Two Medicine Formation

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Two Medicine Formation
Stratigraphic range: Campanian, 83.5–70.6Ma

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 ± 3.4 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.

Contents

History of research [edit]

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]

Geology [edit]

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 [edit]

The Two Medicine Formation spans from 74 to 80 ma, nearly the entire length of the Campanian stage.[5] The formation has been dated using 40Ar/39Ar dating at volcanic ash layers located 10 m below the top and 105 meters above the base.[5] The deposition of the formation may be diachronous.[6] The Lower Two Medicine dates to late Santonian - early Campanian times. The Upper Two Medicine dates to middle-late Campanian times.

Equivalents [edit]

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 Group in southwest Alberta, and the Pakowki Formation eastward.

Stratigraphy [edit]

The Two Medicine overlies the Virgelle Sandstone, which formed from the beach sands exposed on northern and western shores of the receding Colorado Sea.[7] 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.[8] 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.[8] This portion is stratigraphically equivalent to the Judith River Formation and Judith River Group.[8] The sediments are mainly bentonitic siltstones and mudstones with "occasional sandstone lenses."[8] These sediments are though to be the remains of a coastal plain "far removed" from the interior sea.[8] The upper portion is about one half of the formation.[9] Its sediments are similar to the middle portions but punctuated by extensive red beds and caliche horizons.[9] The uppermost 80 m were deposited after the inundation of the Judith River equivalent sediments by the Bearpaw Sea.[9] They are thought to have been deposited in only 500,000 years.[9] Bentonitic ash is common in the Two Medicine.[9] To the south extrusive volcanic activity occurred in association with the Boulder Batholith collectively called the Elkhorn Volcanics.[9]

Taphonomy [edit]

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 [edit]

Climate [edit]

Reconstruction image of a herd of Maiasaurs 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.[9] Some of the dinosaurs from the formation have been speculated to have shown signs of drought related death.[9]

Elevation [edit]

A more upland environment existed in the south of the Two Medicine formation.[8] Paleostreams had a northeasterly flow away from these southwestern uplands.[8] The southern part of the Two medicine formation grades into brackish water silstone/sandstone series called the Horsethief Formation.[9] The sediments of the Horsethief represent shallower water deposits than the Bearpaw Shale adding further evidence of higher elevation areas existing in the south.[9]

Egg Mountain site [edit]

Illustration of life of the Egg Mountain.

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.

Biostratigraphy [edit]

In 2001 David Trexler published a review of evidence for alleged faunal turnover in the Two Medicine Formation of Montana.[10] Most dinosaur-bearing rock formations do not contain multiple distinct stratigraphically separated faunas.[10] Typically dinosaur faunas are static throughout a formation or change piecemeal over time.[10] Faunal turnover usually occurs between formations.[10]

The fossil record can give an appearance of faunal turnover due to multiple causes including evolution, migration, or changing preservational biases.[6] Turnover events can have extremely minor causes like the migration of a taxon to a new area or extremely conspicuous ones like an ecosystem destroying catastrophe.[6] Since the fossil record is incomplete assessing the nature and causes of faunal turnovers is fraught with difficulty, except in cases where the fossil record is "unusually complete."[6] However, fossils recovered from the lower and middle part of the Two Medicine Formation tend to be scrappy and harder to identify than those from its higher strata.[6] In fact, identifiable skeletons from the lowest 100m of the formation are almost completely unknown and rare from the middle of the formation.[6] Despite the low quality of the remains, however, the fossils present an extremely diverse ecosystem.[6] Hypacrosaurus coexisted with Maiasaura for some time, as Hypacrosaurus remains have been found lower in the formation than was earlier known.[11] The discovery of Gryposaurus latidens in Maiasaura's range has shown that the border between hypothesized distinct faunas in the upper and middle is less distinct than once thought.[11] Faunal divisions between the lower and middle parts of the formation are very speculative due to the relative absence of fossils in the lowest strata.[11] Further, there are tooth remains that seem to show the presence of certain taxa are unbroken throughout the whole of the formation.[11] However, there seems to have been a major diversification in ornithischian taxa after the appearance of Maiasaura.[11] 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 rater than a result of preservational biases.[11] Erosional loss of chronologically equivalent Montanan strata makes comparing the Two Medicine formation to the Judith River Group difficult.[11]

Achelousaurus.

