crews coordinated their efforts to provide interpretation of the site to all that visited (Figure5). With careful planning, the excavation continued unimpeded by the throngs of people watching as the bones were removed from the ground. The BLM also provided law enforcement protection of the site and helped limit after hours visitation. This coordinated effort was one of the most intensive dinosaur excavation, research, and public interpretation endeavors ever undertaken in the field. Because of the careful planning, coordination of efforts, and excellent team work, the project ran smoothly from beginning to end.
Over the course of the next two years, MOR crews returned to the site and removed additional bones of this Allosaurus. These bones were also mapped and included on the map of the quarry (Figure 6). Eventually, nearly all of this theropod was recovered from the site. Although originally called the Siber Quarry, this site was renamed the "Big Al" Quarry and the skeleton nicknamed "Big Al." It was understood that "Big Al" was a small individual measuring roughly only 8 meters, and as such was only 60-70% of its maximum size. However, it was a very "big" find because of its completeness, preservation, and scientific importance.
|Figure 6. "Big Al" Quarry map. Author's original field map redrafted by UW student Scott Hartmon and National Science and Technology Center photogrammetrist Neffra Matthews and cartographer Paul Graves.|
|Figure 7. Preparation of "Big Al" skeleton at the Museum of the Rockies by Bob Harmon. Photo by MOR photographer Bruce Selyem.|
Once in the preparation lab at the Museum of the Rockies, the bones were carefully removed from the rock and plaster jackets. Delicate cleaning tools including brushes, files and compressed air devices were used to uncover fossil bones (Figure 7). Preservatives were applied to the fossil surface and the cracks were filled to increase the bones' strength. As the bones were carefully prepared, reconstructed, and preserved, it became apparent that about 95% of the specimen had been found. In addition, the skull was the most complete and best preserved Allosaurus skull ever discovered (Figure 8). Detailed observation of the bones also revealed that there were a number of interesting abnormalities. Closer inspection indicated these resulted from injuries sustained in life by this Allosaurus. Rebecca (Laws) Hanna studied this specimen and described 19 pathologic bones; more known than from any other Allosaurus (Laws, 1996). These injuries were found throughout the body.
|Figure 8. The left side of "Big Al" skull as it was being prepared out from the surrounding rock while still in its protective plaster jacket. Photo by Bruce Selyem.|
As additional bones were being prepared from the rock, each bone was examined for abnormalities and any information that could be provided about the life, death, and preservation of this dinosaur. In addition, each bone was molded and cast by the MOR. After polyurethane copies were made of the numerous bones of this animal, they were painstakingly painted and put in place in a lifelike skeletal mount of this animal. With the development of new resins in recent years, using casts instead of the actual fossils is a popular way for museums to create dinosaur displays. The casts are much lighter and easier to manipulate, creating much more interesting and lifelike poses. An important consideration is that by using a cast instead of the original fossil, scientists can continue to study actual fossils while the public enjoys the display cast.
Figure 9. Bob Harmon with "Big Al's" skeletal cast at the Museum of the Rockies, Montana State University. Photo by Bruce Selyem.
In 1995, the cast was completed and stood in the vertebrate paleontology lab at the Museum of the Rockies (Figure 9). Immediately after the cast was assembled, the author went to Bozeman to oversee the disassembly, packing and transportation of the cast to Laramie. Robert Harmon (MOR chief preparator of paleontology), UW students, and the author reassembled this specimen in the exhibit hall at the UW Geological Museum (Figure 10) tail-to-tail with its Jurassic contemporary, Apatosaurus. For the first time in 145 million years "Big Al" again stood in Wyoming (in a fashion) in December, 1995. The dedication of this cast and the celebration of the end of the first phase of this project occurred in January, 1996.
|Figure 10. "Big Al's" skeletal cast at the University of Wyoming Geology Museum. Photo by Ted Brummond.|
However, the "Big Al" project continued. A variety of interpretive and education displays were developed associated with this Allosaurus by the UW Geological Museum. Over the course of the next several years, plans were made and materials amassed for the second phase of the "Big Al" project. In 1999 private donors (especially, Al Wolfe and J. David Love) and the BLM provided funding for a series of exhibits interpreting "Big Al's" life. The museum hired students, commissioned artists, and accumulated a variety of casts to tell a unique story about this dinosaur. In 2001, the displays on this specimen will be unveiled to the public.
