Two Medicine Formation

Last updated
Two Medicine Formation
Stratigraphic range: Campanian
~82.4–74.4  Ma
Two Medicine.jpg
Outcrop of the Two Medicine Formation near "Egg Mountain" in northern Montana
Type Geological formation
Sub-unitsRock City Member, Shields Crossing Member, Hagans Crossing Member, Flag Butte Member
Underlies Bearpaw Shale
Overlies Virgelle Sandstone
Thickness600 m (2,000 ft) (western portion)
Lithology
Primary Sandstone
Location
Coordinates 48°04′27″N112°17′58″W / 48.07417°N 112.29944°W / 48.07417; -112.29944 (Two Medicine)
Approximate paleocoordinates 55°18′N77°48′W / 55.3°N 77.8°W / 55.3; -77.8 (Two Medicine)
RegionFlag of Montana.svg  Montana
Flag of Alberta.svg  Alberta
CountryFlag of the United States (23px).png  United States
Flag of Canada (Pantone).svg  Canada
Type section
Named for Two Medicine River
Named by Stebinger
Year defined1914
Usa edcp relief location map.png
Lightgreen pog.svg
Two Medicine Formation (the United States)
USA Montana relief location map.jpg
Lightgreen pog.svg
Two Medicine Formation (Montana)

The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountain Overthrust Belt, and the western portion (about 600 metres or 2,000 feet thick) of this formation is folded and faulted while the eastern part, which thins out into the Sweetgrass Arch, is mostly undeformed plains. Below the formation are the nearshore (beach and tidal zone) deposits of the Virgelle Sandstone, and above it is the marine Bearpaw Shale. Throughout the Campanian, the Two Medicine Formation was deposited between the western shoreline of the Late Cretaceous Interior Seaway and the eastward advancing margin of the Cordilleran Overthrust Belt. The Two Medicine Formation is mostly sandstone, deposited by rivers and deltas.

Contents

History of research

In 1913, a US Geological Survey crew headed by Eugene Stebinger and a US National Museum crew headed by Charles Gilmore worked together to excavate the first dinosaur of the formation. [1] Stebinger was the first to identify the Two Medicine Formation and formally described the first fossils in a scientific paper published in 1914. [1] Gilmore returned to the Formation in 1928 and 1935. [1] During this time frame only three species were named and of these only Styracosaurus ovatus and Edmontonia rugosidens are still regarded as valid. [1] Barnum Brown prospected the formation in 1933, but found nothing significant. [1] Both of their research were interrupted by World War II. [1] In 1977, Trexler reports finding hadrosaur remains west of Choteau, Montana. [1] During the next year baby hadrosaurs were discovered. [1] In 1979, Horner and Makela referred these hadrosaur bones to Maiasaura peeblesorum . [1] The announcement attracted renewed scientific interest to the formation and many new kinds of dinosaurs were discovered. [2] More nesting sites were discovered later, including the Devil's Coulee site yielding Hypacrosaurus stebingeri in southern Alberta in 1987.

Geology

The loosely consolidated fine grain sediments composing the formation allow for fast plant growth in badland areas, limiting the number of exposed outcrops. [3] Paleosols, fluvial deposits and bentonitic layers are common in the Two Medicine Formation. [4]

Age

The Two Medicine Formation spans from 82.4 to 74.4 Ma, nearly the entire length of the Campanian stage. [5]

Equivalents

There are several equivalents to the Two Medicine Formation, as with many geologic formations (most of which are named after their type locality). The Sweetgrass Arch in Montana divides the Two Medicine from the Judith River Formation, Bearpaw Shale, Claggett Shale, and Eagle Sandstone. Across the Canada–United States border, the Two Medicine Formation correlates to the Belly River Group in southwest Alberta, and the Pakowki Formation eastward.

