Morrison Formation

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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.

The distictive 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.
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The distictive 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.

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.

Contents

[edit] 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 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.
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DINO 11541, an Allosaurus skull from the Salt Wash Member of the Morrison Formation.
  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 155 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 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 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.
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Workers inside the Dinosaur Quarry building, at the Dinosaur National Monument.
  • Bone Cabin, Wyoming
  • Cañon 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 Stokesosaurus 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.

[edit] Vertebrate Faunal List

(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)

Actinopterygii (ray-finned fish)

  • Hulettia hawesi
  • Morrolepis schaefferi

(unidentified actinopterygian remains are very common, mostly from amiids [bowfins])

Sarcopterygii (lobe-finned fish)

Dipnoi (lungfish)
  • Ceratodus guentheri (common)
  • C. fossanovum
  • C. robustus

Anura (frogs)

  • Enneabratrachus hechti
  • Rhadinosteus parvus

Caudata (salamanders)

  • Comonecturoides marshi (dubious, but other anuran material is known, including two skeletons from Dinosaur National Monument)

Chelonia (turtles)

  • Glyptops plicatus (very common)
  • Dinochelys whitei (also common, but not as common as Glyptops)
  • Dorsetochelys buzzops
  • Uluops uluops

Sphenodontia (tuataras)

  • Opisthias rarus (common)
  • Eilenodon robustus
  • Theretairus antiquus

Squamata (lizards)

  • Dorsetisaurus sp.
  • Paramacellodus sp.
  • Parviraptor gilmorei
  • Saurillodon sp.
  • Schilleria utahensis

Choristodera (champsosaurs)

  • Cteniogenys antiquus (common)

Crocodylia

(cursorial mesosuchians)
(more derived crocodilians)

Pterosauria ("flying reptiles")

"Rhamphorhynchoids" ("long-tailed pterosaurs")
"Pterodactyloids" ("short-tailed pterosaurs")

Dinosauria

Theropoda
Ceratosauria
Megalosauridae
uncertain Tetanurae
Marshosaurus bicentesimus
Allosauridae
Coelurosauria
Tyrannosauroidea
Troodontidae
Sauropoda
uncertain family
Brachiosauridae
Camarasauridae
Diplodocoidea
Diplodocidae
Ornithischia
Stegosauridae
Ankylosauria
Ornithopoda
"Hypsilophodontidae"
Iguanodontia
Dryosauridae
Camptosauridae

Mammalia

Docodonta
  • Docodon victor (common genus)
  • D. striatus
  • D. superbus
  • Peraiocynodon sp.
Multituberculata
Triconodonta
  • Aploconodon comoensis
  • Conodon gidleyi (AKA Phascolodon and Phascolotheridium)
  • Priacodon ferox
  • P. fruitaensis
  • P. gradaevus
  • P. lulli
  • P. robustus
  • Triconolestes curvicuspis
  • Trioracodon bisulcus
Symmetrodonta
  • Amphidon superstes
  • Eurylambda aequicrurius (probably Tinodon)
  • Tinodon bellus (including T. lepidus)
Dryolestoidea
Paurodontidae
  • Araeodon intermissus
  • Archaeotrigon brevimaxillus
  • A. distgamus
  • Comotherium richi
  • Euthlastus cordiformis
  • Foxraptor atrox
  • Paurodon valens
  • Pelicopsis dubius
  • Tathiodon agilis
Dryolestidae
  • Amblotherium gracilis
  • Dryolestes obtusus (common genus)
  • D. priscus
  • D. vorax
  • Laolestes eminens
  • L. grandis
  • Miccylotyrans minimus

[edit] External links and references

[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.
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