Morrison Formation
The Morrison Formation is a distinctive body of rock in the western United States and Canada that has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.
It is centered in Wyoming and Colorado, and includes parts of Montana, Saskatchewan, Alberta, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. It covers an area of 1.5 million square km (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east and much of the rest was destroyed by erosion as the Rocky Mountains rose to the west.
It was named for Morrison, Colorado, where the first fossils were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope.
Geologic History
According to radiometric dating, the Morrison Formation is between 148 and 155 million years old (Ma), which places it in the latest Oxfordian, Kimmeridgian, and earliest Tithonian stages of the late Jurassic. This is similar in age to the Solnhofen Limestone Formation in Germany and the Tendaguru Formation in Tanzania. Throughout the western USA, it variously overlays the Middle Jurassic Summerville, Sundance, Bell Ranch, Wanakah, and Stump Formations.
At the time, the supercontinent of Laurasia had recently split into the continents of North America and Eurasia, although they were still connected by land bridges. North America moved north and was passing through the subtropical regions.
The Morrison Basin, which stretched from New Mexico in the south to Saskatchewan in the north, was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from the basin, carried by streams and rivers from the Elko Highlands (along the borders of present-day Nevada and Utah) and deposited in swampy lowlands, lakes, river channels and floodplains, became the Morrison Formation.
In the north, the Sundance Sea, an extension of the Arctic Ocean, stretched through Canada down to the United States. Coal is found in the Morrison Formation of Montana, which means that the northern part of the formation, along the shores of the sea, was wet and swampy, with more vegetation. Eolian sandstones are found in the southwestern part, which indicates it was much more arid — a desert, with sand dunes.
In the Colorado Plateau region, the Morrison Formation is further broken into four sub-divisions, or members. From the oldest to the most recent, they are:
- Windy Hill Member: The oldest member. At the time, the Morrison basin was characterized by shallow marine and tidal flat deposition along the southern shore of the Sundance Sea.
- Tidwell Member: The Sundance Sea receded to Wyoming during this member and was replaced by lakes and mudflats.
- Salt Wash Member: The first purely terrestrial member. The basin was a semi-arid alluvial plain, with seasonal mudflats.
- Brushy Basin Member: Much finer-grained than the Salt Wash Member, the Brushy Basin Member is dominated by volcanic ash-rich mudstone. Rivers flowed from the west into a basin that contained a giant, saline alkaline lake called Lake T'oo'dichi' and extensive wetlands that were located just west of the modern Uncompaghre plateau.
Deposition in the Morrison Formation ended about 155 Ma. It is followed by a 30 million year gap in the geologic record. The overlying units are the Lower Cretaceous Cedar Mountain, Burro Canyon, Lytle, and Cloverly Formations.
Fossil finds
Though many of the fossils are fragmentary, they are sufficient to provide a good picture of the flora and fauna in the Morrison Basin during the Kimmeridgian. Overall, the climate was dry, similar to a savanna but, since there were no grasses and no flowering plants or trees (angiosperms), the flora was quite different. Conifers were the dominant species of plant life at the time, with relatives of the modern ginkgo, cycads, tree ferns, and horsetail rushes. Much of the vegetation was riparian, living along the river valleys. Insects were very similar to modern species, with termites building 30 m (100 ft.) tall nests. Along the rivers, there were fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crayfish, clams, and monotremes (prototherian mammals, the largest of which was about the size of rat).
The dinosaurs were most likely riparian, as well. Hundreds of dinosaur fossils have been discovered, such as Camptosaurus, Ornitholestes, several stegosaurs comprising at least two species of Stegosaurus and the slightly older Hesperosaurus, and the early ankylosaurs, Mymoorapelta and Gargoyleosaurus, most notably a very broad range of sauropods (the super-giants of the Mesozoic era). Since at least some of species are known to have nested in the area (Camptosaurus embryoes have been discovered), there are indications that it was a good environment for dinosaurs and not just home to migratory, seasonal populations.
Sauropods that have been discovered include the Diplodocus (most famously, the first nearly-complete specimen of D. carnegiei, which is now exhibited at the Carnegie Museum of Natural History, in Pittsburgh, Pennsylvania), Camarasaurus (the most commonly found sauropod), Brachiosaurus, Apatosaurus (also wrongly known as Brontosaurus), Barosaurus, the uncommon Haplocanthosaurus and the Seismosaurus. The very diversity of the sauropods has raised some questions about how they could all co-exist. While the body shape is very similar (long neck, long tail, huge elephant-like body), they appeared to have had very different feeding strategies, in order for all to have existed in the same time frame and similar environment.
Roughly three quarters of all Allosaurus fossils known have also been recovered from the Morrison Formation. The total is more than 60 partial and nearly-complete skeletons, including the first one ever unearthed (the holotype specimen).
Sites and quarries
Locations where significant Morrison Formation fossil discoveries have been made include:
- Bone Cabin, Wyoming
- Canon City, Colorado: One of the three major sites excavated by the paleontologists Othniel Charles Marsh and Edward Drinker Cope during the Bone Wars in 1877, though most of the specimens were too incomplete to classify (nomina dubia). In 1992, a specimen of Stegosaurus stenops was discovered with its armor still in place, which confirmed that the dinosaur had two rows of plates on its back.
- Cleveland-Lloyd Quarry, Utah: First excavated by Lee Stokes in 1937. In the Jurassic, the quarry was a mudhole where several enormous sauropods got stuck and apparently caused a feeding frenzy that lured and trapped many carnivorous dinosaurs. Most of the allosaurs are from this site, as well as the unique Stokeosaurus and Marshosaurus.
- Como Bluff, Wyoming: One of the most renown fossil sites in North America. It was first worked by Cope and particularly Marsh in 1877 and has been the source of many different sauropods and non-dinosaur species. The Cloverly Formation from the Cretaceous and some Triassic strata are also exposed at this location.
- Dinosaur National Monument, Utah
- Dry Mesa Quarry, Colorado: A wide variety of fauna, as well as the most diverse set of dinosaurs from any Morrison Formation quarry. The first dig was in 1972, by the Brigham Young University. Unique specimens include the longest dinosaur known, the Supersaurus, the chimeric Ultrasauros, and the largest carnivore on the continent, the Torvosaurus.
External links and references
- Morrison Natural History Museum home page.
- Dinosaurs and the History of Life, Columbia University lecture on the Morrison Formation.
- Geology of the (Dinosaur National Monument) Quarry, from the National Park Service.
- The Morrison Formation, from the Dinosaur Encyclopedia, including data on the major sites.
- Paleobiology Database: Dalton Well Dinosaur Track Site (Morrison Formation): Late/Upper Jurassic, Utah
- Paleobiology Database: Black Ridge Dinosaur Track Site (Morrison Formation): Late/Upper Jurassic, Colorado