Two Medicine Formation
Two Medicine Formation | ||
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Ma | ||
Approximate paleocoordinates 55°18′N 77°48′W / 55.3°N 77.8°W | | |
Region | Montana Alberta | |
Country | United States Canada | |
Type section | ||
Named for | Two Medicine River | |
Named by | Stebinger | |
Year defined | 1914 | |
The Two Medicine Formation is a
History of research
In
.Geology
The loosely consolidated fine grain sediments composing the formation allow for fast plant growth in
Age
The Two Medicine Formation spans from 80 to 74 Ma, nearly the entire length of the
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
Stratigraphy
The Two Medicine overlies the
Taphonomy
Most of the vertebrate fossils are preserved by
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 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
A more upland environment existed in the south of the Two Medicine Formation.
Egg Mountain site
Egg Mountain, which is near
Biostratigraphy
Most
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.[11] 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.[6] 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.[11]
The upper portion of the formation is more diverse and preserves better quality fossils.
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.[11] 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.[11]
The timeline below follows the stratigraphic chart presented by Horner et al. 2001.[13]
Dinosaurs
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.[14] Early studies assumed that the Two Medicine Formation would have the same dinosaurs as the Judith River Formation.[14] It was only in 1978, that it was discovered that the formation had endemic dinosaurs.[14] Even some genera regarded as wide-ranging predators exhibited a species difference between the Two Medicine and other formations.[15] No ecological barriers have been postulated apart from upland/lowland habitat preference differences between the Two Medicine and Judith River Formation.[12] There is no unequivocal evidence for intermingling between the wildlife of the Two Medicine and geographically adjacent contemporary formations.[12] Dinosaur remains are more common in the upper part of the Two Medicine.[12]
Ankylosaurs
Ankylosaurs of the Two Medicine Formation
| ||||||
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Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Edmontonia[16] | E. rugosidens[16] |
|
Upper[16] | A skull with right mandible, cervical vertebrae, dorsal vertebrae, sacral vertebrae, caudal vertebrae, ribs, partial right ilium, left and right ischia, right pubis and osteoderms. | A nodosaurid also known from the Horseshoe Canyon Formation and Dinosaur Park Formation .
|
|
Euoplocephalus[16] | E. tutus[16] | Upper[16] | Misclassified, actually represent Scolosaurus | |||
Indeterminate |
|
Upper | Misclassified, probably Scolosaurus | |||
Oohkotokia[17] | O. horneri[17] | Upper[17] | [Four] skulls, cervical vertebra, proximal caudal vertebra, ribs partial scapula, distal humerus, a left scapulocoracoid, ischia, [two] tail clubs, cervical half-ring, osteoderms, fragments and an undescribed specimen. | Penkalski (2013) referred to Oohkotokia all ankylosaurine specimens from this formation.[17] Arbour and Currie (2013) later referred Oohkotokia to Scolosaurus.
|
||
Scolosaurus[18] | S. cutleri[18] | Upper[18] | [Four] skulls, cervical vertebra, proximal caudal vertebra, ribs partial scapula, distal humerus, a left scapulocoracoid, ischia, [two] tail clubs, cervical half-ring, osteoderms, fragments and an undescribed specimen. | An ankylosaurid . These remains were previously considered to represent Euoplocephalus and then referred to Oohkotokia before being placed in Scolosaurus.
|
Avialans
Avialans of the Two Medicine Formation
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Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Gettyia[16][19] | G. gloriae[16] | Upper[16] | "Tarsometatarsus"[20] | An enantiornithean
|
Ceratopsians
Ceratopsians of the Two Medicine Formation
| ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Achelousaurus[16] | A. horneri[16] | Landslide Butte | Upper[16] | "[Three] partial skulls, [one] partial skeleton."[21] | A ceratopsid
|
|
Brachyceratops[16] | B. montanensis[16] | Upper[16] | "[Six] partial skulls, skeletons, subadult."[21] | A centrosaurine ceratopsid. Might be a juvenile Styracosaurus | ||
Cerasinops | C. hodgskissi | Lower[22] | A leptoceratopsid
|
|||
Einiosaurus[16] | E. procurvicornis[16] | Landslide Butte | Upper[16] | "[Three] adult skulls, juvenile and subadult cranial and postcranial elements."[21] | A centrosaurine ceratopsid | |
Prenoceratops | P. pieganensis | Upper | A leptoceratopsid | |||
Stellasaurus[23] | S. ancellae | Landslide Butte | Upper[16] | "Nasal horn and fragmentary parietal frill." | A centrosaurine ceratopsid | |
Styracosaurus[24] | S. ovatus | Fragmentary parietal frill. | A centrosaurine ceratopsid |
Non-avialan eumaniraptorans
Non- eumaniraptorans of the Two Medicine Formation
| ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Bambiraptor[16] | B. feinbergorum[16] | Upper[16] | "Almost complete skull and postcrania," type specimen
|
A dromaeosaur
|
||
Dromaeosaurus[26] | Indeterminate[26] | Lower[26] | ||||
Richardoestesia[16]
|
Indeterminate[16] | Upper[16] | Teeth | |||
Saurornitholestes[27] | Indeterminate[27] |
|
Partial skeleton, isolated pedal elements | A saurornitholestine dromaeosaur | ||
