Oohkotokia

Oohkotokia (/ˌoʊ.oʊkəˈtoʊkiə/ OH-oh-kə-TOH-kee-ə) is a genus of ankylosaurid dinosaur within the subfamily Ankylosaurinae. It is known from the upper levels of the Two Medicine Formation (late Campanian stage, about 74 Ma ago) of Montana, United States. The discovery of Oohkotokia supports that Ankylosaurine dinosaurs existed and flourished continuously in Montana and/or Alberta throughout the late Campanian and early Maastrichtian stages in the Late Cretaceous period. It was a large, heavily built, quadrupedal, herbivore, that could grow up to 6 m (19.7 ft) long.

Oohkotokia
Temporal range: Late Cretaceous, 74 Ma
Oohkotokia
Skull of the holotype, MOR 433
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Family: Ankylosauridae
Subfamily: Ankylosaurinae
Genus: Oohkotokia
Penkalski, 2013
Type species
Oohkotokia horneri
Penkalski, 2013

Discovery and naming

The generic name, Oohkotokia, is derived from the Blackfoot animate noun "ooh’kotoka", meaning "large stone" and the Latin suffix "ia" meaning "derived from"; thus "child of stone", which is a reference to its extensive body armor. The generic name also honors the Blackfeet people, on whose land the specimen was found. The specific name, O. horneri, refers to John R. Horner of the Museum of the Rockies, who collected the type specimen.[1] Oohkotokia contains a single type species, Oohkotokia horneri, named and described in 2013 by Paul Penkalski. Penkalski described this genus after finding it in the collection of Montana's Museum of the Rockies where it had been stored for more than three decades.

Description

Oohkotokia skull
Skull of MOR 433 from below and in profile

The holotype of Oohkotokia, MOR 433 consists of a crushed but rather well preserved skull that measured 375 mm, axial material, a partial scapula, several thin-walled osteoderms, cervical armour, a very large humerus, and other fragments. Estimates suggest that Oohkotokia was at best 6 m (19.7 ft) long and weighed 2 tons at most.[2] The skull of this genus bears great similarity to skulls that have been referred to Euoplocephalus, but is significant because it has a relatively smooth overall surface texture when compared to the skulls of most other ankylosaurids from the same time period.

Referred specimens[1]

  • Specimen MOR 363, which consists of fragmentary skull with supraorbital and quadratojugal bosses identical to those observed in holotype MOR 433, which was also found in the Two Medicine Formation, was referred to Oohkotokia.
  • Specimen NSM PV 20381, which was collected in 1995 but not described, and includes an incomplete skull, a pelvic bone, one keeled osteoderm, forelimbs, and hindlimbs without feet. This specimen was once thought to belong to Euoplocephalus.[3]
  • Specimen TMP 2001.42.19, from the Two Medicine Formation, included a partial skull without teeth, squamosal horns, a distal humerus, osteoderms, a left scapulocoracoid, both ischia, and a complete and well preserved "tail club", which is average-sized - approximately 320 mm wide.
  • Specimen MOR 538, a partial tail club similar to that of TMP 2001.42.19.
  • Specimen USNM 7943, a partial cervical half-ring found in 1917 near the locality in Montana where the holotype of Brachyceratops montanensis was discovered.
  • Specimen USNM 11892, a partial skull with five teeth first described and referred to Dyoplosaurus by Gilmore in 1930. This specimen has very large squamosal horns and teeth with shelf-like labial cingula and unique Z-shaped carinae in occlusal view.

Tail

Oohkotokia horneri
Referred tail club, TMP 2001.42.19

Like other ankylosaurids, Oohkotokia had an enlarged mass of bone forming a "club" on the end of its tail, made of two enlarged bone lumps. The presence of this type of tail club separates ankylosaurids like Oohkotokia from its close relatives the nodosaurids, who do not have this tail feature. Its tail club was made of several plates of bone, permeated by soft tissue, which allowed the absorption of thousands of pounds of force. The large clubs at the end of their tails may have been used in self-defense, by swinging the club at predators or rivals, or in sexual selection.

Classification

Penkalski (2013) assigned this genus to Ankylosaurinae based on anatomical features present in the skull and the morphology of its body armor. There was sufficient morphological detail to conclude that Oohkotokia is a different animal than its relatives, Euoplocephalus tutus, Dyoplosaurus acutosquameus, and Scolosaurus cutleri.[1]

Arbour and Currie (2013)[4] found that Oohkotokia shares diagnostic features with Scolosaurus, including cervical half ring morphology and squamosal horn shape. They also noted that the rostrum is broken in all specimens referred to Oohkotokia (with some being reconstructed) and therefore the nasal plate morphology does not provide strong evidence for the separation of Oohkotokia. They concluded that Oohkotokia is a junior synonym of Scolosaurus.

Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.

According to Penkalski (2013), Oohkotokia can be distinguished based on the following characteristics:[1]

  • The median, middle, plate on the nasal area of the skull roof is small and is not distinguished from surrounding osteoderms
  • On the rear of the skull the squamosal bosses are prominent, horn-like, and "trihedral"
  • The keel on the squamosal boss is flat rostrally. on the front side, grading into a blunt keel dorsally, on top; and the caudal, rear, surface of squamosal boss is flat to gently rounded and is unkeeled
  • The quadratojugal bosses on the cheek are broad and smooth with strong caudal curvature
  • The apex of the quadratojugal boss is rounded and unkeeled, and is situated caudally;
  • The transverse nuchal crest on the rear skull roof is not visible in lateral, side, view
  • The occipital condyle is small, approximately 16% of basal skull length
  • The orbit, eye socket, is relatively large
  • The rump osteoderms are basally excavated with a smooth, weakly ornamented external surface texture; and steeply-pitched, triangular caudal, tail, osteoderms are present

Paleoecology

Provenance and occurrence

Oohkotokia postcrania
Postcranial material of the holotype

The remains of Oohkotokia, MOR 433 were recovered in the Upper Member of the Two Medicine Formation, in Montana, which radiometric dating has shown to be approximately 74 million years old.[5] The remains were collected in 1986-1987 in grey siltstone that was deposited during the Campanian stage of the Cretaceous period.[1] The specimen is housed in the collection of the Museum of the Rockies in Bozeman, Montana.

Fauna and habitat

Studies suggest that the paleoenvironment of the Two Medicine Formation featured a seasonal, semi-arid climate with possible rainshadows from the Cordilleran highlands. Lithologies, invertebrate faunas, and plant and pollen data suggest that during the Campanian, this region experienced a long dry season and warm temperatures. The extensive red beds and caliche horizons of the upper Two Medicine are evidence of at least seasonally arid conditions.

The Two Medicine Formation has produced the remains of oviraptorosaurs, ornithopods, the nodosaur Edmontonia, the ceratopsians Achelousaurus, Brachyceratops, Cerasinops, and Einiosaurus among others, Troodon, the dromaeosaurids Bambiraptor, Dromaeosaurus and Saurornitholestes, and the tyrannosauroids Albertosaurus, Daspletosaurus and Gorgosaurus. These dinosaurs and Oohkotokia shared the same ancient paleoenvironment with freshwater bivalves, gastropods, turtles, lizards, and champsosaurs. Some of the dinosaurs from this formation have been speculated to have shown signs of drought related death. The Upper Two Medicine Formation is particular significant for discoveries of a range of ontogenetic stages in dinosaurs, which has included hadrosaur nests with eggs and hatchlings, and Troodon eggs with intact embryos.[6][7][8][9]

Taphonomy

Oohkotokia quarry
Quarry map of the holotype in situ

The skull of Oohkotokia, specimen MOR 433 is crushed but is overall reasonably well preserved on its dorsal and lateral surfaces, but its midline is offset to the animal's left side. The premaxillary beak is missing and most of the palate has eroded away.[1] The crushing suggests that it was the result of dinoturbation, which is the trampling of soils, sediments and bones by passing dinosaurs. A roughly triangular area is present around which the bones of the skull have been splayed outward, as if the skull was stepped on prior to fossilization. The alveolar borders of the maxillae were eroded and no teeth were preserved. The recovered fossil material ranges from red to grey, but the coloration is not thought to bear any taphonomic significance, and the variation most likely arose from differential weathering.[1]

