The Two Medicine Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

, or rock body, in northwestern

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...

and southern

Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...

that was deposited between and (million years ago), during

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...

(

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

) time. It crops out to the east of the

Rocky Mountain The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

Overthrust Belt, and the western portion (about thick) of this formation is folded and faulted while the eastern part, which thins out into the

Sweetgrass Arch Sweet grass or sweetgrass may refer to: Plants * '' Hierochloe odorata'' (sweet grass or holy grass), from northern North America and Eurasia * Sweet-grass or mannagrass, any of the many species in the genus ''Glyceria'' * ''Anthoxanthum odoratum' ...

, 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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

, deposited by rivers and deltas.

History of research

1913 Events January * January 5 – First Balkan War: Battle of Lemnos – Greek admiral Pavlos Kountouriotis forces the Turkish fleet to retreat to its base within the Dardanelles, from which it will not venture for the rest of the ...

, a

US Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and ...

crew headed by Eugene Stebinger and a

US National Museum The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7 ...

crew headed by Charles Gilmore worked together to excavate the first dinosaur of the formation. Stebinger was the first to identify the Two Medicine Formation and formally described the first fossils in a scientific paper published in

1914 This year saw the beginning of what became known as World War I, after Archduke Franz Ferdinand of Austria, heir to the Austrian throne was Assassination of Archduke Franz Ferdinand, assassinated by Serbian nationalist Gavrilo Princip. It als ...

. Gilmore returned to the Formation in 1928 and 1935. During this time frame only three species were named and of these only '' Styracosaurus ovatus'' and '' Edmontonia rugosidens'' are still regarded as valid. Barnum Brown prospected the formation in 1933, but found nothing significant. Both of their research were interrupted by

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

. In

1977 Events January * January 8 – Three bombs explode in Moscow within 37 minutes, killing seven. The bombings are attributed to an Armenian separatist group. * January 10 – Mount Nyiragongo erupts in eastern Zaire (now the Democrat ...

, Trexler reports finding

hadrosaur Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which incl ...

remains west of

Choteau, Montana Choteau is a city in and the county seat of Teton County, Montana, United States. It lies along U.S. Routes 89 and 287, (the latter terminating at the former in this city) approximately east of the Rocky Mountains, near Flathead National F ...

. During the next year baby hadrosaurs were discovered. In

1979 Events January * January 1 ** United Nations Secretary-General Kurt Waldheim heralds the start of the '' International Year of the Child''. Many musicians donate to the '' Music for UNICEF Concert'' fund, among them ABBA, who write the so ...

, Horner and Makela referred these hadrosaur bones to ''

Maiasaura peeblesorum ''Maiasaura'' (from the Greek ''μαῖα'', meaning "good mother" and ''σαύρα'', the feminine form of ''saurus'', meaning "reptile") is a large herbivorous saurolophine hadrosaurid ("duck-billed") dinosaur genus that lived in the area curr ...

''. The announcement attracted renewed scientific interest to the formation and many new kinds of dinosaurs were discovered. More nesting sites were discovered later, including the Devil's Coulee site yielding '' Hypacrosaurus stebingeri'' in southern

1987 File:1987 Events Collage.png, From top left, clockwise: The MS Herald of Free Enterprise capsizes after leaving the Port of Zeebrugge in Belgium, killing 193; Northwest Airlines Flight 255 crashes after takeoff from Detroit Metropolitan Airport, ...

Geology

The loosely consolidated fine grain sediments composing the formation allow for fast plant growth in

badland Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded."Badlands" in ''Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 2, p. 47. They are characterized by steep slopes, m ...

areas, limiting the number of exposed outcrops. Paleosols, fluvial deposits and bentonitic layers are common in the Two Medicine Formation.

Age

The Two Medicine Formation spans from 80 to 74 Ma, nearly the entire length of the

stage. The formation has been dated using ⁴⁰Ar/³⁹Ar dating at volcanic ash layers located below the top and above the base. The deposition of the formation may be diachronous. The Lower Two Medicine dates to late Santonian to early

times. The Upper Two Medicine dates to middle-late Campanian times.

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 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 ...

, Bearpaw Shale, Claggett Shale, and Eagle Sandstone. Across the Canada–US border, the Two Medicine Formation correlates to the

Belly River Group The Belly River Group is a stratigraphical unit of Late Cretaceous age in the Western Canadian Sedimentary Basin. It takes the name from the Belly River, a tributary of the Oldman River in southern Alberta, and was first described in outcrop o ...

