[D] Oohkotokia horneri [sG] [T] [Su]
Cretaceous Late Campanian
Ornithischia Thyreophora Eurypoda Ankylosauria Ankylosauridae
Two Medicine Formation, MOR Locality TM-034, northwest of Cut Bank, Montana, USA
Described based on a specimen in the collections of the Museum of the Rockies, Montana, USA. Oohkotokia exhibits a unique combination of characters not seen in other late Campanian North American ankylosaurids: prominent, horn-like, trihedral squamosal bosses, a small, undistinguished median nasal plate on the dorsal surface of the rostrum, a relatively small occipital condyle, a smooth osteoderm external texture, and triangular lateral osteoderms. Other specimens from the Two Medicine Formation are referable to Oohkotokia. O. horneri, Euoplocephalus tutus, Dyoplosaurus acutosquameus, and Scolosaurus cutleri separate stratigraphically.
From the Blackfoot animate noun ooh’kotoka, meaning large stone or rock, plus the Latin –ia, indicating made of or derived from, literally “child of stone,” an allusion to the allencompassing armour. Intended pronunciation: “O-OH-ko-toke-ee-uh”. The generic name honours the Blackfeet people, on whose land the specimen was found. | The specific name honours John R. Horner for his work on dinosaurs from Montana.
MOR 433, was found disarticulated in a grey siltstone. Recovered in 1986-87 by field crews from the MOR, the material as catalogued includes an ankylosaurid
skull, typical ankylosaur axial material, a partial scapula, several thin-walled, ankylosaurid-type osteoderms, nodosaurid-like cervical armour, a very large humerus, and various fragments from Group 1 area of quarry.
MOR 363, a fragmentary skull from 60 m below the top of the Upper Two Medicine Formation (Horner, pers. comm., 2004), with supraorbital and quadratojugal bosses identical to those of the holotype; MOR 538; NSM PV 20381, an undescribed specimen recovered in 1995 from a bonebed (Tanoue 2005) that includes a partial skull, vertebrae, a partial pelvis, forelimb and hindlimb elements without feet, and one keeled osteoderm; FPDM V-35, another undescribed specimen from the Upper Two Medicine Formation; TMP 2001.42.19; USNM 7943; USNM 11892.
Upper Two Medicine Formation (upper Campanian) of northwestern Montana, about 55 m below the contact with the Bearpaw Shale. The Two Medicine Formation is about 650 m thick and consists primarily of mudstones, siltstones, and fine-grained lenticular sandstones, interbedded with bentonites of varying thickness (Dawson 1885; Stebinger 1914; Rogers et al. 1993). The Upper Two Medicine has been dated by Rogers et al. (1993) using radioisotopes at 74 Ma (10 m below top of formation), an age accepted by Eberth (1997) and Trexler (2001). The top of the Dinosaur Park Formation was given an age of 74-74.5 Ma by Eberth (1997), but of 75 Ma or older by Hamblin (1994). Eberth (2005) refined this to around 74.9 Ma. Thus, the uppermostTwo Medicine sediments are slightly younger than the latest Dinosaur Park beds.
An ankylosaurine diagnosed by the following unique combination of characters: median plate on nasal area of skull roof small (<50 mm), not distinguished from surrounding osteoderms; prominent, horn-like, trihedral squamosal bosses; keel on squamosal boss flat rostrally, grading into a blunt keel dorsally; apex rounded and unkeeled, situated caudally; caudal surface of squamosal boss flat to gently rounded and unkeeled; broad, smooth quadratojugal bosses with strong caudal curvature; nuchal crest not visible in lateral view; occipital condyle small (≤16% basal skull length); orbit large; osteoderms basally excavated with a smooth, weakly ornamented external surface texture; steeply-pitched, triangular caudal osteoderms. The small nasal plate, caudally curved quadratojugal bosses, and horn-like squamosal bosses distinguish O. horneri from Euoplocephalus tutus. Nuchal crest morphology, supraorbital boss shape, and vertebral morphology separate O. horneri from Dyoplosaurus acutosquameus, and osteoderm shape and texture distinguish O. horneri from D. acutosquameus and Scolosaurus cutleri.