Morrison formation

The characteristic layers of the Morrison Formation

The Morrison Formation is a series of marginal marine and continental sedimentary rocks of the late Jurassic in the western United States and Canada and the most abundant source of dinosaur fossils in North America. It consists of claystones , sandstones , siltstones and limestones and has a light gray, greenish gray or reddish color. Most of the fossils come from the green siltstone deposits and from the lower sandstones - formations of the rivers and flood plains of the Jurassic period.

The base of this formation is exposed in Wyoming and Colorado , further outcrops can be found in Montana , Saskatchewan , Alberta , North Dakota , South Dakota , Nebraska , Kansas , Oklahoma , Texas , New Mexico , Arizona , Utah and Idaho . The rocks of the Morrison Formation extend over an area of ​​1.5 million square kilometers. However, only a fraction strikes the surface and can be examined by geologists and paleontologists . Over 75% - including the Great Plains - is covered by younger sediments, and much of the Morrison rocks were destroyed by erosion when the Rocky Mountains were lifted.

The Morrison Formation was named after the town of Morrison , Colorado, where the first fossils were discovered by Arthur Lakes in 1877 . That same year, the formation became the center of the famous Bone Wars, fought by rivals Othniel Charles Marsh and Edward Drinker Cope , which resulted in 142 re-descriptions of dinosaur species . The Morrison Formation was also a significant source of uranium ore in Colorado, New Mexico and Utah .

Geological history

According to the results of radiometric dating , the Morrison Formation is 156.3 ± 2 million years old at its base and 146.8 ± 1 million years old at its upper end, which is why it belongs to the late Oxfordian , Kimmeridgian and early Tithonian ( geological stages ) of the late Jurassic. In her age she resembles the Solnhofen limestone limestone in Germany and the Tendaguru layers in Tanzania . In the western United States it lies partly over the Central Jurassic layers of the Summerville, Sundance, Bell Ranch, Wanakah, and Stump Formations.

At the time of the deposits, the northern landmass of Laurasia had just broken apart into what is now the continents of North America and Eurasia, although there were still land bridges connecting them. North America moved northward and passed the subtropical regions.

In the north, the Sundance Sea, an extension of the Arctic Ocean, stretched through what is now Canada to the United States. Coal was discovered in Montana's Morrison Formation , indicating that the northern portion of the formation along the coastal strip was wet and marshy. In the southwest of the formation, sandstones were found that were formed by Aeolian transport (e.g. sandstorms), which indicates a much drier, desert-like environment with sand dunes.

In the Colorado Plateau, the Morrison Formation is divided into the following layers (subformations):

1. Windy Hill Member: The oldest shift member. At the time, there were coastal deposits on the southern coast of the Sundance Sea.
2. Tidwell Member: During the deposition of this layer, the Sundance Sea retreated into Wyoming and was replaced by a landscape of lakes and mud flats.

1. Salt Wash Member: The first purely terrestrial layer. The region was a semi-arid floodplain with seasonal mud flats.
2. Brushy Basin Member: This layer is much finer-grained than the "Salt Wash Member" and is dominated by claystone, which contains a lot of volcanic ash. Rivers poured from the west into a basin with a large salt lake known as Lake T'oo'dichi 'and extensive wetlands.

The Morrison Formation deposition ended about 147 million years ago. Above the rock unit there is a discordance - no sedimentary rocks have been documented for a period of 30 million years after the Morrison Formation was formed. The overlying layers are from the Early Cretaceous and include the Cedar Mountain, Burro Canyon, Lytle, and Cloverly Formations.

Fossil finds

Although many of the fossils are only fragmentary, they provide a good picture of the flora and fauna of the region during the Kimmeridgian. Overall, the climate was dry, the region resembled a savannah , although the vegetation was different: There were no grasses or other flowering plants , but conifers , ginkgos , tree ferns and horsetail . Many of the fossilized plants lived in river valleys. Fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crabs, mussels and mammals lived along the rivers, the largest of which were as big as rats.

The dinosaurs probably lived along the rivers as well. Hundreds of dinosaur fossils have been discovered, including Camptosaurus , Ornitholestes , various stegosaurs (at least Stegosaurus and the slightly older Hesperosaurus ), early ankylosaurs such as Mymoora Delta and Gargoyleosaurus , and a number of giant sauropods. Since at least some species are known to have nested in this area (e.g. Camptosaurus embryos have been discovered), it is believed that it was a good habitat for dinosaurs that did not only live there seasonally.

Sauropods from the Morrison Formation include the famous Diplodocus , known for an almost completely preserved skeleton , the most commonly found Camarasaurus , Apatosaurus (formerly known as Brontosaurus), Brachiosaurus and rarer genera such as Barosaurus , Haplocanthosaurus and Suuwassea . This diversity of sauropods begs the question of how these giant herbivores could all have existed in this habitat. While they hardly differ in their physique (long neck, long tail, and elephant-like body), they likely had different feeding habits.