See also

References

  1. ^ a b c d e f g 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.
  2. ^ "Oohkotokia". DinoChecker.com. Retrieved 29 May 2013.
  3. ^ Tanoue, K (2005). "Postcranial skeleton and ontogeny of Euoplocephalus tutus (Ornithischia, Ankylosauridae)". Journal of Vertebrate Paleontology. 25 (3): 121A. doi:10.1080/02724634.2005.10009942.
  4. ^ Arbour, V. M.; Currie, P. J. (2013). Farke, Andrew A (ed.). "Euoplocephalus tutus and the Diversity of Ankylosaurid Dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA". PLoS ONE. 8 (5): e62421. doi:10.1371/journal.pone.0062421. PMC 3648582. PMID 23690940.
  5. ^ Rogers, R.R.; Swisher III, C.C.; Horner, J.R. (1993). "40Ar/39Ar age and correlation of the nonmarine Two Medicine Formation (Upper Cretaceous), northwestern Montana, U.S.A". Canadian Journal of Earth Sciences. 30: 1066–1075. doi:10.1139/e93-090.
  6. ^ Horner, J.R.; Makela, R. (1979). "Nest of juveniles provides evidence of family structure among dinosaurs". Nature. 282: 296–298. Bibcode:1979Natur.282..296H. doi:10.1038/282296a0.
  7. ^ Horner, J.R. (1999a). "Egg clutches and embryos of two hadrosaurian dinosaurs". Journal of Vertebrate Paleontology. 19: 607–611. doi:10.1080/02724634.1999.10011174.
  8. ^ Horner, J.R.; Weishampel, D.B. (1988). "A comparative embryological study of two ornithischian dinosaurs". Nature. 332: 256–257. doi:10.1038/332256a0.
  9. ^ Varricchio, D.J.; Horner, J.R.; Jackson, F.D. (2002). "Embryos and eggs for the Cretaceous theropod dinosaur Troodon formosus". Journal of Vertebrate Paleontology. 22: 564–576. doi:10.1671/0272-4634(2002)022[0564:EAEFTC]2.0.CO;2.
Acantholipan

Acantholipan is a genus of herbivorous nodosaurid dinosaur from Mexico from the early Santonian age of the Late Cretaceous. It includes one species, Acantholipan gonzalezi.

Ankylosaurinae

Ankylosaurinae is a subfamily of ankylosaurid dinosaurs, existing from the Early Cretaceous about 105 million years ago until the end of the Late Cretaceous, about 66 mya. Many genera are included in the clade, such as Ankylosaurus, Pinacosaurus, Euoplocephalus, and Saichania.

Anodontosaurus

Anodontosaurus is an extinct genus of ankylosaurid dinosaurs within the subfamily Ankylosaurinae. It is known from the entire span of the Late Cretaceous Horseshoe Canyon Formation (mid Late Campanian to "middle" Maastrichtian stage, about 72.8-67 Ma ago) of southern Alberta, Canada. It contains two species, A. lambei and A. inceptus.

Bienosaurus

Bienosaurus (meaning "Bien's lizard") is a genus of thyreophoran dinosaur from the Lower Jurassic (probably Sinemurian) Lower Lufeng Formation in Yunnan Province in China.

Bissektipelta

Bissektipelta is a genus of herbivorous ankylosaurid dinosaur from the Upper Cretaceous of Uzbekistan. Bissektipelta is monospecific, containing only the species B. archibaldi.

Chuanqilong

Chuanqilong is an extinct genus of ankylosaurid dinosaur from the Early Cretaceous of China. It is known from the type species, Chuanqilong chaoyangensis. It lived during the Aptian stage of early Cretaceous period (125 - 112 mya) and was about 4.5 meters long. Its weight is estimated at some 450 kg.

Dongyangopelta

Dongyangopelta is an extinct genus of nodosaurid ankylosaurian dinosaur known from the "middle" Cretaceous Chaochuan Formation (Albian or Cenomanian stage) of Dongyang, Zhejiang Province, China. Dongyangopelta was first named by Rongjun Chen, Wenjie Zheng, Yoichi Azuma, Masateru Shibata, Tianliang Lou, Qiang Jin and Xingsheng Jin in 2013 and the type species is Dongyangopelta yangyanensis. It differs from Zhejiangosaurus, the second nodosaurid from southeast China, in the characters of presacral rod, ilium, and femur. Donyangopelta is distinguishable from Zhejiangosaurus only on the basis of the morphology of its pelvic shield.

Euoplocephalus

Euoplocephalus ( yoo-OP-loh-KEF-ə-ləs) is a genus of very large, herbivorous ankylosaurian dinosaurs, living during the Late Cretaceous of Canada. It has only one named species, Euoplocephalus tutus.