Stratigraphy

The Two Medicine overlies the Virgelle Sandstone, which formed from the beach sands exposed on northern and western shores of the receding Colorado Sea. [6] A Cretaceous Interior Seaway transgression submerged the area briefly early on in Two Medicine history leaving anomalous paralic sediments and isolated shale bodies about 100 m above the base of the formation. [7] The Middle portion of the two medicine formation is about 225 m thick, deposited while the Clagette Sea was receding and the Bearpaw Sea transgressing. [7] This portion is stratigraphically equivalent to the Judith River Formation and Judith River Group. [7] The sediments are mainly bentonitic siltstones and mudstones with "occasional sandstone lenses." [7] These sediments are thought to be the remains of a coastal plain "far removed" from the interior sea. [7] The upper portion is about one half of the formation. [8] Its sediments are similar to the middle portions but punctuated by extensive red beds and caliche horizons. [8] The uppermost 80 m were deposited after the inundation of the Judith River equivalent sediments by the Bearpaw Sea. [8] They are thought to have been deposited in only 500,000 years. [8] Bentonitic ash is common in the Two Medicine. [8] To the south extrusive volcanic activity occurred in association with the Boulder Batholith collectively called the Elkhorn Volcanics. [8]

Taphonomy

Most of the vertebrate fossils are preserved by CaCo3 permineralization. [3] This type of preservation preserves high levels of detail, even down to the microscopic level. [3] However, it also leaves specimens vulnerable to weathering when exposed to the surface. [3]

Paleoenvironment

Climate

Reconstruction image of a herd of Maiasaura walking along a creek-bed in Two Medicine Formation. Shown are the region's typical conifer, fern and horsetail vegetation, and a volcano erupting in the distance is evocative of the ash layers found in the Two Medicine Formation. Maiasaur Pano-v1.jpg
Reconstruction image of a herd of Maiasaura walking along a creek-bed in Two Medicine Formation. Shown are the region's typical conifer, fern and horsetail vegetation, and a volcano erupting in the distance is evocative of the ash layers found in the Two Medicine Formation.

The Two Medicine Formation was deposited in a seasonal, semi-arid climate with possible rainshadows from the Cordilleran highlands. This region during the Campanian experienced a long dry season and warm temperatures. Lithologies, invertebrate faunas, and plant and pollen data support the above interpretation. The extensive red beds and caliche horizons of the upper Two Medicine are evidence of at least seasonally arid conditions. [8] Some of the dinosaurs from the formation have been speculated to have shown signs of drought-related death. [8]

Elevation

A more upland environment existed in the south of the Two Medicine Formation. [7] Streams had a northeasterly flow away from these southwestern uplands. [7] The southern part of the Two medicine formation grades into brackish water siltstone/sandstone series called the Horsethief Formation. [8] The sediments of the Horsethief represent shallower water deposits than the Bearpaw Shale adding further evidence of higher elevation areas existing in the south. [8]

Egg Mountain site

Egg Mountain, which is near Choteau, Montana, was discovered in 1977 by Marion Brandvold, owner of the Trex Agate Rock Shop in Bynum, Montana, who discovered the bones of juvenile dinosaurs at this site. It is a colonial nesting site on the Willow Creek Anticline in the Two Medicine Formation that is famous for its fossil eggs of Maiasaura, which demonstrated for the first time that at least some dinosaurs cared for their young. The eggs were arranged in dug-out earthen nests, each nest about a parent's body length from the next, and baby dinosaurs were also found with skeletons too cartilaginous for them to walk - similar to those of altricial (helpless) baby birds. The parent(s) must then have brought food to the young, and there is plant matter in the nests that may be evidence of either this or for incubation of the eggs. Maiasaura also grew extremely fast, at rates comparable to modern birds. Skeletons of Orodromeus and skeletons and eggs of Troodon were also found at Egg Mountain.

Biostratigraphy

Achelousaurus Achelousaurus BW.jpg
Achelousaurus

Most dinosaur-bearing rock formations do not contain multiple distinct faunas at different positions within the formation's stratigraphic column. [9] Usually the lower sediments of a given formation will contain the same kinds of dinosaurs as the upper sediments, or the species composition changes only gradually. [9] However, some researchers had argued that the Two Medicine Formation was an exception, preserving multiple distinct dinosaur faunas. [9]

Later research came to find that the supposedly distinct dinosaur faunas at different levels of the formations were more similar than had been previously thought. [10] While the dinosaur fauna of the lower and middle sections Two Medicine was apparently diverse, the quality of preservation was low and few of these remains can be referred to individual species. [11] The middle Two Medicine is a better source of fossils, but still poor overall. This makes it difficult to argue that these sections of the formation preserve distinct faunas. [10]

The upper portion of the formation is more diverse and preserves better quality fossils. [12] However, many of the taxa that supposedly distinguished it as a separate fauna have since been found in older sediments. In particular, Gryposaurus latidens and Hypacrosaurus have been found to coexist with Maiasaura . [10] Further, there are fossil teeth that seem to show the presence of certain taxa are unbroken throughout the whole formation. [10]

