Many experienced fossil collectors begin by looking at a geologic map, which shows the name and the age of the rock units exposed on the earth’s surface. If the rocks are metamorphic or igneous, the paleontologist ignores them. They focus instead on sedimentary rocks, especially those in mountainous areas where they may have been uplifted and eroded.
The next thing you must determine is whether the exposed sedimentary rocks are the right age to contain fossils. Again, a paleontologist begins by looking at a geologic map that not only shows the location of sedimentary rocks but also the age of the rock formations (fig. 4.1). If you are looking for dinosaurs, for example, you would only look in rocks from the Mesozoic Era, or the “Age of Dinosaurs” (250–66 million years ago). Most dinosaurs come from rocks of the Jurassic Period (140–200 million years ago) or the Cretaceous Period (66–140 million years ago). This greatly limits the number of places you can look because only a small percentage of the exposed rock on the earth’s surface are Jurassic or Cretaceous in age.
Figure 4.1 ▲
A geologic map of an area shows what kind of bedrock (shown by the bands of different shades) is found beneath the plants and loose soils and sediments. Certain key fossils (as shown in the photo) are typical of most fossiliferous sedimentary formations, and the occurrence of these index fossils allows a paleontologist to identify the formation, correlate it with other formations, and determine its age. (Photograph by the author)
In addition, the geologic map may also indicate what sedimentary environment formed the rocks. Dinosaurs are often found prehistoric river floodplains or possibly in ancient lakes, where they lived and also died and were buried (see fig. 3.1). You would not expect to find good dinosaur fossils in rocks that represented the open ocean because all dinosaurs were land creatures. (Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs lived in the seas of the Mesozoic, but they are not dinosaurs, even if the public thinks so.) This quickly limits your options even further because most of the good exposures of Jurassic and Cretaceous rocks are marine rocks, which rarely produce dinosaur fossils. Only a handful of places on Earth have exposures of nonmarine Jurassic and Cretaceous rocks that produce dinosaur fossils, and they are in desert badlands.
A good example of how paleontologists select a site was described by my friend, classmate, and coauthor Neil Shubin in his best-selling book, Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. He and his colleagues were looking for a fossil that was intermediate in age between the earliest amphibians that had been collected and the most advanced youngest amphibian fossils known. They looked on geologic maps for rocks of a certain time span (385–365 million years ago) in the Late Devonian Period, which already had good fossils of very primitive amphibian-like fish from 385 million years ago as well as fossils of more advanced fish from 365 million years ago. After scouring the geologic maps of the world, they found only three areas of the right age, and the right sedimentary environment (shallow marine sandstones and shales, with some from rivers or deltas). Two of them (in Pennsylvania, and in Spitsbergen and Greenland in the Arctic) had already been explored and collected. But the third place, in the Canadian Arctic, had never been studied. They raised money for a quick visit to the area, and they found bone scraps. To mount a full-scale Arctic expedition, they had to raise millions of dollars more, which they did for several summers in a row. Only after the third year of very hard, very expensive, very dangerous work dealing with harsh weather and marauding polar bears did they find the fossils of Tiktaalik, the famous transitional fossil between fish and amphibians (fig. 4.2). After this bonanza, they went back several more times, finding many more specimens of Tiktaalik along with a number of other fish and animals that lived in this ancient river delta 375 million years ago.
Figure 4.2 ▲
The transitional fossil between fish and amphibians is known as Tiktaalik. Neil Shubin and the crew found it in the Canadian Arctic after consulting geologic maps to see where rocks of the right age for this transition might be preserved. (Courtesy of Wikimedia Commons)
If all this seems like very hard work, it is. To discover rare fossils like dinosaurs or Tiktaalik, you can spend months in the field looking and spend a lot of money and you may still find nothing at all. Most paleontologists use a much more reliable method—they go to places where the fossils they are looking for have already been found. The best way to make sure you find something is not to gamble on unknown areas on a geologic map but to go to known fossil beds (many websites list these fossil beds) and see what you can find.
In the western United States, many localities are known to produce lots of fossils. For example, the Big Badlands of South Dakota have been producing fossil mammals and turtles every year since 1848. And the Miocene rocks of western Nebraska are legendary for their richness and long sequence of Oligocene and Miocene fossil mammals. For example, dense bone beds mostly filled with the bones of the small rhinoceros Menoceras (fig. 4.3A) can be found at Agate Springs Fossil Beds National Monument in Nebraska.