Troodontidae | Indeterminate | Upper | Partial skulls, several vertebrae, ribs, gastralia, chevrons, a sacrum, partial pelvis, and partial fore and hind limbs | Remains probably referable to Stenonychosaurus. Formerly referred to Troodon, now a potentially dubious genus |
Ornithopods
An unidentified
parksosaurs of the Two Medicine Formation
| ||||||
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Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Acristavus[29] | A. gagslarsoni | Lower | A hadrosaur
|
|||
Glishades | G. ericksoni | A | ||||
Gryposaurus[27] | G. latidens[26] | 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.[12] | A saurolophine hadrosaur. The isolated G. latidens teeth are a rare component of channel lag deposits in the middle portion of the formation.[12] | |
Indeterminate[16] | Upper[16] | |||||
Hypacrosaurus[16] | H. stebingeri[16] |
|
Upper[16] | A very abundant species of lambeosaurine hadrosaur.[33]
|
||
Indeterminate | "Choteau/Bynum" | Upper | ||||
Maiasaura[16] | M. peeblesorum[16] |
|
Upper[16] | "More than [two hundred] specimens including articulated skull and postcrania, embryo to adult."[32] | A saurolophine hadrosaur. | |
Orodromeus[16] | O. makelai[16] | "Choteau/Bynum" | Upper[16] | An | ||
Prosaurolophus[16] | P. maximus[16] |
|
Upper[16] | "Disarticulated, associated skull and postcrania pertaining to at least [four] individuals."[32] | A saurolophine hadrosaur. Prosaurolophus blackfeetensis, erected for Two Medicine fossils, is a synonym of P. maximus.[37] |
Oviraptorosaurs
The first find of an oviraptorosaur in Montana was an
Oviraptorosaurs of the Two Medicine Formation
| ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Chirostenotes[39] | C. pergracilis[39] | Known from the articular region of a lower jaw, catalogued as MOR 1107[39] | Was previously referred to Caenagnathus sternbergi (a synonym of Chirostenotes) |
Tyrannosauroids
Tyrannosauroids of the Two Medicine Formation
| ||||||
---|---|---|---|---|---|---|
Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
Daspletosaurus[16] | D. horneri[16] | Upper | Bonebed[40] | |||
D. wilsoni | Upper | Bonebed[41] | More closely related to D. torosus and the Oldman/Dinosaur Park species than D. horneri.[42] | |||
Gorgosaurus | G. libratus? | "Choteau/Bynum" | Upper |
Color key
|
Notes Uncertain or tentative taxa are in small text; |
Other fauna
Many other fossil animals have been found, such as freshwater
See also
- List of dinosaur-bearing rock formations
- List of fossil sites (with link directory)
References
- ^ a b c d e f g h i "Previous Work". Trexler (2001); page 300.
- ^ "Introduction". Trexler (2001); pages 299-300.
- ^ a b c d "Introduction". Trexler (2001); page 299.
- ^ "Introduction". Trexler (2001); pages 298-299.
- ^ a b "Geological Setting". Trexler (2001); page 300.
- ^ a b c "Faunal Turnover, Migration, and Evolution". Trexler (2001); page 304.
- ^ "Geological Setting". Trexler (2001); pages 300-301.
- ^ a b c d e f g "Geological Setting". Trexler (2001); page 301.
- ^ a b c d e f g h i j k "Geological Setting". Trexler (2001); page 302.
- ^ a b c "Abstract". Trexler (2001); page 298.
- ^ a b c d e f "Faunal Turnover, Migration, and Evolution". Trexler (2001); page 306.
- ^ a b c d e f "Two Medicine Fauna". Trexler (2001); page 303.
- ^ 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).
- ^ a b c "Two Medicine Fauna". Trexler (2001); page 302.
- ^ "Two Medicine Fauna". Trexler (2001); pages 302-303.
- ^ 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 "3.11 Montana, United States; 6. Upper Two Medicine Formation". in Weishampel, et al. (2004). Page 583.
- ^ .
- ^ PMID 23690940.
- PMID 30479894.
- ^ "Table 11.1". in Weishampel, et al. (2004). Page 212.
- ^ a b c "Table 23.1". in Weishampel, et al. (2004). Page 495.
- S2CID 86091277.
- PMID 32431910.
- S2CID 210260909.
- ^ "Table 10.1". in Weishampel, et al. (2004). Page 198.
- ^ a b c d e f "3.11 Montana, United States; 2. Lower Two Medicine Formation". in Weishampel, et al. (2004). Pages 582-583.
- ^ a b c "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.
- ^ "Two Medicine Fauna". Trexler (2001); page 304.
- S2CID 8878474.
- .
- S2CID 128568454.
- ^ a b c "Table 20.1". in Weishampel, et al. (2004). Page 440.
- ISSN 0031-0182.
- ^ Dawson, John. "Egg Mountain, the Two Medicine, and the Caring Mother Dinosaur". National Park Service. U. S. Department of the Interior. Retrieved 20 March 2021.
- . Retrieved 25 April 2021.
- S2CID 4329316. Retrieved 25 April 2021.
- .
- ^ a b c "Abstract". Varricchio (2001); page 42.
- ^ a b c "Table 5.1". in Varricchio (2001). Page 44.
- PMID 28358353.
- ^ 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.
- ^ "Tyrannosauroidea". www.theropoddatabase.com. Retrieved 2020-11-06.
- S2CID 56002643.
- ^ Naish, Darren (January 30, 2013). "A new azhdarchid pterosaur: the view from Europe becomes ever more interesting". Tetrapod Zoology. Retrieved February 6, 2013.
- ^ Carroll, N. (2015). "Reassignment of Montanazhdarcho minor as a non-azhdarchid member of the Azhdarchoidea". Journal of Vertebrate Paleontology, Programs and Abstracts. 104.
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.
- 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 (5): 394–413. JSTOR 3514834.
- 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" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 114 (2–4): 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.
- ISBN 0-520-24209-2. Retrieved 2019-02-21.