Nevertheless, some true changes in faunal composition seem to occur in the upper Two Medicine. The appearance of Maiasaura in the formation precedes the arrival of a diverse variety of other ornithischians. [10] According to David Trexler, thorough examination of strata found along the Two Medicine River (which exposes the entire upper half of the Two Medicine Formation) indicates that the apparent diversification was a real event rather than a result of preservational biases. [10]

The timeline below follows the stratigraphic chart presented by Horner et al. 2001. [13]

Bearpaw FormationAchelousaurus horneriEiniosaurus procurvicornisRubeosaurus ovatusScolosaurus cutleriProsaurolophus maximusHypacrosaurus stebingeriBambiraptor feinbergorumTroodon formosusOrodromeus makelaiMaiasaura peeblesorumAcristavus gagslarsoniSaurornitholestesRichardoestesiaDromaeosaurusGryposaurus latidensTwo Medicine Formation

Dinosaurs

Some of the dinosaurs from the formation have been speculated to show signs of drought related death. [8] Very few articulated dinosaurs have been found in the formation; most specimens are isolated, bone bed, poorly preserved or broken remains. [14] Early studies assumed that the Two Medicine Formation would have the same dinosaurs as the Judith River Formation. [14] It was only in 1978, that it was discovered that the formation had endemic dinosaurs. [14] Even some genera regarded as wide-ranging predators exhibited a species difference between the Two Medicine and other formations. [15] No ecological barriers have been postulated apart from upland/lowland habitat preference differences between the Two Medicine and Judith River Formation. [12] There is no unequivocal evidence for intermingling between the wildlife of the Two Medicine and geographically adjacent contemporary formations. [12] Dinosaur remains are more common in the upper part of the Two Medicine. [12]

Ankylosaurs

Ankylosaurs of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Edmontonia [16] E. rugosidens [16]
  • Landslide Butte
  • Two Medicine River
Upper [16] A skull with right mandible, cervical vertebrae, dorsal vertebrae, sacral vertebrae, caudal vertebrae, ribs, partial right ilium, left and right ischia, right pubis and osteoderms.A nodosaurid also known from the Horseshoe Canyon Formation and Dinosaur Park Formation.
Edmontonia rugosidens armour AMNH 5381.jpg
Euoplocephalus [16] E. tutus [16] Upper [16] Misclassified, actually represent Scolosaurus
Indeterminate
  • Landslide Butte
  • Two Medicine River
UpperMisclassified, probably Scolosaurus
Oohkotokia [17] O. horneri [17] Flag Butte Member [5] [Four] skulls, cervical vertebra, proximal caudal vertebra, ribs partial scapula, distal humerus, a left scapulocoracoid, ischia, [two] tail clubs, cervical half-ring, osteoderms, fragments and an undescribed specimen.Penkalski (2013) referred to Oohkotokia all ankylosaurine specimens from this formation. [17] Arbour and Currie (2013) later referred Oohkotokia to Scolosaurus. [18]
Oohkotokia skull.jpg
Scolosaurus [19] S. cutleri [19] Upper [19] [Four] skulls, cervical vertebra, proximal caudal vertebra, ribs partial scapula, distal humerus, a left scapulocoracoid, ischia, [two] tail clubs, cervical half-ring, osteoderms, fragments and an undescribed specimen.An ankylosaurine ankylosaurid. These remains were previously considered to represent Euoplocephalus and then referred to Oohkotokia before being placed in Scolosaurus.
Oohkotokia horneri.jpg

Avialans

Avialans of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Gettyia [16] [20] G. gloriae [16] Upper [16] "Tarsometatarsus" [21] An avisaurid enantiornithean