The "Big Al" exhibit will become a highlight display at the UW Geological Museum. The exhibit is planned to be one of the most comprehensive displays on Allosaurus anywhere in the world. The history of discoveries of Allosaurus in the Rocky Mountain West will be told, ranging from the 1869 discovery of the single fragment of a tail bone to the nearly complete skeleton of "Big Al" in 1991. Interactive displays will teach the museum visitor about the process of scientific investigation. An educational exercise (What Killed "Big Al"?: Investigating the Evidence) is also being developed. This exercise will be utilized by Kindergarten - College age students that visit the museum. The displays will engage the students to think like scientists, gather data and make appropriate interpretations. Tens of thousands of visitors (including thousands of students) from around the world visit the museum each year. Eventually, this exercise will be put on the museum's website (http://www.uwyo.edu/geomuseum/) so that students unable to visit the museum can still learn from the exhibit.
While the UW Geological Museum developed displays and educational programs around the cast of "Big Al," a team of "paleodetectives" was studying the original fossil bones of this dinosaur. Rebecca (Laws) Hanna continued her work on the pathologies of "Big Al" (Laws, 1996; Hanna, 2000; Hanna, in press). Dan Chure incorporated the important osteological features of this dinosaur into his doctoral dissertation work on Allosaurus (Chure 2000). Paul Bybee (1996) utilized this specimen in his dissertation growth study on Allosaurus. John Foster and Chure included "Big Al" in their hind limb proportion allometric study on Allosaurus (Foster and Chure, 1999). Recently, Cambridge paleontologist Emily Rayfield CT scanned the skull of "Big Al" to determine the cranial biomechanics of its skull (Rayfield et al, 2001). A study which she will also incorporate into her doctoral dissertation. Taphonomic studies dealing with ancient beetle traces on dinosaur bones have also included "Big Al's" skeleton (Laws et al., 1996; Hasiotis et al., 1999). Building on his photodocumentation and mapping work, the author continues his taphonomic analysis of the specimen. As "Big Al" is one of the most complete Allosaurus specimens ever discovered and is stored accessibly in a federal repository, it will continue to provide paleontologists around the world with exciting investigative opportunities. The Museum of the Rockies cast a second "Big Al" mount for their highly acclaimed traveling exhibit "T. rex on Trial." As numerous presentations were given on this specimen, "Big Al" became the focus of media attention around the world. In a case of serendipity, as the displays were being developed at the UW Geological Museum, the British Broadcasting Corporation (BBC) contacted the museum with plans to do a special high lighting "Big Al," as part of their hugely successful "Walking with Dinosaurs" series. BBC's "The Ballad of "Big Al'" aired in Europe and Australia in December 2000 and in the United States in April 2001 (Salamon, 2001). "Big Al" continues to be known worldwide, as the UW mount is seen by people on a variety of websites currently available (e.g., BBC, Discovery Channel, UW Geological Museum). Continued projects, exhibits, research, and education activities will proceed on this unique specimen for years to come (Leiggi and Breithaupt, in press).
