Bearpaw Formation

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.[2] 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.[3]

Bearpaw Formation
Stratigraphic range: Campanian-Maastrichtian, 75–72 Ma
↓
Contact (red arrow) between the underlying marine shales of the Bearpaw Formation and the coastal Horseshoe Canyon Formation.
TypeGeological formation
Unit ofMontana Group
UnderliesHorseshoe Canyon Formation, St. Mary River Formation, Eastend Formation, and others
OverliesDinosaur Park Formation,
Judith River Formation
ThicknessUp to 350 meters (1,150 ft)[1]
Lithology
PrimaryShale, claystone
OtherSiltstone, sandstone, concretionary beds
Location
Coordinates48°15′0″N 109°30′0″W
RegionAlberta, Saskatchewan, Montana
CountryCanada, United States
ExtentNorthern Montana to central Alberta and southern Saskatchewan
Type section
Named forBear Paw Mountains, Montana
Named byHatcher and Stanton, 1903[2]

Lithology and depositional environment
Bearpaw shale being excavated to recover ammonites for ammolite production.

The formation was deposited in the Bearpaw Sea, which was part of the Western Interior Seaway that advanced and then retreated across the region during Campanian time.[4] It is composed primarily of dark grey shales, claystones, silty claystones and siltstones, with subordinate silty sandstones. It also includes bedded and nodular concretions (both calcareous and ironstone concretions) and thin beds of bentonite. As the seaway retreated toward the southwest, the marine sediments of the Bearpaw became covered by the deltaic and coastal plain sediments of the overlying formations.[1][5][6]

Relationship to other units

The Bearpaw Formation conformably overlies the Dinosaur Park Formation of the Belly River Group in central Alberta, and the Judith River Formation in the plains to the east and Montana. It is overlain by the Horseshoe Canyon Formation in central Alberta; by the Blood Reserve Formation and the St. Mary River Formation in southern Alberta; by the Eastend Formation in southern Saskatchewan; and by the Fox Hills Formation in Montana. To the east, it merges into the Pierre Shale.[1]

Fauna
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.
A specimen of Placenticeras ammolite from the Bearpaw Formation.

The Bearpaw Formation is famous for its well-preserved ammonite fossils. These include Placenticeras meeki and Placenticeras intercalare, and the baculite Baculites compressus, some of which are mined south-central Alberta to produce the organic gemstone ammolite.[3]

Other fossils found in this formation include many types of shellfish, bony fish, sharks, rays, birds, and marine reptiles like mosasaurs such as Prognathodon overtoni and Plioplatecarpus peckensis, plesiosaurs such as Dolichorhynchops herschelensis, Albertonectes and Nakonanectes, and sea turtles. Dinosaur remains have occasionally been discovered, presumably from carcasses that washed out to sea.[7][8]

Dinosaurs
Dinosaurs from the Bearpaw Formation
GenusSpeciesLocationMemberMaterialNotesImages
Brachylophosaurus[7] Indeterminate[7]
Brachylophosaurus
Daspletosaurus
Daspletosaurus
Edmontonia[7] Indeterminate[7]
Edmontonia
cf. Kritosaurus "Nearly complete skull and postcranium."[9] A hadrosaurid
Kritosaurus
Prosaurolophus[7][10] P. maximus[10] Three juvenile specimens[10] A Saurolophinae hadrosaurid, also known from the Dinosaur Park and Two Medicine Formations
Prosaurolophus
Stegoceras[7] Indeterminate[7]
Stegoceras

Plesiosaurs
Plesiosaurs from the Bearpaw Formation
GenusSpeciesLocationMemberMaterialNotesImages
Albertonectes[11] A. vanderveldei Alberta A complete, well-preserved postcranial specimen, missing only the skull. An elasmosaurid plesiosaur. Albertonectes has the longest neck of any known plesiosaur.
Albertonectes
Nakonanectes[12] N. bradti Montana A nearly complete skeleton including the skull. A small elasmosaurid plesiosaur with an unusually short neck.
Terminonatator[13] T. ponteixensis Saskatchewan A partially articulated incomplete skeleton, including a skull. An elasmosaurid plesiosaur.
Dolichorhynchops[14] D. herschelensis Saskatchewan An incomplete skeleton One of the latest known polycotylids.
Dolichorhynchops

Mosasaurs
Mosasaurs from the Bearpaw Formation
GenusSpeciesLocationMemberMaterialNotesImages
Mosasaurus[15] M. missouriensis Alberta and Montana Several specimens, including a near complete skeleton with stomach contents A large mosasaurine mosasaur.
Mosasaurus missouriensis
M. conodon Saskatchewan A large mosasaurine mosasaur.
Mosasaurus conodon
Prognathodon[16] P. overtoni Alberta Several exceptionally preserved specimens A large mosasaurine mosasaur.
Prognathodon overtoni
Plioplatecarpus[17] P. primaevus Saskatchewan A widespread genus of plioplatecarpine mosasaur.
Plioplatecarpus
P. peckensis Montana
Tylosaurus[18] T. saskatchewanensis Saskatchewan A single semi-complete skeleton A large tylosaurine mosasaur.
Tylosaurus