Ceratopsians

Ceratopsians of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Achelousaurus [16] A. horneri [16] Landslide ButteUpper Flag Butte Member [5] "[Three] partial skulls, [one] partial skeleton." [22] A centrosaurine ceratopsid
Achelousaurus dinosaur.png
Brachyceratops [16] B. montanensis [16] Flag Butte Member [5] "[Six] partial skulls, skeletons, subadult." [22] A centrosaurine ceratopsid. Might be a juvenile Styracosaurus
Brachyceratops BW.jpg
Cerasinops C. hodgskissi [23] Lower Hagans Crossing Member [5] A leptoceratopsid
Cerasinops BW.jpg
Einiosaurus [16] E. procurvicornis [16] Landslide ButteUpper Flag Butte Member [5] "[Three] adult skulls, juvenile and subadult cranial and postcranial elements." [22] A centrosaurine ceratopsid
Einiosaurus BW.jpg
Prenoceratops P. pieganensisUpper Flag Butte Member [5] A leptoceratopsid
Prenoceratops BW.jpg
Stellasaurus [24] S. ancellaeLandslide ButteFlag Butte Member [5] "Nasal horn and fragmentary parietal frill."A centrosaurine ceratopsid
Stellasaurus parietal.jpg
Styracosaurus [25] S. ovatusFlag Butte Member [5] Fragmentary parietal frill.A centrosaurine ceratopsid
Rubeosaurus ovatus.jpg

Non-avialan eumaniraptorans

Non-avialan eumaniraptorans of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Bambiraptor [16] B. feinbergorum [16] Flag Butte Member [5] "Almost complete skull and postcrania," [26] type specimen A saurornitholestine dromaeosaur
Bambiraptor reconstruction.jpg
Dromaeosaurus [27] Indeterminate [27] Lower [27]
Dromaeosaurus Restoration.png
Richardoestesia [16] Indeterminate [16] Upper [16] Teeth
Saurornitholestes [28] Indeterminate [28]
  • "Choteau/Bynum"
  • Landslide Butte
  • Two Medicine River
Partial skeleton, isolated pedal elementsA saurornitholestine dromaeosaur
Saurornitholestes digging Burrows wahweap.jpg
Troodontidae IndeterminateUpperPartial skulls, several vertebrae, ribs, gastralia, chevrons, a sacrum, partial pelvis, and partial fore and hind limbsRemains probably referable to Stenonychosaurus . Formerly referred to Troodon , now a potentially dubious genus

Ornithopods

An unidentified lambeosaurine has been collected from the same stratigraphic placement, west of Bynum, and is in preparation at The Montana Dinosaur Center [29]

Ornithopods and parksosaurs of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Acristavus [30] A. gagslarsoniLower Hagans Crossing Member [5] A saurolophine hadrosaur
Glishades G. ericksoniA hadrosauroid [31] or an indeterminate juvenile saurolophine hadrosaur. [32]
Gryposaurus [28] G. latidens [27] Two Medicine River
  • Lower Hagans Crossing Member [5]
"Several partial skulls and postcranial skeletons." [33] Also known from isolated teeth which may have been redeposited fossils, although this explanation is unlikely. [12] A saurolophine hadrosaur. The isolated G. latidens teeth are a rare component of channel lag deposits in the middle portion of the formation. [12]
Gryposaurus BW.jpg
Indeterminate [16] Upper [16]
Hypacrosaurus [16] H. stebingeri [16]
  • Landslide Butte
  • Two Medicine River
Flag Butte Member [5] A very abundant species of lambeosaurine hadrosaur. [34]
Hypracosaurus NT.png
Indeterminate"Choteau/Bynum"Upper
Maiasaura [16] M. peeblesorum [16]
  • "Choteau/Bynum"
  • Two Medicine River
Upper Hagans Crossing Member [5] "More than [two hundred] specimens including articulated skull and postcrania, embryo to adult." [33] A saurolophine hadrosaur. Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts. [11] It is the most common dinosaur found in the Egg Mountain locality. [35]
Maiasaura BW.jpg
Orodromeus [16] O. makelai [16] "Choteau/Bynum"Lacustrine Interval, Lower Flag Butte Member [5] An orodromine thescelosaur which was the most common small herbivore in the Egg Mountain area. [36] [37]
Orodromeus (pencil 2013).png
Prosaurolophus [16] P. maximus [16]
  • Landslide Butte
  • Two Medicine River
Upper Flag Butte Member [5] "Disarticulated, associated skull and postcrania pertaining to at least [four] individuals." [33] A saurolophine hadrosaur. Prosaurolophus blackfeetensis, erected for Two Medicine fossils, is a synonym of P. maximus. [38]
Prosaurolophus Maximus.jpg

Oviraptorosaurs

The first find of an oviraptorosaur in Montana was an articular region from the lower jaw of Caenagnathus sternbergi , from the Two Medicine Formation, according to a 2001 paper by David J. Varrichio. [39] This species had previously only been known from the Canadian province of Alberta. [39] Varricchio observes that during the late Campanian, Alberta and Montana had very similar theropods despite significant differences in the types of herbivorous dinosaur faunas. [39]