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. 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. 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. This portion is stratigraphically equivalent to the

and Judith River Group. The sediments are mainly bentonitic

siltstones Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and mudstones with "occasional

lenses." These sediments are thought to be the remains of a coastal plain "far removed" from the interior sea. The upper portion is about one half of the formation. Its sediments are similar to the middle portions but punctuated by extensive red beds and

caliche Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions ...

horizons. The uppermost 80 m were deposited after the inundation of the Judith River equivalent sediments by the Bearpaw Sea. They are thought to have been deposited in only 500,000 years. Bentonitic ash is common in the Two Medicine. To the south extrusive volcanic activity occurred in association with the Boulder Batholith collectively called the Elkhorn Volcanics.

Taphonomy

Most of the vertebrate fossils are preserved by CaCo₃

permineralization Permineralization is a process of fossilization of bones and tissues in which mineral deposits form internal casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, perminera ...

. This type of preservation preserves high levels of detail, even down to the microscopic level. However, it also leaves specimens vulnerable to

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

when exposed to the surface.

Paleoenvironment

Climate

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 Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

faunas, and plant and pollen data support the above interpretation. The extensive red beds and

horizons of the upper Two Medicine are evidence of at least seasonally arid conditions. Some of the dinosaurs from the formation have been speculated to have shown signs of drought-related death.

Elevation

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

Egg Mountain site

Egg Mountain, which is near

, 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 structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

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 In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...

(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 ''Orodromeus'' (meaning "Mountain Runner") 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''. Discovery and naming The rem ...

'' and skeletons and eggs of ''

Troodon ''Troodon'' ( ; ''Troödon'' in older sources) is a wastebasket taxon and a dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Late Cretaceous period (about 77 mya). It includes at leas ...

'' were also found at Egg Mountain.

Biostratigraphy

Most dinosaur-bearing rock formations do not contain multiple distinct faunas at different positions within the formation's stratigraphic column. 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. However, some researchers had argued that the Two Medicine Formation was an exception, preserving multiple distinct dinosaur faunas. 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. 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. 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. The upper portion of the formation is more diverse and preserves better quality fossils. 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''. Further, there are fossil teeth that seem to show the presence of certain taxa are unbroken throughout the whole formation. 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. 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. The timeline below follows the stratigraphic chart presented by Horner ''et al.'' 2001.Horner, J. R., Schmitt, J. G., Jackson, F., & Hanna, R. (2001). Bones and rocks of the Upper Cretaceous Two Medicine-Judith River clastic wedge complex, Montana. In Field trip guidebook, Society of Vertebrate Paleontology 61st Annual Meeting: Mesozoic and Cenozoic Paleontology in the Western Plains and Rocky Mountains. Museum of the Rockies Occasional Paper (Vol. 3, pp. 3-14). ImageSize = width:1000px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px Period = from:-81 till:-72.5 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:1 start:-81 ScaleMinor = unit:year increment:1 start:-74 TimeAxis = orientation:hor AlignBars = justify Colors = #legends id:CAR value:claret id:ANK value:rgb(0.4,0.3,0.196) id:HER value:teal id:HAD value:green id:OMN value:blue id:black value:black id:white value:white id:cretaceous value:rgb(0.675,0.675,0.35) id:jurassic value:rgb(0.753,0.753,0.486) id:triassic value:rgb(0.827,0.827,0.65) id:mesozoic value:rgb(0.54,0.54,0.258) BarData= bar:eratop bar:space bar:NAM1 bar:NAM2 bar:NAM3 bar:NAM4 bar:NAM15 bar:NAM5 bar:NAM6 bar:NAM7 bar:NAM8 bar:NAM9 bar:NAM10 bar:NAM11 bar:NAM12 bar:NAM13 bar:NAM14 bar:space bar:period bar:space bar:era PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 shift:(7,-4) bar:eratop from: -81 till: -72.5 color:mesozoic text:Late Cretaceous PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left color:HAD bar:NAM1 from: -80.6 till: -76.9 text: Gryposaurus latidens color:CAR bar:NAM2 from: -80.6 till: -76.9 text: Dromaeosaurus sp. color:CAR bar:NAM3 from: -80.6 till: -76.9 text:

Richardoestesia ''Richardoestesia'' is a morphogenus of theropod dinosaur teeth, originally described from the Late Cretaceous of what is now North America. It currently contains two species, ''R. gilmorei'' and ''R. isosceles''. It has been used as a morphota ...

sp. color:CAR bar:NAM4 from: -80.6 till: -76.6 text: Saurornitholestes sp. color:HAD bar:NAM15 from: -79.2 till: -79 text: Acristavus gagslarsoni color:HAD bar:NAM5 from: -76.9 till: -76.6 text:

color:HAD bar:NAM6 from: -76.9 till: -76.6 text: Orodromeus makelai color:CAR bar:NAM7 from: -76.9 till: -76.6 text:

Troodon formosus ''Troodon'' ( ; ''Troödon'' in older sources) is a wastebasket taxon and a dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Late Cretaceous period (about 77 mya). It includes at leas ...

