Morrison Formation

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The distinctive banding of the Morrison Formation, a group of rock layers that occur throughout Dinosaur National Monument and the source of fossils like those found at the Dinosaur Quarry.

The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States and Canada, which 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, with outcrops in 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 fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope.

Morrison at the type locality (the location that defines the formation) at Dinosaur Ridge, west of Denver, Colorado.

Close-up of the Morrison at the type locality.

In Colorado, New Mexico, and Utah, the Morrison Formation was a major source of uranium ore. (See Uranium mining in the United States.)

[edit] Geologic History

According to radiometric dating, the Morrison Formation dates from 156.3 ± 2 million years old (Ma) at its base,^[1] to 146.8 ± 1 million years old at the top,^[2] which places it in the latest Oxfordian, Kimmeridgian, and early 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 their east-facing drainage basins, 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, or wind-deposited 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:

DINO 11541, an Allosaurus skull from the Salt Wash Member of the Morrison Formation.

Reddish mudstones of the Tidwell Member underlying the whitish sandstones of the Saltwash Member, south of Cisco, Utah.

Brushy Basin Member on the Colorado Plateau

"Popcorn" texture that characterizes the Brushy Basin Member. The texture is due to bentonite, or volcanic ash.

1. 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.
2. Tidwell Member: The Sundance Sea receded to Wyoming during this member and was replaced by lakes and mudflats.
3. Salt Wash Member: The first purely terrestrial member. The basin was a semi-arid alluvial plain, with seasonal mudflats.
4. Brushy Basin Member: Much finer-grained than the Salt Wash Member, the Brushy Basin Member is dominated by mudstone rich in volcanic ash. 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 147 Ma. The latest Morrison strata are followed by a thirty-million year gap in the geologic record. The overlying units are the Lower Cretaceous Cedar Mountain, Burro Canyon, Lytle, and Cloverly Formations.

[edit] 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 angiosperms (grasses, flowers, and some trees), the flora was quite different. Conifers, the dominant plants of the time, were to be found with ginkgos, cycads, tree ferns, and horsetail rushes. Much of the fossilized 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 a 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 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 their body shapes are very similar (long neck, long tail, huge elephant-like body), they are assumed 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 sixty partial and nearly-complete skeletons, including the first one ever unearthed (the holotype specimen).

[edit] Sites and quarries

Locations where significant Morrison Formation fossil discoveries have been made include:

Workers inside the Dinosaur Quarry building, at the Dinosaur National Monument.

• Bone Cabin, Wyoming
• Garden Park, 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 Stokesosaurus and Marshosaurus.
• Como Bluff, Wyoming: One of the most renowned 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.
• Fruita Paleontological Resource Area: Badlands located south of Fruita, Colorado, was actively worked by George Callison from the University of California, Los Angeles. Numerous specimens of mammals, lizards, and crocodiles were found. Most recently, Fruitafossor windscheffelia was described from the area.
  

  

  Fruita Paleontological Resource Area. One of the sites is denoted by the arrow.

[edit] Vertebrate fauna

(mostly from Foster [2003], with additional material in the mammal section after [1]; the higher-level classifications will vary as new finds are made. Only described taxa are included, so several nomen nudums and undescribed taxa are left out)

[edit] Fish

Both ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii) were present. Ray-finned fish included Hulettia hawesi, Morrolepis schaefferi, and unidentified actinopterygian remains, mostly from amiids (bowfins). The lobe-finned fish present were lungfish (Dipnoi) of the genus Ceratodus, including the common Ceratodus guentheri, C. fossanovum, and C. robustus.

[edit] Amphibians

Amphibians of the Morrison Formation included both frogs Anura and salamanders (Caudata). Frogs included Enneabratrachus hechti and Rhadinosteus parvus. Salamanders were represented by Comonecturoides marshi (dubious), and unnamed material including two skeletons from Dinosaur National Monument.

[edit] Turtles

Turtles (Testudines) are very common fossils in the Morrison, due to their bony shells. The most common were Glyptops plicatus (very common) and Dinochelys whitei (also common, but not as common as Glyptops). Also present were Dorsetochelys buzzops and Uluops uluops.

[edit] Sphenodonts, squamates, and champsosaurs

A variety of small reptiles were present in the Morrison Formation. Sphenodontia (the modern tuataras) were represented by the common Opisthias rarus, and Eilenodon robustus and Theretairus antiquus. Lizards (Squamata) were represented by Dorsetisaurus sp., Paramacellodus sp., Parviraptor gilmorei, Saurillodon sp., and Schilleria utahensis. Champsosaurs (Choristodera), a type of aquatic reptile superficially similar to crocodiles, were represented by Cteniogenys antiquus, a common genus.

[edit] Crocodylia

Crocodiles of a variety of sizes and habitats were common Morrison animals. Cursorial mesosuchians), or small terrestrial running crocs, included Hallopus victor and "Fruitachampsa callisoni". More derived crocodilians included Goniopholis felix (common), G. gilmorei, G. lucasi, G. stovalli, Hoplosuchus kayi, and Macelognathus vagans.

[edit] Pterosaurs

Pterosaurs, or flying reptiles, are very uncommon fossils in the Morrison, due to their fragility. Several types are known, from both rhamphorhynchoids (long-tailed pterosaurs) and Pterodactyloids (short-tailed pterosaurs). Rhamphorhynchoids included Comodactylus ostromi, Harpactognathus gentryii, and Utahdactylus kateae. Pterodactyloids included Dermodactylus montanus, Kepodactylus inseperatus, Laopteryx priscus, and Mesadactylus ornithosphyos.