Turtles
Turtles from the Bearpaw Formation
GenusSpeciesLocationMemberMaterialNotesImages
Nichollsemys N. baieri Alberta Known from skulls A basal chelonioid sea turtle.
Nichollsemys

References
  1. Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. ISBN 0-920230-23-7.
  2. Hatcher, J.B. and Stanton, T.W., 1903. The stratigraphic position of the Judith River beds and their correlation with the Belly River beds. Science, no. 5, v. 18, p. 211-212.
  3. Mychaluk, K.A.; Levinson, A.A. & Hall, R.H. "Ammolite: Iridescent fossil ammonite from southern Alberta, Canada" (PDF). Gems & Gemology. 37 (1): 4-25. Retrieved 2015-01-11.
  4. "Latest Cretaceous Western Interior Seaway". Archived from the original on 2011-09-27. Retrieved 2007-06-22.
  5. Canadian Society of Petroleum Geologists (1994). "The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 24: Upper Cretaceous and Tertiary strata of the Western Canada Sedimentary Basin". Compiled by Mossop, G.D. and Shetsen, I. Archived from the original on 2016-07-01. Retrieved 2016-06-20.
  6. Wall, J.H., Sweet, A.R. and Hills, L.V. 1971. Paleoecology of the Bearpaw and contiguous Upper Cretaceous formations in the C.P.O.G. Strathmore well, southern Alberta. Bulletin of Canadian Petroleum Geology, vol. 19, no. 3, p. 691-702.
  7. Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Cretaceous, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 574-588. ISBN 0-520-24209-2.
  8. Bearpaw fauna in Alberta
  9. "Table 20.1," in Weishampel, et al. (2004). Page 441.
  10. Eamon T. Drysdale; François Therrien; Darla K. Zelenitsky; David B. Weishampel; David C. Evans (2019). "Description of juvenile specimens of Prosaurolophus maximus (Hadrosauridae: Saurolophinae) from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada, reveals ontogenetic changes in crest morphology". Journal of Vertebrate Paleontology. in press: e1547310. doi:10.1080/02724634.2018.1547310.
  11. Kubo, T.; Mitchell, M. T.; Henderson, D. M. (2012). "Albertonectes vanderveldei, a new elasmosaur (Reptilia, Sauropterygia) from the Upper Cretaceous of Alberta". Journal of Vertebrate Paleontology. 32 (3): 557–572. doi:10.1080/02724634.2012.658124.
  12. Serratos, Danielle J.; Druckenmiller, Patrick; Benson, Roger B.J. (2017). "A new elasmosaurid (Sauropterygia, Plesiosauria) from the Bearpaw Shale (Late Cretaceous, Maastrichtian) of Montana demonstrates multiple evolutionary reductions of neck length within Elasmosauridae". Journal of Vertebrate Paleontology. 37 (2): e1278608. doi:10.1080/02724634.2017.1278608. S2CID 132717607.
  13. Sato, Tamaki (2003). "Terminonatator ponteixensis, a new elasmosaur (Reptilia:Sauropterygia) from the Upper Cretaceous of Saskatchewan". Journal of Vertebrate Paleontology. 23 (1): 89–103. doi:10.1671/0272-4634(2003)23[89:TPANES]2.0.CO;2. ISSN 0272-4634.
  14. Sato, Tamaki (1 September 2005). "A new Polycotylid Plesiosaur (Reptilia: Sauropterygia) from the Upper Cretaceous Bearpaw Formation in Saskatchewan, Canada". Journal of Paleontology. 79 (5). doi:10.1666/0022-3360(2005)079[0969:ANPPRS]2.0.CO;2.
  15. Takuya Konishi; Michael Newbrey; Michael Caldwell (2014). "A small, exquisitely preserved specimen of Mosasaurus missouriensis (Squamata, Mosasauridae) from the upper Campanian of the Bearpaw Formation, western Canada, and the first stomach contents for the genus". Journal of Vertebrate Paleontology. 34 (4): 802–819. doi:10.1080/02724634.2014.838573. S2CID 86325001.
  16. Konishi, Takuya; Brinkman, Donald; Massare, Judy A.; Caldwell, Michael W. (2011-09-01). "New exceptional specimens of Prognathodon overtoni (Squamata, Mosasauridae) from the upper Campanian of Alberta, Canada, and the systematics and ecology of the genus". Journal of Vertebrate Paleontology. 31 (5): 1026–1046. doi:10.1080/02724634.2011.601714. ISSN 0272-4634. S2CID 129001212.
  17. Cuthbertson, Robin S.; Holmes, Robert B. (22 April 2015). "A new species of Plioplatecarpus (Mosasauridae, Plioplatecarpinae) from the Bearpaw Formation (Campanian, Upper Cretaceous) of Montana, U.S.A." Journal of Vertebrate Paleontology. 35 (3). doi:10.1080/02724634.2014.922980.
  18. Jiménez-Huidobro, P.; Caldwell, M.W.; Paparella, I.; Bullard, T.S. (2018). "A new species of tylosaurine mosasaur from the upper Campanian Bearpaw Formation of Saskatchewan, Canada". Journal of Systematic Palaeontology. 17 (10): 1–16. doi:10.1080/14772019.2018.1471744. S2CID 90533033.
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