The first fossil of Euoplocephalus was found in 1897 in Alberta. In 1902, it was named Stereocephalus, but that name had already been given to an insect, so it was changed in 1910. Later, many more ankylosaurid remains were found from the Campanian of North America and often made separate genera. In 1971, Walter Coombs concluded that they all belonged to Euoplocephalus which then would be one of the best-known dinosaurs. Recently however, experts have come to the opposite conclusion, limiting the authentic finds of Euoplocephalus to about a dozen specimens. These include a number of almost complete skeletons, so much is nevertheless known about the build of the animal.

Euoplocephalus was about five and a half meters long and weighed about two and a half tonnes. Its body was low-slung and very flat and wide, standing on four sturdy legs. Its head had a short drooping snout with a horny beak to bite off plants that were digested in the large gut. Like other ankylosaurids, Euoplocephalus was largely covered by bony armor plates, among them rows of large high-ridged oval scutes. The neck was protected by two bone rings. It could also actively defend itself against predators like Gorgosaurus using a heavy club-like tail end.

Invictarx

Invictarx is a genus of herbivorous nodosaurid dinosaur from New Mexico dating from the early Campanian epoch of the Late Cretaceous.

Mongolostegus

Mongolostegus is a genus of stegosaur from the Early Cretaceous (Aptian-Albian) of Mongolia. The type and only species is M. exspectabilis, known from a single specimen previously under the nomen nudum Wuerhosaurus mongoliensis.

Nodosaurinae

Nodosaurinae is a group of ankylosaurian dinosaurs named in 1919 by Othenio Abel.

Nodosaurus

Nodosaurus (meaning "knobbed lizard") is a genus of herbivorous ankylosaurian dinosaur from the Late Cretaceous, the fossils of which are found in North America.

Scolosaurus

Scolosaurus is an extinct genus of ankylosaurid dinosaurs within the subfamily Ankylosaurinae. It is known from either the lower levels of the Dinosaur Park Formation or upper levels of the Oldman Formation (the location of the type specimen's quarry is uncertain) in the Late Cretaceous (latest middle Campanian stage, about 76.5 Ma ago) of Alberta, Canada. It contains two species, S. cutleri and S. thronus.

Silvisaurus

Silvisaurus, from the Latin silva "woodland" and Greek sauros "lizard", is a nodosaurid ankylosaur from the middle Cretaceous period.

Tatisaurus

Tatisaurus is a genus of ornithischian dinosaur from the Early Jurassic from the Lower Lufeng Formation in Yunnan Province in China. Little is known as the remains are fragmentary.

Texasetes

Texasetes (meaning "Texas resident") is a genus of ankylosaurian dinosaurs from the late Lower Cretaceous of North America. This poorly known genus has been recovered from the Paw Paw Formation (late Albian) near Haslet, Tarrant County, Texas, which has also produced the nodosaurid ankylosaur Pawpawsaurus. Texasetes is estimated to have been 2.5–3 m (8–10 ft) in length. It was named by Coombs in 1995.

Tianzhenosaurus

Tianzhenosaurus (Tianzhen + Greek sauros="lizard") is a genus of ankylosaurid dinosaurs discovered in Tianzhen County, at Kangdailiang near Zhaojiagou Village, in Shanxi Province, China, in the Late Cretaceous Huiquanpu Formation. Thus far, a virtually complete skull and postcranial skeleton have been assigned to the genus, which is monotypic (T. youngi Pang & Cheng, 1998).

This was a medium-sized ankylosaurian, the skull measuring 28 cm (11 in) in length, with a total body length around 4 m (13 ft).

Vickaryous et al. (2004) placed Tianzhenosaurus within the Ankylosauridae, nested as the sister group to Pinacosaurus. Some authors have suggested that Tianzhenosaurus is actually a junior synonym of Saichania chulsanensis.

Tsagantegia

Tsagantegia (; meaning "of Tsagan-Teg"; Tumanova, 1993) is a genus of medium-sized ankylosaurid dinosaur from the Late Cretaceous of Mongolia, during the Cenomanian stage.

The holotype specimen (GI SPS N 700/17), a complete skull, was recovered from the Bayan Shireh Formation (Cenomanian-Santonian), at the Tsagan-Teg ("White Mountain") locality, Dzun-Bayan, in the southeastern Gobi Desert, Mongolia. The genus is monotypic, including only the type species, T. longicranialis.

Two Medicine Formation

The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma and 70.6 ± 3.4 Ma (million years ago), during Campanian (Late Cretaceous) time, and is located in northwestern Montana and southern Alberta. It crops out to the east of the Rocky Mountain Overthrust Belt, and the western portion (about 600 metres (2,000 ft) 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 formation 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.

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