[edit] Theropod dinosaurs

Theropod dinosaurs, the carnivorous dinosaurs, came in several different types. The less derived types, the ceratosaurs and megalosaurids, included Ceratosaurus nasicornis, C. dentisulcatus, C. magnicornis, Elaphrosaurus sp., and the megalosaur Torvosaurus tanneri (?including Edmarka rex). Allosaurids included the common Allosaurus fragilis (including Epanterias amplexus), A. new species, Antrodemus valens, and giant Saurophaganax maximus. Coelurosaurs, more derived types more closely related to birds, included Coelurus fragilis, Ornitholestes hermanni, Tanycolagreus topwilsoni, the possible troodontid Koparion douglassi, and the definite troodontid WDC DML 001. There was also the possible early tyrannosaur relative Stokesosaurus clevelandi. Marshosaurus bicentesimus was a medium-sized theropod of uncertain type that may have been related to the allosaurids.

[edit] Sauropod dinosaurs

Sauropods, the giant long-necked long-tailed four-legged herbivorous dinosaurs, are among the most common and famous Morrison fossils. A few have uncertain relationships, like "Apatosaurus" minimus (possibly an early titanosaur) and Haplocanthosaurus. Sauropods including Haplocanthosaurus priscus, H. delfsi, and the diplodocids Dystrophaeus and Eobrontosaurus appeared in the early stages of the Morrison. The middle stages were dominated by familiar forms such as the Giraffe-like Brachiosaurus altithorax, which were uncommon, but related camarasaurids, like Camarasaurus supremus, C. grandis, C. lentus, and C. lewisi, were very common. Also common were long, low diplodocids, like Apatosaurus ajax, A. excelsus (formerly "Brontosaurus"), A. louisae, A. parvus, Atlantosaurus montanus, Barosaurus lentus, Diplodocus longus, D. carnegii, "D." hayi, "D." lacustris, Dyslocosaurus polyonychius. By the late Morrison, gigantic diplodocids (or likely diplodocids) had appeared, including Diplodocus hallorum (formerly Seismosaurus), Supersaurus vivianae, Amphicoelias altus, and the largest of all, A. fragilimus. Smaller sauropods, such as Suuwassea emiliae from Montana, tend to be found in the northern reaches of the Morrison, near the shores of the ancient Sundance Sea, suggesting ecological niches favoring smaller body size there compared with the giants found further south.^[3]

[edit] Ornithischian ("bird-hipped") dinosaurs

The herbivorous ornithischian dinosaurs were diverse but not as common as sauropods in the Morrison. Unclassified members include the "Fruita Echinodon", a possible heterodontosaurid, and the dubious Tichosteus lucasanus and T. aequifacies. Plate-backed stegosaurids included Hesperosaurus mjosi, Hypsirophus discursus, Stegosaurus armatus (?including S. ungulatus), S. stenops, and "S." longispinus. Armored dinosaurs that weren't stegosaurs were unknown in the formation until the 1990s. Two have been named: Gargoyleosaurus parkpinorum and Mymoorapelta maysi. Ornithopods, bipedal herbivores, came in several types. Small "hypsilophodonts" included Drinker nisti, Laosaurus celer, "L." gracilis, Nanosaurus agilis, Othnielia rex, and Othnielosaurus consors. Larger but similar-looking dryosaurids were represented by Dryosaurus altus and the camptosaurid Camptosaurus aphanoecetes, which is currently known only from Dinosaur National Monument. Still larger was the more common Camptosaurus dispar, probably including Brachyrophus altarkansanus and Symphyrophus musculosus. Dryosaurids and camptosaurids were early iguanodonts, a group that would later spawn the duck-billed dinosaurs.

[edit] Mammals

Many types of early mammals are known from the Morrison; almost all of them were small, rodent-like animals. Unclassified types include the digger Fruitafossor windscheffelia. Docodonts included the common genus Docodon, represented by D. victor, D. striatus, and D. superbus, and Peraiocynodon sp. Multituberculates, a common type of early mammal, were represented by Ctenacodon serratus, C. laticeps, C. scindens, "C." brentbaatar, Glirodon grandis, Psalodon fortis, ?P. marshi, P. potens, and Zofiabaatar pulcher. Triconodonts present included Aploconodon comoensis, Conodon gidleyi (AKA Phascolodon and Phascolotheridium), Priacodon ferox, P. fruitaensis, P. gradaevus, P. lulli, P. robustus, Triconolestes curvicuspis, and Trioracodon bisulcus.

Symmetrodonts were represented by Amphidon superstes, Eurylambda aequicrurius (probably Tinodon), and Tinodon bellus (including T. lepidus). Finally, two families of Dryolestoidea were present: Paurodontidae, including Araeodon intermissus, Archaeotrigon brevimaxillus, A. distgamus, Comotherium richi, Euthlastus cordiformis, Foxraptor atrox, Paurodon valens, Pelicopsis dubius, and Tathiodon agilis; and Dryolestidae, including Amblotherium gracilis, Dryolestes obtusus (common genus), D. priscus, D. vorax, Laolestes eminens, L. grandis, and Miccylotyrans minimus.

[edit] See also

• List of fossil sites (with link directory)

[edit] References

[edit] 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
• Catalogue of Morrison mammals (bottom of the page)

[edit] Other References

• Foster, J.R. 2003. Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. Bulletin 23.