Late Cretaceous Interior Seaway

Two Medicine Formation

The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma (million years ago), during Campanian (Late Cretaceous) time, and is located in northwestern Montana. It crops out to the east of the Rocky Mountain Overthrust Belt, and the western portion (about 600 m 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 Two Medicine Fm. 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 Fm. 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 Fm. is mostly sandstone, deposited by rivers and deltas.

Geologic Equivalents

There are several equivalents to the Two Medicine Fm., 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 Canadian border, the Two Medicine Fm. correlates to the Belly River and Bearpaw Formations in southwest Alberta, and the Milk River, Pakowki, and Judith River Formations eastward.

Paleoclimate

The Two Medicine Fm. 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.

Fossils

Dinosaurs (including birds)

The Two Medicine Fm. is one of the most important dinosaur-bearing formations in the world. Below is a list of dinosaurs that have been found in this formation.

Lower Two Medicine Fm. (late Santonian - early Campanian)

Dinosaurs

Theropoda (Currie, 2005)
Coelurosauria incerate sedis
Family unknown
*Ricardoestesia
Tyrannosauroidea
Tyrannosauridae
*Gorgosaurus
*Daspletosaurus
Maniraptora
Troodontidae
*Troodon
Dromaeosauridae
*Dromaeosaurus
*Saurornitholestes

Ornithischia (Ryan and Evans, 2005)

Ceratopsia
Leptoceratopsidae
*Cerasinops hodgskissi
Hadrosauridae
*Gryposaurus latidens

Upper Two Medicine Fm. (middle-late Campanian)

Theropoda (Currie, 2005)

Coelurosauria incertae sedis
Family unknown
*Ricardoestesia
Tyrannosauridae
*Daspletosaurus sp.
Caenagnathidae
*Chirostenotes
Troodontidae
*Troodon
Dromaeosauridae
*Bambiraptor feinbergorum
*Dromaeosaurus
*Saurornitholestes
Avisauridae
*Avisaurus gloriae
Aves incertae sedis
*Piksi

Ornithischia (Ryan and Evans, 2005)

Ankylosauria
Ankylosauridae
*Euoplocephalus tutus
Nodosauridae
*Edmontonia rugosidens
Ceratopsia
Leptoceratopsidae
*Cerasinops hodgskissi
*Prenoceratops pieganensis
Ceratopsidae
Centrosaurinae
*Achelousaurus horneri
*Brachyceratops montanensis (might be juvenile of other centrosaurine species)
*Einiosaurus procurvicornis
*Styracosaurus ovatus

Ornithopoda
*Orodromeus makelai
Hadrosauridae
Hadrosaurinae
*Brachylophosaurus canadensis
*Gryposaurus sp. indet.
*Maiasaura peeblesorum
*Prosaurolophus blackfeetensis
Lambeosaurinae
*Hypacrosaurus stebingeri

Placeholders of the Placeholder Formation
Taxa Presence Description Images
Genus:

Genus:

Genus:

Genus:

Egg Mountain

Egg Mountain, discovered and named by Jack Horner and Bob Makela in 1979, is a colonial nesting site on the Willow Creek Anticline in the Two Medicine Fm. 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.

Other Fossils

Many other fossil animals have been found, such as freshwater bivalves, gastropods, turtles, a varanid lizard, and champsosaurs. The multituberculate mammal Cimexomys has been found on Egg Mountain. Insect and mammal burrows have also been discovered, as well as dinosaur coprolites.

See also

Footnotes

References

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.

Rogers, R.R. 1990. Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana: evidence for drought-related mortality. Palaios 5:394-413.

Varricchio, D.J. 1995. Taphonomy of Jack's Birthday Site, a diverse dinosaur bonebed from the Upper Cretaceous Two Medicine Formation of Montana. Palaeogeography, Palaeoclimatology, Palaeoecology 114:297-323.

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