Figure 4.3 ▲
(A) Portion of the bone bed from Agate Springs National Monument, full of the bones of the little rhinoceros Menoceras, which are shown in articulated skeletons behind the bone bed slab. (B) The famous “Rhino Pompeii” at Ashfall Fossil Bed State Park in northeastern Nebraska, with complete skeletons of the rhinoceros Teleoceras found in their death poses as they suffocated in volcanic ash. ([A] Photograph by the author; [B] photograph courtesy of S. Tucker)
Discoveries also occur when the paleontologist is hard working and well prepared. Mike Voorhies of the University of Nebraska State Museum was prospecting for fossils among the riverbank outcrops in northeastern Nebraska one day in 1977 when he found a bit of rhino skull poking out of the rock. He dug in further and found the skull of a baby rhinoceros. As he kept digging, he found its complete skeleton, then he found the skeleton of its mother next to it. Within a few months, he and his crew had found a complete bone bed of hippo-like rhinos know as Teleoceras that had died in volcanic ash, which suffocated them and buried them in life poses (fig. 4.3B). They were so well preserved that even the tiniest bones in the throat were still there, as well as the fossilized seeds of their last meals. Some of the female rhinos had fetuses still in them, and others had baby rhinos in nursing position when they died. In addition to rhinos, this locality produces a few horses, extinct musk deer, and even cranes. The locality was nicknamed “Rhino Pompeii,” but today it is called Ashfall Fossil Bed State Monument. You can visit the bone bed as it is still being excavated, protected from the weather and vandals by a huge building known as the “Rhino Barn.”
Most of the rich dinosaur-bearing beds of the world are well known and have been collected off and on for years. For example, in 1909 paleontologists Earl Douglass and William J. Holland of the Carnegie Museum of Natural History in Pittsburgh were prospecting outcrops of the Morrison Formation, a well-known Upper Jurassic dinosaur-bearing unit in the Uinta Basin of northeastern Utah. Holland described their work this way:
We decided that we would set forth early the next day with our teams of mules and visit the foot-hills, where Hayden had indicated the presence of Jurassic exposures. We started shortly after dawn and spent a long day on the cactus-covered ridge of Dean Man’s Bench, in making our way through the gullies and ravines to the north…. The next day we went forward through the broken foot-hills which lie east and south of the great gorge through which the Green River emerges from the Uinta Mountains on its course to the Grand Canyon of Arizona. As we slowly made our way through the stunted groves of pine we realized that we were upon Jurassic beds. We tethered our mules in the forest. Douglass went to the right and I to the left, scrambling up and down through the gullies in search of Jurassic fossils, with the understanding that, if he found anything he was to discharge the shotgun which he carried, and if I found anything, I would fire the rifle, which I carried. His shotgun was presently heard and after a somewhat toilsome walk in the direction of the sound I heard him shout. I came up to him standing beside the weathered-out femur of a Diplodocus lying in the bottom of a very narrow ravine in which it was difficult to descend. Whence this perfectly preserved bone had fallen, from what stratum of the many above us it had been washed, we failed to ascertain. But there it was, as clean and perfect as if it had been worked out from the matrix in the laboratory. It was too heavy for us to shoulder and carry away, and possibly even too heavy for the light-wheeled vehicle in which we were traveling. So we left it there, proof positive that in that general region search for dinosaurian remains would probably be successful.
Holland’s prediction came true a year later, on August 17, 1909, when Douglass was working in the same area with a local Mormon farmer, George “Dad” Goodrich. Douglass climbed the ridge above where the femur had been found the year before and looked down. “At last, in the top of the ledge where the softer overlying beds form a divide—a kind of saddle—I saw eight of the tail bones of Brontosaurus in exact position. It was a beautiful sight.” He and Goodrich went back to town and recruited more helpers, then they began to quarry out the bones. “I have discovered a huge dinosaur Brontosaurus and if the skeleton is as perfect as the portions we have exposed, the task of excavating will be enormous and will cost a lot of money, but the rock is that kind to get perfect bones from.” This message brought Holland to Utah, and when he saw what Douglass had, he immediately telegraphed Andrew Carnegie, the rich man who had founded their museum, to get him to agree to fund the excavation.