color:CAR bar:NAM8 from: -76.9 till: -76.6 text: Bambiraptor feinbergorum color:HAD bar:NAM9 from: -76.6 till: -74 text: Hypacrosaurus stebingeri color:HAD bar:NAM10 from: -76.6 till: -74 text: Prosaurolophus maximus color:ANK bar:NAM11 from: -76.6 till: -74 text: Scolosaurus cutleri color:HER bar:NAM12 from: -74.7 till: -74.5 text:

Rubeosaurus ovatus ''Styracosaurus'' ( ; meaning "spiked lizard" from the Ancient Greek / "spike at the butt-end of a spear-shaft" and / "lizard") is a genus of herbivorous ceratopsian dinosaur from the Cretaceous Period (Campanian stage), about 75.5 to 74.5&nb ...

color:HER bar:NAM13 from: -74.5 till: -74 text:

Einiosaurus procurvicornis ''Einiosaurus'' is a genus of herbivorous centrosaurine ceratopsian dinosaur from the Upper Cretaceous (Campanian stage) of northwestern Montana. The name means 'buffalo lizard', in a combination of Blackfeet Indian ''eini'' and Latinized Ancien ...

color:HER bar:NAM14 from: -74.2 till: -74 text: Achelousaurus horneri PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:period from: -72.5 till: -74 color:triassic text: BPF from: -76.6 till: -74 color:triassic text:Unit 5 from: -76.9 till: -76.6 color:jurassic text:4 from: -80.6 till: -76.9 color:triassic text:Unit 3 from: -80.8 till: -80.6 color:jurassic text:2 from: -81 till: -80.8 color:triassic text:1 bar:era from: -81 till: -80.5 color:mesozoic text:E. Camp. from: -80.5 till: -75.9 color:cretaceous text:Middle Campanian from: -75.9 till: -72.5 color:jurassic text:Late Campanian

Dinosaurs

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

Ankylosaurs

Avialans

Ceratopsians

Non-avialan eumaniraptorans

Ornithopods

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

Oviraptorosaurs

The first find of an oviraptorosaur in Montana was an

articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two oth ...

region from the lower jaw of '' Caenagnathus sternbergi'', from the Two Medicine Formation, according to a 2001 paper by David J. Varrichio. This species had previously only been known from the Canadian province of

. Varrichio observes that during the late

, Alberta and Montana had very similar

theropods Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ca ...

despite significant differences in the types of herbivorous dinosaur faunas.

Tyrannosauroids

Other fauna

Many other fossil animals have been found, such as freshwater bivalves, gastropods,

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

s, 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 Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

, but subsequently it was reinterpreted as a

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

, likely a member of

Ornithocheiroidea Ornithocheiroidea (or ornithocheiroids) is a group of pterosaurs within the extinct suborder Pterodactyloidea. They were typically large pterosaurs that lived from the Early to Late Cretaceous periods (Valanginian to Maastrichtian stages), with fo ...

Azhdarchoid Azhdarchoidea (or azhdarchoids) is a group of pterosaurs within the suborder Pterodactyloidea, more specifically within the group Ornithocheiroidea. Pterosaurs belonging to this group lived throughout the Early and Late Cretaceous periods, with o ...

pterosaurs are also known from the Two Medicine Formation, including a very large, yet-unnamed

azhdarchid Azhdarchidae (from the Persian word , , a dragon-like creature in Persian mythology) is a family of pterosaurs known primarily from the Late Cretaceous Period, though an isolated vertebra apparently from an azhdarchid is known from the Early Cre ...

, the estimated wingspan of which was , and smaller '' Montanazhdarcho minor'', a non-azhdarchid azhdarchoid.

Insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

and mammal burrows have also been discovered, as well as dinosaur

coprolites A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name is d ...

References

Bibliography

* Dodson, P., C.A. Forster, and S.D. Sampson. 2004. Ceratopsidae in Weishampel, D.B., P. Dodson, and H. Osmolska (eds.) ''The Dinosauria''. 2nd Edition, University of California Press. * * Trexler, D., 2001, Two Medicine Formation, Montana: geology and fauna: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 298–309. * * Varricchio, D. J. 2001. Late Cretaceous oviraptorosaur (Theropoda) dinosaurs from Montana. pp. 42–57 in D. H. Tanke and K. Carpenter (eds.), Mesozoic Vertebrate Life. Indiana University Press, Indianapolis, Indiana * {{ISBN, 0-520-24209-2 Geologic formations of Alberta Geologic formations of Montana Upper Cretaceous Series of North America Cretaceous Alberta Cretaceous Montana Campanian Stage Sandstone formations of Canada Sandstone formations of the United States Fluvial deposits Deltaic deposits Ichnofossiliferous formations Ooliferous formations Fossiliferous stratigraphic units of North America Paleontology in Alberta Paleontology in Montana Glacier National Park (U.S.)