The largest carnivore in the Morrison Formation is the popular Allosaurus ; three quarters of all Allosaurus finds come from this formation: more than 60 partial and almost complete skeletons, including the holotype , of the first fossil discovered.

Famous localities

Important fossils come from the following sites, among others:

At the Dinosaur National Monument during a guided tour

• Bone Cabin, Wyoming
• Garden Park, Colorado: One of the three major sites discovered by paleontologists Othniel Charles Marsh and Edward Drinker Cope during the Bone Wars in 1877, though most finds are too incomplete to classify ( nomina dubia ). In 1992 a skeleton of Stegosaurus stenops was discovered here, showing its bone plates in their original position - this confirms that the bone plates were arranged in two rows over the animal's back.
• Cleveland-Lloyd Quarry, Utah: Discovered by Lee Stokes in 1937, this site was once a mud hole where several sauropods got stuck. The sauropods attracted carnivores, but they also sank and died. Most of the Allosaurus finds come from this site, but also the unique Stokesosaurus and Marshosaurus finds .
• Como Bluff , Wyoming: One of the most famous localities in North America. It was first studied by Cope, and then more specifically by Marsh in 1877. Como Bluff has come from many different sauropods and non-dinosaur species. In addition to the Morrison Formation, the Cloverly Formation of the early Cretaceous and some Triassic strata are exposed in this locality.

• Dinosaur National Monument , Utah
• Dry Mesa Quarry, Colorado: This site is known for very diverse finds. The first dig was carried out by Brigham Young University in 1972 . Unique finds include Supersaurus , one of the longest known dinosaurs; other finds are the chimera Ultrasauros and the large carnivore Torvosaurus .
• Fruita Paleontological Resource Area: This area is badlands south of Fruita , Colorado, explored by George Callison of the University of California. Numerous finds of mammals, lizards and crocodiles come from this deposit . Fruitafossor has recently been described from this area.

Vertebrate fauna

The following is a list of the vertebrates found, whereby only described taxa are listed. Most of the information comes from and.

fishes

Both ray fins and meat fins have been found. The ray fins found include Hulettia hawesi , Morrolepis schaefferi and other unidentified finds, while lungfish (Dipnoi) of the genus Ceratodus , including Ceratodus guentheri , C. fossanovum and C. robustus , are found among the meat fins .

Amphibians

Amphibians of the Morrison Formation include frogs as well as salamanders . The frogs include the finds Enneabratrachus hechti and Rhadinosteus parvus , while salamanders with Comonecturoides marshi and two other, as yet unnamed skeletons from the Dinosaur National Monument are represented.

Turtles

Turtles are very common fossils in the Morrison Formation as their shields are the most likely to be preserved. The most common is Glyptops plicatus , not quite as often is Dinochelys whitei . Other turtles are Dorsetochelys buzzops and Uluops uluops .

Sphenodonts, scale reptiles and choristodera

The Morrison Formation is home to various small reptiles: Sphenodonts are represented by Opisthias rarus , Eilenodon robustus and Theretairus antiquus , Scaly creepers (Squamata) by Dorsetisaurus sp., Paramacellodus sp., Parviraptor gilmorei , Saurillodon sp., And Schilleria utahensis . The champsosaurids (aquatic reptiles with superficial similarities to the crocodiles) belonging to the Choristodera are represented by Cteniogenys antiquus .

Crocodiles

The crocodiles were common with a wide range of different sizes and habitats. Among the Mesosuchiern include Hall Opus victor and Fruitachampsa calli soni , more modern crocodiles close Goniopholis felix (common), G. gilmorei , G. lucasi , G. stovalli , Hoplosuchus Kayi , and Macelognathus vagans with one.

Pterosaurs

The pterosaurs (or pterosaurs ) are very rare in the Morrison formation due to their fragile bones. Several species are known, both long-tailed species ( Rhamphorhynchoideen ) and short-tailed species ( Pterodactyloids ). The rhamphorhynchoids include Comodactylus ostromi , Harpactognathus gentryii , and Utahdactylus kateae , while the pterodactyloids were represented by Dermodactylus montanus , Kepodactylus inseperatus , Laopteryx priscus, and Mesadactylus ornithosphyos .

Theropod dinosaur

Theropods , the carnivorous dinosaurs, come in several types. The more primitive forms, the ceratosaurs and the megalosaurids , include Ceratosaurus nasicornis , C. dentisulcatus , C. magnicornis , Elaphrosaurus sp., Torvosaurus tanneri (possibly including Edmarka rex ). The allosaurids were Allosaurus fragilis (including Epanterias amplexus ), A. new species, Antrodemus valens , and the gigantic Saurophaganax maximus . Coelurosaurs , more advanced theropods, were Coelurus fragilis , Ornitholestes hermanni , Tanycolagreus topwilsoni , the possible troodontid Koparion douglassi , and the definitive troodontid WDC DML 001. Furthermore, Stokesosaurus clevelandi was found, which was perhaps an early tyrannosauroid . Marshosaurus bicentesimus was a medium-sized theropod of unknown type, perhaps related to the allosaurids.

