Two Medicine Formation

Two Medicine Formation
Two Medicine Formation
Stratigraphic range: Late Cretaceous
	Exposure of the Two Medicine Formation near "Egg Mountain" in northern Montana.
Type	Geological formation
Underlies	Bearpaw Shale
Overlies	Virgelle Sandstone
Lithology
Primary	Sandstone
Location
Region	Montana
Country	U. S. A.

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 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 Canadian border, the Two Medicine Formation 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 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.

Egg Mountain site

Egg Mountain 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.

Fauna

The Two Medicine Formation is an important dinosaur-bearing formation. Below is a list of dinosaurs that have been found in this formation.

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

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.

Ankylosaurs

Ankylosaurs reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Edmontonia^[1]	E. rugosidens^[1] Two Medicine River	Landslide Butte	Upper^[1]			Edmontonia Euoplocephalus
Euoplocephalus^[1]	E. tutus^[1]		Upper^[1]
Euoplocephalus^[1]	Indeterminate	Landslide Butte Two Medicine River	Upper

Avialans

Avialans reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes
Avisaurus^[1]	A. gloriae^[1]		Upper^[1]	"Tarsometatarsus."^[2]
Piksi	P. barbarulna		Upper		Aves incertae sedis

Ceratopsians

Ceratopsians reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Achelousaurus^[1]	A. horneri^[1]	Landslide Butte	Upper^[1]	"[Three] partial skulls, [one] partial skeleton."^[3]		Achelousaurus Cerasinops
Brachyceratops^[1]	B. montanensis^[1]		Upper^[1]	"[Six] partial skulls, skeletons, subadult."^[3]	Might be juvenile of other centrosaurine species.
Cerasinops	C. hodgskissi		Upper Lower
Einiosaurus^[1]	E. procurvicornis^[1]	Landslide Butte	Upper^[1]	"[Three] adult skulls, juvenile and subadult cranial and postcranial elements."^[3]
Leptoceratops	Indeterminate	Landslide Butte	Upper
Prenoceratops	P. pieganensis		Upper
Rubeosaurus^[1]	R. ovatus^[1]		Upper^[1]
Styracosaurus	S. ovatus	Landslide Butte	Upper	"Fragmentary parietal frill."^[4]

Deinonychosaurs

Deinonychosaurs reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Bambiraptor^[1]	B. feinbergorum^[1]		Upper^[1]	"Almost complete skull and postcrania."^[5]		Bambiraptor Saurornitholestes File:Troodont.jpg Troodon
Dromaeosaurus^[6]	Indeterminate^[6]		Lower^[6]
Ricardoestesia^[1]	Indeterminate^[1]		Upper^[1]
Saurornitholestes^[7]	Indeterminate^[7]	"Choteau/Bynum" Landslide Butte Two Medicine River	Upper^[1] Middle Lower^[6]
Troodon^[7]	T. formosus^[1]		Upper^[1]
Troodon^[7]	Indeterminate^[6]	"Choteau/Bynum" Landslide Butte Two Medicine River	Upper Middle Lower^[6]

Oviraptorosaurs

Oviraptorosaurs reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Caenagnathus^[8]	C. sternbergi^[8]			Known from the articular region of a lower jaw, catalogued as MOR 1107^[8]		Chirostenotes
Chirostenotes^[1]	C. pergracilis^[1]		Upper^[1]

Ornithopods

Ornithopods reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Brachylophosaurus	B. canadensis		Upper			Hypacrosaurus Maiasaura
Glishades	G. ericksoni
Gryposaurus^[7]	G. latidens^[6]	Two Medicine River	Middle Lower^[6]	"Several partial skulls and postcranial skeletons."^[9]
Gryposaurus^[7]	Indeterminate^[1]		Upper^[1]
Hypacrosaurus^[1]	H. stebingeri^[1]	Landslide Butte Two Medicine River	Upper^[1]
Hypacrosaurus^[1]	Indeterminate	"Choteau/Bynum"	Upper
Maiasaura^[1]	M. peeblesorum^[1]	"Choteau/Bynum" Two Medicine River	Upper^[1]	"More than [two hundred] specimens including articulated skull and postcrania, embryo to adult."^[9]
Orodromeus^[1]	O. makelai^[1]	"Choteau/Bynum"	Upper^[1]
Prosaurolophus^[1]	P. blackfeetensis^[1]	Landslide Butte Two Medicine River	Upper^[1]	"Disarticulated, associated skull and postcrania pertaining to at least [four] individuals."^[9]