Oviraptorosaurs of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Chirostenotes [40] C. pergracilis [40] Known from the articular region of a lower jaw, catalogued as MOR 1107 [40] Was previously referred to Caenagnathus sternbergi (a synonym of Chirostenotes)

Tyrannosauroids

Tyrannosauroids of the Two Medicine Formation
GenusSpeciesLocationStratigraphic positionMaterialNotesImages
Daspletosaurus [16] D. horneri [16] Flag Butte Member [5] Bonebed [41]
Daspletosaurus torosus steveoc flipped.jpg
D. wilsoniUpperBonebed [42]
Gorgosaurus G. libratus?"Choteau/Bynum"UpperIncludes TCM 2001.89.1., the undescribed skeleton
Gorgosaurus.png
Teratophoneini IndeterminateFrontal [43] Phylogenetic analysis recovers it in Teratophoneini [44]
Color key
Taxon Reclassified taxonTaxon falsely reported as presentDubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon
Notes
Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Other fauna

Many other fossil animals have been found, such as freshwater bivalves, gastropods, turtles, lizards such as Magnuviator , and champsosaurs. The multituberculate mammal Cimexomys has been found on Egg Mountain. The species Piksi barbarulna was described based on forelimb bones from the Two Medicine Formation; it was initially thought to be a bird, but subsequently it was reinterpreted as a pterosaur, likely a member of Ornithocheiroidea. [45] Azhdarchoid pterosaurs are also known from the Two Medicine Formation, including a very large, yet-unnamed azhdarchid, the estimated wingspan of which was 8 metres (26 ft), and smaller Montanazhdarcho minor , a non-azhdarchid azhdarchoid. [46] [47] Insect and mammal burrows have also been discovered, as well as dinosaur coprolites.

See also

Related Research Articles

<i>Maiasaura</i> Extinct genus of reptiles

Maiasaura is a large herbivorous saurolophine hadrosaurid ("duck-billed") dinosaur genus that lived in the area currently covered by the state of Montana and the Canadian province of Alberta. in the Upper Cretaceous Period, from 86.3 to 70.6 million years ago. Maiasaura peeblesorum is the state fossil of Montana.

<i>Montanoceratops</i> Extinct genus of dinosaurs

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<i>Orodromeus</i> Extinct genus of dinosaurs

Orodromeus is a genus of herbivorous orodromine thescelosaurid dinosaur from the Late Cretaceous of North America. Only one species is known, the type species Orodromeus makelai.

<span class="mw-page-title-main">Lance Formation</span> Geological formation in the United States

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<i>Scolosaurus</i> Extinct genus of dinosaurs

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<span class="mw-page-title-main">Dinosaur Park Formation</span> Uppermost member of the Belly River Group geologic unit in Alberta, Canada

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<span class="mw-page-title-main">Judith River Formation</span> Fossil-bearing geologic formation in Montana, part of the Judith River Group

The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the Late Cretaceous, between 79 and 75.3 million years ago, corresponding to the "Judithian" land vertebrate age. It was laid down during the same time period as portions of the Two Medicine Formation of Montana and the Oldman Formation of Alberta. It is an historically important formation, explored by early American paleontologists such as Edward Drinker Cope, who named several dinosaurs from scrappy remains found here on his 1876 expedition. Modern work has found nearly complete skeletons of the hadrosaurid Brachylophosaurus.

<span class="mw-page-title-main">Horseshoe Canyon Formation</span> Geological formation in Canada

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<span class="mw-page-title-main">Cloverly Formation</span> Geological formation in the United States

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<span class="mw-page-title-main">Bearpaw Formation</span> Geologic formation in North America

The Bearpaw Formation, also called the Bearpaw Shale, is a geologic formation of Late Cretaceous (Campanian) age. It outcrops in the U.S. state of Montana, as well as the Canadian provinces of Alberta and Saskatchewan, and was named for the Bear Paw Mountains in Montana. It includes a wide range of marine fossils, as well as the remains of a few dinosaurs. It is known for its fossil ammonites, some of which are mined in Alberta to produce the organic gemstone ammolite.

<span class="mw-page-title-main">Wayan Formation</span> Stratigraphic Unit in Idaho

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The Wapiti Formation is a geological formation of the Western Canada Sedimentary Basin in northwestern Alberta, and northeastern British Columbia, Canada. Its deposition spanned the time interval from the lower Campanian through to the upper Maastrichtian, between approximately 80 and 68 Ma. It was named by G.M. Dawson in 1881, presumably for exposures along the lower part of the Wapiti River and downstream along the Smoky River in Alberta.