Earlier 1992 work by Jack Horner speculated that transitional species evolved in the uppermost part of the Two Medicine Formation during the Bearpaw Transgression for a half-million year span as the transgression inundated the Judith River Formation and later the Two Medicine area, gradually destroying the dinosaurs' preferred habitats.[12] Horner cited certain ceratopsid and pachycephalosaurid species as possible evidence for his hypothesis.[12] Trexler accepted that the proposal might be true for some taxa, but notes that the effected hadrosaurid and tyrannosaurid taxa had actually evolved before the Bearpaw Transgression began.[12] The most likely cause in the difference between dinosaur faunas in the Two Medicine Formation and Judith River Group is the latter being adapted to moist coastal lowlands and the former being a higher elevation somewhat arid region.[13] Ecological separation between the different areas is the most likely explanation for faunal differences if there was no natural barrier.[13] However there are many observations about the regions that this explanation fails to account for.[13] Post mortem carcass transport or migration should result in shared taxa between formations but none are known.[13] No evidence of gradual migration of Judith River taxa to the Two Medicine Formation exists even though the Two Medicine Formation should have gradually become coastal lowlands as the Judith River region was inundated by the Bearpaw Transgression. The absence of evidence for migration is conspicuous given the discovery of hundreds of dinosaur specimens in the upper Two Medicine.[13] The Judith river formations and Two Medicine Formation shared the same northeasterly paleoflow direction in their streams, meaning they should have the same erosional source material, yet there is no evidence of this reflected in their respective facies.[14] The Two Medicine and Judith River regions being arid and moist respectively despite exist within 100 km of each other and within the same rainshadow cast by the high mountains to the west.[15] Trexler concludes that previously hypothesized faunal turnover boundaries may actually be due to preservational biases.[15]

Dinosaurs [edit]

Some of the dinosaurs from the formation have been speculated to show signs of drought related death.[9] Very few articulated dinosaurs have been found in the formation; most specimens are isolated, bone bed, poorly preserved or broken remains.[16] Early studies assumed that the Two Medicine Formation would have the same dinosaurs as the Judith River Formation.[16] It was only in 1978, that it was discovered that the formation had endemic dinosaurs.[16] Even some genera regarded as wide ranging predators exhibited a species difference between the Two Medicine and other formations.[17] No ecological barriers have been postulated apart from upland/lowland habitat preference differences between the Two Medicine and Judith River Formation.[18] There is no unequivocal evidence for intermingling between the wildlife of the Two Medicine and geographically adjacent contemporary formations.[18] Dinosaur remains are more common in the upper part of the Two Medicine.[18]

Color key
Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon
Notes
Uncertain or tentative data are in small text; crossed out data are discredited.

Ankylosaurs [edit]

Ankylosaurs reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes Images

Edmontonia[19]

E. rugosidens[19]
  • Landslide Butte
  • Two Medicine River

A nodosaurid.
Edmontonia

Euoplocephalus[19]

E. tutus[19]

Misclassified, actually represent the new genus Oohkotokia.[20]

Indeterminate
  • Landslide Butte
  • Two Medicine River
  • Upper

Misclassified, probably referable to Oohkotokia.[20]

Oohkotokia[20]

O. horneri[20]

Penkalski (2013) referred to Oohkotokia all ankylosaurine specimens from this formation.[20]

Avialans [edit]

Avialans reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes

Avisaurus[19]

A. gloriae[19]

"Tarsometatarsus."[21]

Ceratopsians [edit]

Ceratopsians reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes Images

Achelousaurus[19]

A. horneri[19]
  • Landslide Butte

"[Three] partial skulls, [one] partial skeleton."[22]
Achelousaurus
Brachyceratops
Cerasinops
Einiosaurus
Leptoceratops
Prenoceratops
Rubeosaurus

Brachyceratops[19]

B. montanensis[19]

"[Six] partial skulls, skeletons, subadult."[22]

Might be juvenile of other centrosaurine species.