STORY OF "BIG AL'S" LIFE AND DEATH
After nearly a decade of "paleodetective" investigative work on this specimen, the story of "Big Al" is finally coming to light. The interpreted scenario of the final days of "Big Al's" life, its death, and subsequent burial are as follows. A young (subadult) Allosaurus of indeterminate gender, approximately 60-70% of its adult size, struggled to survive on a floodplain of what is now northern Wyoming approximately 145 million years ago. Its exact age is difficult to determine, as the physiology and growth rates of dinosaurs are still under study. Individual specimens like "Big Al" may provide important clues on these topics, as bone histology studies are done. Paul Bybee (personal communication) has speculated that "Big Al" was no more than 7 years old. However, this author feels that it was probably older. Interestingly, Allosaurus is the only allosaurid genus represented by immature specimens, as well as adults.
|Figure 11. "Big Al's" pathologic right ribs.|
Wyoming's environment 145 million years ago was experiencing dramatic wet and dry seasons. During the dry season, the struggle for survival was increased as "Big Al" had to obtain food and water. It was a young animal and many of its prey were much larger. Smaller prey were quick and some could put up a considerable fight. Complicating "Big Al's" struggle were the many injuries that this dinosaur suffered. Although the causes of these injuries are unknown,the complications of such pathologies can be correlated with similar abnormalities seen in modern animals. Continued study on these pathologies may lend some important insights into dinosaur behavior. Were these injuries inflicted from some interspecific battle with prey animals like Apatosaurus, Barosaurus, or Stegosaurus? Were they the cause of some intraspecific fight with another Allosaurus over food, pack hierarchy, or mating? Or was this Allosaurus a clumsy, diseased individual who injured itself as it stepped or fell on sharp rocks and logs? "Big Al" assuredly had numerous injuries to its soft tissue (i.e., skin and muscle), as well. Various fractures, breaks, infections, and intergrowths in "Big Al's" foot, tail, hand, ribs (Figure 11), hips, back, and arm dramatically affected its effectiveness as a predator. Thus, hunting would have been difficult. Undoubtedly, it was a weaker, less efficient predatory dinosaur than its healthy contemporaries. Although no one injury may have caused "Big Al's" premature death (although a systemic infection could be fatal), the combination of all of these injuries and those not preserved osteologically may have been the ultimate cause of "Big Al's" demise, as it struggled to find food and water during the dry season. "Big Al's" infected toe bone (which preserves a major open abscess) is unquestionably one of its most painful injuries. This pathology on the major weight bearing toe (Figure 12) would have made walking very painful and may have resulted in "Big Al" limping.
|Figure 12. "Big Al's" pathologic right foot.|
As "Big Al" struggled to find food and water, it collapsed in a dry river bed (Figure 13), leading to its rapid envelopment by sand and mud. The completeness of the specimen indicates that the carcass was not transported very far from where the animal expired, representing essentially an autochthonous deposit. As "Big Al" lay in the dry stream bed, its carcass desiccated in the hot sun. The muscles, tendons, and other soft tissue dried, and the animal's head pulled back over its tail in a characteristic "death pose"(opisthotonic body position) often seen in articulated fossil vertebrates in arid environments (Weigelt, 1989). Because "Big Al's" preserved skeleton still had many of its bones in articulation, this carcass was buried soon after death, prior to the soft tissue completely decaying away. Once buried, decay of the remaining soft tissue continued, but the bones remained locked in life position by the entombing sediments. The beautiful preservation of the bones of the left side of "Big Al" (i.e., limb, hip, and skull) indicates that as this dinosaur lay on its left side, partially buried in sand soon after death. Perhaps this initial burial was by sediment laden flood waters associated with the start of the rainy season. "Big Al's" corpse may have been anchored from additional stream transport by its left limb, which was buried first and embedded into the sediment. Although the left side of the carcass was quickly engulfed with sediment, the right side remained exposed. As the soft tissue decayed away the bones began to disassociate. The right leg was moved by current action anterodorsally to the dorsal vertebrae and buried on the left side of the specimen. The femur, tibia, and fibula remained in association, as the ligaments have not completely decayed. The right ilium was disassociated from the hip region