<span class="mw-page-title-main">Gosau Group</span> Geological group in Austria, Germany and Slovakia

The Gosau Group is a geological stratigraphic group in Austria, Germany and western Slovakia whose strata date back to the Late Cretaceous to Eocene. It is exposed in numerous sporadic isolated basins within the Northern Calcareous Alps. It is divided into two subgroups, the Lower Gosau Subgroup which dates from the Turonian to Campanian, approximately 90 to 75 Ma and the Upper Gosau Subgroup which dates to the Santonian to Eocene, about 83.5 to 50 Ma. The formations within each subunit vary significantly between basins. The sequence is largely marine, but the Grünbach Formation represents a terrestrial deposit. Many of the units of the group are fossiliferous, typically providing marine fossils such as ammonites, though terrestrial remains including those of dinosaurs are known from the Grünbach Formation and Schönleiten Formation.

The Horsethief Sandstone is a Mesozoic geologic formation in Montana. Dinosaur remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus. The southern part of the Two Medicine Formation grades into the brackish water siltstone/sandstone series that compose the Horsethief Formation. To the north the Horsethief Sandstone is equivalent to the Blood Reserve Formation of Alberta, Canada.

Continuoolithus is an oogenus of dinosaur egg found in the late Cretaceous of North America. It is most commonly known from the late Campanian of Alberta and Montana, but specimens have also been found dating to the older Santonian and the younger Maastrichtian. It was laid by an unknown type of theropod. These small eggs are similar to the eggs of oviraptorid dinosaurs, but have a distinctive type of ornamentation.

<span class="mw-page-title-main">Paleontology in Montana</span> Paleontological research in the U.S. state of Montana

Paleontology in Montana refers to paleontological research occurring within or conducted by people from the U.S. state of Montana. The fossil record in Montana stretches all the way out to sea where local bacteria formed stromatolites and bottom-dwelling marine life left tracks on the sediment that would later fossilize. This sea remained in place during the early Paleozoic, although withdrew during the Silurian and Early Devonian, leaving a gap in the local rock record until its return. This sea was home to creatures including brachiopods, conodonts, crinoids, fish, and trilobites. During the Carboniferous the state was home to an unusual cartilaginous fish fauna. Later in the Paleozoic the sea began to withdraw, but with a brief return during the Permian.

<i>Oohkotokia</i> Extinct genus of dinosaurs

Oohkotokia is a genus of ankylosaurid dinosaur within the subfamily Ankylosaurinae. It is known from the upper levels of the Two Medicine Formation of Montana, United States. The discovery of Oohkotokia supports that Ankylosaurine dinosaurs existed and flourished continuously in Montana and/or Alberta throughout the late Campanian and early Maastrichtian stages in the Late Cretaceous period. It was a large, heavily built, quadrupedal, herbivore, that could grow up to 5 metres (16 ft) long and weigh up to 2 metric tons.

<span class="mw-page-title-main">Timeline of hadrosaur research</span>

This timeline of hadrosaur research is a chronological listing of events in the history of paleontology focused on the hadrosauroids, a group of herbivorous ornithopod dinosaurs popularly known as the duck-billed dinosaurs. Scientific research on hadrosaurs began in the 1850s, when Joseph Leidy described the genera Thespesius and Trachodon based on scrappy fossils discovered in the western United States. Just two years later he published a description of the much better-preserved remains of an animal from New Jersey that he named Hadrosaurus.

References

  1. 1 2 3 4 5 6 7 8 9 "Previous Work". Trexler (2001); page 300.
  2. "Introduction". Trexler (2001); pages 299-300.
  3. 1 2 3 4 "Introduction". Trexler (2001); page 299.
  4. "Introduction". Trexler (2001); pages 298-299.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Rogers, Raymond R.; Horner, John R.; Ramezani, Jahandar; Roberts, Eric M.; Varricchio, David J. (2024). "Updating the Upper Cretaceous (Campanian) Two Medicine Formation of Montana: Lithostratigraphic revisions, new CA-ID-TIMS U-Pb ages, and a calibrated framework for dinosaur occurrences". Geological Society of America Bulletin. doi:10.1130/B37498.1.
  6. "Geological Setting". Trexler (2001); pages 300-301.
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