Cerasinops

C. hodgskissi
  • Upper
  • Lower

Einiosaurus[19]

E. procurvicornis[19]
  • Landslide Butte

"[Three] adult skulls, juvenile and subadult cranial and postcranial elements."[22]

Leptoceratops

Indeterminate
  • Landslide Butte
  • Upper

Prenoceratops

P. pieganensis
  • Upper

Rubeosaurus[19]

R. ovatus[19]
  • Landslide Butte

"Fragmentary parietal frill."[23]

Deinonychosaurs [edit]

Deinonychosaurs reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes Images

Bambiraptor[19]

B. feinbergorum[19]

"Almost complete skull and postcrania,"[24] type specimen
Bambiraptor
Dromaeosaurus
Saurornitholestes
Troodon formosus

Dromaeosaurus[25]

Indeterminate[25]

Ricardoestesia[19]

Indeterminate[19]

Teeth

Saurornitholestes[26]

S. langstoni[27]
  • "Choteau/Bynum"
  • Landslide Butte
  • Two Medicine River

Partial skeleton, isolated pedal elements

Troodon[26]

T. formosus[19]

A troodontid, also found in the Dinosaur Park, Judith River, Oldman and Lance Formations

Indeterminate[25]
  • "Choteau/Bynum"
  • Landslide Butte
  • Two Medicine River
  • Upper
  • Middle
  • Lower[25]

Ornithopods [edit]

An unidentified lambeosaurine has been collected from the same stratigraphic placement, west of Bynum, and is in preparation for the Timescale Adventures collection.[28]

Ornithopods reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes Images

Acristavus[29]

A. gagslarsoni
Brachylophosaurus
Gryposaurus latidens
Hypacrosaurus stebingeri
Maiasaura
Orodromeus
Prosaurolophus

Brachylophosaurus

B. canadensis
  • Upper

Glishades

G. ericksoni

A non-hadrosaurid hadrosauroid[30] or an indeterminate juvenile saurolophine hadrosaurid.[31]

Gryposaurus[26]

G. latidens[25]
  • Two Medicine River

"Several partial skulls and postcranial skeletons."[32] Also known from isolated teeth which may have been redeposited fossils, although this explanation is unlikely.[18]

The isolated G. latidens teeth are a rare component of channel lag deposits in the middle portion of the formation.[18]

Indeterminate[19]

Hypacrosaurus[19]

H. stebingeri[19]
  • Landslide Butte
  • Two Medicine River

Indeterminate
  • "Choteau/Bynum"
  • Upper

Maiasaura[19]

M. peeblesorum[19]
  • "Choteau/Bynum"
  • Two Medicine River

"More than [two hundred] specimens including articulated skull and postcrania, embryo to adult."[32]

Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts.[6]

Orodromeus[19]

O. makelai[19]
  • "Choteau/Bynum"

Prosaurolophus[19]

P. blackfeetensis[19]
  • Landslide Butte
  • Two Medicine River

"Disarticulated, associated skull and postcrania pertaining to at least [four] individuals."[32]

Oviraptorosaurs [edit]

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.[33] This species had previously only been known from the Canadian province of Alberta.[33] Varrichio observes that during the late Campanian, Alberta and Montana had very similar theropods despite significant differences in the types of herbivorous dinosaur faunas.[33] Another new Montanan oviraptorosaur specimen was an Elmisaurus elegans foot reported from the Hell Creek Formation.[33]

Oviraptorosaurs reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes Images

Caenagnathus[34]

C. sternbergi[34]

Known from the articular region of a lower jaw, catalogued as MOR 1107[34]

Sometimes considered a synonym of Chirostenotes
Chirostenotes elegans

Chirostenotes[19]

C. elegans

Based on MOR 1107

C. pergracilis[19]

Misclassified, based on MOR 1107

Tyrannosauroids [edit]

Color key
Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon
Notes
Uncertain or tentative data are in small text; crossed out data are discredited.
Tyrannosauroids reported from the Two Medicine Formation
Genus Species Location Stratigraphic position Material Notes Images

Albertosaurus[35]

Indeterminate[35]
Albertoaurus
Aublysodon mirandus
Daspletosaurus torosus
Gorgosaurus libratus

Aublysodon[26]

A. mirandus[19]

Indeterminate[36]
  • "Choteau/Bynum"
  • Landslide Butte
  • Two Medicine River
  • Upper
  • Middle
  • Lower[25]

Daspletosaurus[19]

D. torosus[19]

Misclassified, actually a new species

Unnamed
  • "Choteau/Bynum"
  • Two Medicine River
  • Upper

Bonebed[37]

Gorgosaurus

G. libratus
  • Lower

Indeterminate
  • "Choteau/Bynum"
  • Upper
  • Middle

Other animals [edit]

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. 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.[38] Azhdarchid 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.[39] Insect and mammal burrows have also been discovered, as well as dinosaur coprolites.