Sauropod dinosaur

The sauropods , gigantic long-necked herbivores , are among the most common and popular fossils in the formation. Some have uncertain relationships, such as "Apatosaurus" minimus , Haplocanthosaurus priscus and H. delfsi . The brachiosaurids such as Brachiosaurus altithorax were rare, but related genera such as Camarasaurus supremus , C. grandis , C. lentus, and C. lewisi were very common. The long diplodocids Apatosaurus ajax , A. excelsus (" Brontosaurus "), A. louisae , A. parvus , Atlantosaurus montanus , Barosaurus lentus , Diplodocus longus , D. carnegii , D. hallorum (formerly Seismosaurus ), "D. " hayi , "D." lacustris , Dyslocosaurus polyonychius , Eobrontosaurus yahnahpin , and the gigantic Supersaurus vivianae (including Dystylosaurus edwini and Ultrasauros macintoshi ). Other Diplodocoids related to the Diplodocids were rarer, such as Amphicoelias altus , the gigantic A. fragilimus and Suuwassea emiliae .

Ornithischia

The herbivorous ornithischia (bird's pelvis dinosaur) are very diverse in the Morrison Formation, but were not as common as the sauropods. Non-classifiable members of this group are Echinodon , a possible heterodontosaurid , and the dubious Tichosteus lucasanus and T. aequifacies . The stegosaurids equipped with back plates were with Hesperosaurus mjosi , Hypsirophus discursus , Stegosaurus armatus (possibly including S. ungulatus ), S. stenops and "S." longispinus represented. Ankylosaurs (armored dinosaurs) have only been discovered since the 1990s, so today Gargoyleosaurus parkpinorum and Mymoorapelta maysi are known. Ornithopods were represented in several forms. The small hypsilophodontids include Drinker nisti , Laosaurus celer , "L." gracilis , Nanosaurus agilis , Othnielia rex and Othnielosaurus consors . Larger but similar-looking species were the dryosaurids , which were represented by Dryosaurus altus . Even larger was the common Camptosaurus dispar , which may include Brachyrophus altarkansanus and Symphyrophus musculosus . Dryosaurids and Camptosaurids were early iguanodonts , a group that would later produce the hadrosaurids (duck-billed dinosaurs).

Mammals

Many species of early mammals (Mammalia) are known from the Morrison Formation, almost all of which were small, rodent-like animals. One species that cannot be classified is Fruitafossor windscheffeli . Next are Docodonten as Docodon ( D. victor , D. striatus and D. superbus ) and Peraiocynodon sp. known, as well as Multituberculata such as Ctenacodon serratus ( C. laticeps , C. scindens , "C." brentbaatar ), Glirodon grandis , Psalodon fortis ( ? P. marshi , P. potens ) and Zofiabaatar pulcher . Triconodonts include 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 Amphidon superstes , Eurylambda aequicrurius (possibly Tinodon ), and Tinodon bellus (including T. lepidus ). Finally, two families of the Dryolestoidea were present: the Paurodontidae , including Araeodon intermissus , Archaeotrigon brevimaxillus , A. distgamus , Comotherium richi , Euthlastus cordiformis , Foxraptor atrox , Paurodon valens , Pelicopsis dubius and Tathiodon agilis ; and the Dryolestidae , including Amblotherium gracilis , the commonly found Dryolestes obtusus , D. priscus , D. vorax , Laolestes eminens , L. grandis, and Miccylotyrans minimus .

Web links and references

• Homepage of the Morrison Natural History Museum
• Dinosaurs and the History of Life , Columbia University article on the Morrison Formation.
• Geology of the (Dinosaur National Monument) Quarry , from the National Park Service.
• The Morrison Formation ( April 10, 2005 memento on the Internet Archive ) , from the Dinosaur Encyclopedia, including major site dates.
• Paleobiology Database: Dalton Well Morrison Formation Dinosaur Footprint Site: Late Jurassic, Utah
• Paleobiology Database: Black Ridge Dinosaur Footprint Location (Morrison Formation): Late Jurassic, Colorado
• Collection of Mammals of the Morrison Formation

Individual evidence

1. ↑ KC Trujillo, Chamberlain, KR and Strickland, A .: Oxfordian U / Pb ages from SHRIMP analysis for the Upper Jurassic Morrison Formation of southeastern Wyoming with implications for biostratigraphic correlations . In: Geological Society of America Abstracts with Programs . 38, No. 6, 2006, p. 7.
2. ↑ SA Bilbey: Cleveland-Lloyd Dinosaur Quarry - age, stratigraphy and depositional environments . In: Carpenter, K .; Chure, D .; and Kirkland, JI (Eds.): The Morrison Formation: An Interdisciplinary Study  (= Modern Geology 22). Taylor and Francis Group, 1998, pp. 87-120, ISSN  0026-7775 .
3. ^ Foster, JR 2003. Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, USA Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. Bulletin 23.
4. ^ Mesozoic mammals, in English