and transported several meters laterally. Whereas the left side of the skull was well embedded in the sediment, the right side of the skull was exposed longer, as indicated by the scattering of maxillary teeth and the disarticulation of the lower jaw. The specimen was lying on its left side with its ventral portion slightly tilted upward, probably associated with bloating of the body cavity. This body position allowed for the ischia and associated pubes to be disarticulated from the hip region and moved anteriorly along the axial skeleton. Many of the ribs and gastralia and other smaller elements (e.g., hand and foot bones) were found disarticulated and scattered primarily ventral to the axial skeleton. The scatter pattern of these elements indicates that they may have been disassociated and transported a short distance due to an eddy current caused by "Big Al's" axial skeleton during later flooding events. Although speculative, it is possible that there was some scavenging on the internal organs of this animal. This activity could scatter some of the chest region without showing any visible effect on the bones themselves. Although there is little evidence of vertebrate scavenging on "Big Al's" bones, close inspection of the bones shows the evidence of beetle burrows on some of the elements, suggesting that "Big Al's" carcass was the "dining spot" of hundreds to thousands of beetle larvae (Laws et al., 1996; Hasiotis et al., 1999). Within a half a year after death, "Big Al" was most likely buried completely by sands and muds. The pattern of "Big Al's" skeletal elements indicates that it was buried by a series of flooding events over the course of many weeks. These flood waters contained high quantities of sediment and a few disassociated sauropod bones that were deposited around "Big Al's" skeleton.
Figure 13. "Big Al" dying in a river channel. Drawing by UW student Thomas Adams.
|Figure 14. Skeletal reconstruction of "Big Al". Drawing by UW student Thomas Adams.|
In 1991, a partially articulated skeleton of a sub-adult Allosaurus was discovered near Shell, Wyoming in the Upper Jurassic Morrison Formation. This area of northern Wyoming has been known to contain important dinosaur remains ever since crews from the American Museum of Natural History in New York collected there in the 1930s. In August of that year, a nearly complete (95%), partially articulated, subadult, Allosaurus fragilis (with at least 19 pathologic bones) was uncovered just north of the Howe Quarry. This specimen, nicknamed "Big Al," is beautifully preserved and one of the most complete skeletons of Allosaurus ever discovered. Found on public lands administered by the BLM, the specimen has been studied by scientists from around the world. Research by these "paleodetectives" has unearthed important clues about this specimen and the area in which it lived, as well as providing valuable information on the life and times of Jurassic carnosaurs. An important find, such as this Allosaurus, indicates that significant paleontological material continues to be uncovered in the Rocky Mountain West even after 150 years of discoveries (Breithaupt, 1999). With a high potential for new discoveries to be made in this region, interagency cooperation (to share expertise and resources) is essential to collect, study, interpret, exhibit, and preserve these specimens for future generations.
The excellent cooperation exercised by the various groups and individuals involved with the "Big Al" project allowed for the efficient extraction of the skeleton from the ground. The safeguarding of natural scientific resources, such as this Allosaurus, indicates a realization that fossils have a fascinating story to tell us about past life and environments and is a role model for future paleontology projects. It is hoped that this type of "paleodetective" partnership project will be emulated by other land management agencies and paleontologists in the future.
Appreciation is extended to the students, volunteers, staff, collectors, and researchers who were all "paleodetectives" in the unearthing and understanding of this dinosaur. Gratitude is extended to various BLM representatives, especially those of the Cody Office, for their help in arranging and participating in this project. Thanks to BLM paleontologist Laurie Bryant for her assistance in the project. Thanks goes out to the BLM for assistance in funding various parts of this study. Various reviewers provided valuable suggestions on this manuscript. Most importantly, a thanks to my colleagues at the Museum of Rockies in Montana, especially Pat Leiggi, Shelley McKamey, Bob Harmon, Allison Gentry, Karen Masta, Ellen Lamm, Carrie Ancell, and Jack Horner for spear-heading this cooperative project to preserve and protect this important scientific and educational resource.
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