The nearest town was tiny Vernal, Utah, over 20 miles (32 km) away, so for the next 13 years (1909–1922), Douglass took up permanent residence in the area near Carnegie Quarry. There he and his crews lived and worked year round except when the weather was unbearable. He even brought his young wife and one-year-old baby out to live with him, first in a small tent heated by an iron stove, but eventually they lived in a homesteader’s log cabin with a garden and cow and chickens and everything the family needed. First they exposed the “Brontosaurus” he originally discovered, only to find the neck twisted back into the rock—and the skull missing. Nevertheless, it was nearly complete and about 98 feet (30 meters) long, with a tail over 30 feet (9 meters) in length. When it was shipped to Pittsburgh, cleaned, and mounted, Holland described it and named it Apatosaurus louisae in honor of Carnegie’s wife Louise.
Once the first skeleton had been removed, Douglass and his men found three other sauropod skeletons nearby. They realized that the bone bed was a thick ledge of sandstone that was tilted almost vertically. They blasted away the overburden of soft Morrison shales and trenched down to expose the top surface of the tilted sandstone layer. Eventually the trench was 600 feet (180 meters) long and 80 feet (24 meters) deep. Between 1909 and 1922, Douglass and his men removed the top half of the sandstone wall, which was more than 300 feet (90 meters) long and 75 feet (23 meters) high. They also excavated the east and west side of the huge wall of sandstone.
They recovered 700,000 pounds (315 tonnes) of fossils and took them by buckboard wagon to the nearest rail stop in Dragon, Utah, over 50 miles (80 kilometers) away. Altogether they found more than 20 skeletons and fossils representing about 300 additional individual dinosaurs. Apparently, the sandstone layer had once been a sand bar in a Jurassic river, and portions of carcasses had floated down to that spot and then become buried. In 1922, William Holland retired, and Andrew Carnegie died, so the funding dried up. The Carnegie Museum was crammed to the limit, with more than 300 tons (270 tonnes) of bones that had not yet been prepared or cleaned, so the museum decided to end the excavation and close the quarry. Douglass then began working for the University of Utah and collected another 33 crates of specimens, including a complete Allosaurus. However, they never offered him a position, and he died in poverty in 1931 without seeing his dream come true.
Douglass’s dream was to see the Carnegie Quarry made into a national monument. They had removed only half of the original wall of sandstone, and he knew that the locality had amazing potential. Douglass wanted the rest to be left in place as a permanent monument for people to see dinosaur bones as they are found in the field. As he wrote, “I hope that the Government, for the benefit of science and the people, will uncover a large area, leave the bones and skeletons in relief and house them in. It would make one of the most astounding and instructive sights imaginable.” Douglass tried to buy the mineral rights to protect the site, but the courts rules that dinosaur bones were not minerals. But Holland had a powerful friend, Charles Doolittle Walcott, a paleontologist who was also head of the Smithsonian. Walcott convinced President Woodrow Wilson to designate the quarry area as Dinosaur National Monument in 1915.
The monument was isolated in the middle of the wilderness of Utah and remained primitive and undeveloped for years because there was almost no way to reach it in the days before cars were common and roads paved. During the Depression, crews of unemployed men came from the WPA to deepen and enlarge the quarry. Nothing much was built on the site during the Second World War, but in the 1950s the Park Service surveyed the area and determined it was worth developing. A modern glass-sided building was finished in 1958, with its north wall made of the sandstone layer full of dinosaur bones.
Over the years, the building has become one of the most popular national monuments in the country (fig. 4.4). About 400,000 visitors a year come to northeastern Utah to gawk at the wall of dinosaur bones that are slowly being excavated in relief and left in place (unless they need to be removed to expose more bones). In 2009, the quarry building was closed because it was beginning to sink into the soft Morrison shales below its foundation, and that was tearing it apart. A much larger Visitor’s Center was built down on the flats below the quarry (near where Douglass had his log cabin), and a shuttle service now brings up the flood of visitors coming to the tiny parking lot below. The quarry building was rebuilt with 70-foot steel pilings driven deep into the harder bedrock, and it reopened in 2011. It only houses the paleontological exhibits. The main Visitor’s Center down on the flats has exhibits about the rest of the monument, plus a gift shop, offices, and storage for the specimens removed from the quarry.
Figure 4.4 ▲
Dinosaur National Monument Quarry Visitors Center, showing the wall of dinosaur bones left where they were found but excavated in relief. (Photograph by the author)
The rocks are constantly being eroded and exposed each year when the rains and snows come and go, so some new fossils might be exposed the following spring and summer. The paleontologists who find fossil dinosaurs usually work in a particular area year after year, and sooner or later they begin to find something worth the huge amount of time and expense that has been invested in this difficult work.