See also [edit]

Footnotes [edit]

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  1. ^ a b c d e f g h i "Previous Work," Trexler (2001); page 300.
  2. ^ "Introduction," Trexler (2001); pages 299-300.
  3. ^ a b c d "Introduction," Trexler (2001); page 299.
  4. ^ "Introduction," Trexler (2001); pages 298-299.
  5. ^ a b "Geological Setting," Trexler (2001); page 300.
  6. ^ a b c d e f g h "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 304.
  7. ^ "Geological Setting," Trexler (2001); pages 300-301.
  8. ^ a b c d e f g "Geological Setting," Trexler (2001); page 301.
  9. ^ a b c d e f g h i j k "Geological Setting," Trexler (2001); page 302.
  10. ^ a b c d "Abstract," Trexler (2001); page 298.
  11. ^ a b c d e f g "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 306.
  12. ^ a b c "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 307.
  13. ^ a b c d e "Conclusions," Trexler (2001); page 307.
  14. ^ "Conclusions," Trexler (2001); pages 307-308.
  15. ^ a b "Conclusions," Trexler (2001); page 308.
  16. ^ a b c "Two Medicine Fauna," Trexler (2001); page 302.
  17. ^ "Two Medicine Fauna," Trexler (2001); pages 302-303.
  18. ^ a b c d e "Two Medicine Fauna," Trexler (2001); page 303.
  19. ^ 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 "3.11 Montana, United States; 6. Upper Two Medicine Formation," in Weishampel, et al. (2004). Page 583.
  20. ^ a b c d e f Penkalski, P. (2013). "A new ankylosaurid from the late Cretaceous Two Medicine Formation of Montana, USA". Acta Palaeontologica Polonica. doi:10.4202/app.2012.0125.  edit
  21. ^ "Table 11.1," in Weishampel, et al. (2004). Page 212.
  22. ^ a b c "Table 23.1," in Weishampel, et al. (2004). Page 495.
  23. ^ "Table 23.1," in Weishampel, et al. (2004). Page 496.
  24. ^ "Table 10.1," in Weishampel, et al. (2004). Page 198.
  25. ^ 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.
  26. ^ a b c d "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.
  27. ^ Britt, 1993. "Pneumatic postcranial bones in dinosaurs and other archosaurs." PhD Thesis, University of Calgary (Canada), Alberta.
  28. ^ "Two Medicine Fauna," Trexler (2001); page 304.
  29. ^ Gates, T.A.; Horner, J.R.; Hanna, R.R.; and Nelson, C.R. (2011). "New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America". Journal of Vertebrate Paleontology 31 (4): 798–811. doi:10.1080/02724634.2011.577854. 
  30. ^ Prieto-Márquez, Albert (2010). "Glishades ericksoni, a new hadrosauroid (Dinosauria: Ornithopoda) from the Late Cretaceous of North America". Zootaxa 2452: 1–17. 
  31. ^ Nicolás E. Campione, Kirstin S. Brink, Elizabeth A. Freedman, Christopher T. McGarrity and David C. Evans (2012). "‘Glishades ericksoni’, an indeterminate juvenile hadrosaurid from the Two Medicine Formation of Montana: implications for hadrosauroid diversity in the latest Cretaceous (Campanian-Maastrichtian) of western North America". Palaeobiodiversity and Palaeoenvironments. in press. doi:10.1007/s12549-012-0097-1. 
  32. ^ a b c "Table 20.1," in Weishampel, et al. (2004). Page 440.
  33. ^ a b c d "Abstract," Varricchio (2001); page 42.
  34. ^ a b c "Table 5.1," in Varricchio (2001). Page 44.
  35. ^ 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.
  36. ^ Listed as "cf. Aublysodon sp." in "3.11 Montana, United States; 2. Lower Two Medicine Formation," in Weishampel, et al. (2004). Pages 582-583.
  37. ^ Currie, Trexler, Koppelhus, Wicks and Murphy (2005). "An unusual multi-individual tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA)." P.p. 313-324 in Carpenter, K. (ed.), The Carnivorous Dinosaurs. III. Theropods as living animals.
  38. ^ Federico L. Agnolin and David Varricchio (2012). "Systematic reinterpretation of Piksi barbarulna Varricchio, 2002 from the Two Medicine Formation (Upper Cretaceous) of Western USA (Montana) as a pterosaur rather than a bird". Geodiversitas 34 (4): 883–894. 
  39. ^ Naish, Darren (January 30, 2013). "A new azhdarchid pterosaur: the view from Europe becomes ever more interesting". Tetrapod Zoology. Retrieved February 6, 2013. 

References [edit]

  • 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.
  • 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. 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.

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

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