Tyrannosauroids

Color key

Taxon	Reclassified taxon	Taxon falsely reported as present	Dubious taxon or junior synonym	Ichnotaxon	Ootaxon	Morphotaxon

Notes
Uncertain or tentative taxa are in small text; ~~crossed out~~ taxa are discredited.

Tyrannosauroids reported from the Two Medicine Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Albertosaurus^[10]	Indeterminate^[10]		Upper^[1] Lower^[6]			Daspletosaurus
Aublysodon^[7]	A. mirandus^[1]		Upper^[1]
Aublysodon^[7]	Indeterminate^[11]	"Choteau/Bynum" Landslide Butte Two Medicine River	Upper Middle Lower^[6]
Daspletosaurus^[1]	D. torosus^[1]		Upper^[1]
Daspletosaurus^[1]	Indeterminate	"Choteau/Bynum" Two Medicine River	Upper
Gorgosaurus	G. libratus		Lower
Gorgosaurus	Indeterminate	"Choteau/Bynum"	Upper Middle

Footnotes

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t ^u ^v ^w ^x ^y ^z ^aa ^ab ^ac ^ad ^ae ^af ^ag ^ah ^ai ^aj ^ak ^al ^am ^an ^ao ^ap ^aq ^ar ^as ^at ^au ^av ^aw ^ax ^ay ^az ^ba "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Page 583.
^ "Table 11.1," in Weishampel, et al. (2004). Page 212.
^ ^a ^b ^c "Table 23.1," in Weishampel, et al. (2004). Page 495.
^ "Table 23.1," in Weishampel, et al. (2004). Page 496.
^ "Table 10.1," in Weishampel, et al. (2004). Page 198.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j "3.11 Montana, United States; 2. Lower Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.
^ ^a ^b ^c ^d ^e "3.11 Montana, United States; 2. Lower Two Medicine Formation" and "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.
^ ^a ^b ^c "Table 5.1," in Varricchio (2001). Page 44.
^ ^a ^b ^c "Table 20.1," in Weishampel, et al. (2004). Page 440.
^ ^a ^b Listed as "cf. Albertosaurus sp." in "3.11 Montana, United States; 2. Lower Two Medicine Formation" and "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.
^ Listed as "cf. Aublysodon sp." in "3.11 Montana, United States; 2. Lower Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.

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.
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.
Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.

48°04′27″N 112°17′58″W / 48.07417°N 112.29944°W / 48.07417; -112.29944 (Two Medicine)

[upper-twom-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t ^u ^v ^w ^x ^y ^z ^aa ^ab ^ac ^ad ^ae ^af ^ag ^ah ^ai ^aj ^ak ^al ^am ^an ^ao ^ap ^aq ^ar ^as ^at ^au ^av ^aw ^ax ^ay ^az ^ba "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Page 583.

[table-11-1-212-2] "Table 11.1," in Weishampel, et al. (2004). Page 212.

[table-23-1-495-3] "Table 23.1," in Weishampel, et al. (2004). Page 495.

[table-23-1-496-4] "Table 23.1," in Weishampel, et al. (2004). Page 496.

[table-10-1-198-5] "Table 10.1," in Weishampel, et al. (2004). Page 198.

[lower-twom-6] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j "3.11 Montana, United States; 2. Lower Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.

[both-7] "3.11 Montana, United States; 2. Lower Two Medicine Formation" and "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.

[table-5-1-44-8] "Table 5.1," in Varricchio (2001). Page 44.

[table-20-1-440-9] "Table 20.1," in Weishampel, et al. (2004). Page 440.

[alberto-10] Listed as "cf. Albertosaurus sp." in "3.11 Montana, United States; 2. Lower Two Medicine Formation" and "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.

[lower-aub-11] Listed as "cf. Aublysodon sp." in "3.11 Montana, United States; 2. Lower Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]