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.EdmontoniaE. rugosidensA nodosaurid.EuoplocephalusE. tutusMisclassified, actually represent Scolosaurus.IndeterminateMisclassified, probably Scolosaurus.OohkotokiaO. horneriPenkalski (2013) referred to Oohkotokia all ankylosaurine specimens from this formation. Arbour and Currie (2013) later referred Oohkotokia to Scolosaurus.ScolosaurusS. cutleriAn ankylosaurine ankylosaurid. These remains were previously considered to represent Euoplocephalus and then referred to Oohkotokia before being placed in Scolosaurus.GettyiaG. gloriae'Tarsometatarsus.'An avisaurid enantiornithean.AchelousaurusA. horneri' partial skulls, partial skeleton.'A centrosaurine ceratopsid.BrachyceratopsB. montanensis' partial skulls, skeletons, subadult.'A centrosaurine ceratopsid. Might be a juvenile Rubeosaurus.CerasinopsC. hodgskissiA leptoceratopsid.EiniosaurusE. procurvicornis' adult skulls, juvenile and subadult cranial and postcranial elements.'A centrosaurine ceratopsid.PrenoceratopsP. pieganensisA leptoceratopsid.RubeosaurusR. ovatus'Fragmentary parietal frill.'A centrosaurine ceratopsid.BambiraptorB. feinbergorum'Almost complete skull and postcrania,' type specimenA saurornitholestine dromaeosaurDromaeosaurusIndeterminateRichardoestesiaIndeterminateTeethSaurornitholestesIndeterminatePartial skeleton, isolated pedal elementsA saurornitholestine dromaeosaur.Troodontidae indet.Indeterminate 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 dubious genus.AcristavusA. gagslarsoniLowerA saurolophine hadrosaur.GlishadesG. ericksoniA hadrosauroid or an indeterminate juvenile saurolophine hadrosaur.GryposaurusG. latidens'Several partial skulls and postcranial skeletons.' Also known from isolated teeth which may have been redeposited fossils, although this explanation is unlikely.A saurolophine hadrosaur. The isolated G. latidens teeth are a rare component of channel lag deposits in the middle portion of the formation. IndeterminateHypacrosaurusH. stebingeriA lambeosaurine hadrosaurIndeterminateMaiasauraM. peeblesorum'More than specimens including articulated skull and postcrania, embryo to adult.'A saurolophine hadrosaur. Choteau Maiasaura remains are found in higher strata than their Two Medicine River counterparts.OrodromeusO. makelaiAn orodromine parksosaur.ProsaurolophusP. maximus'Disarticulated, associated skull and postcrania pertaining to at least individuals.'A saurolophine hadrosaur. Prosaurolophus blackfeetensis, erected for Two Medicine fossils, is a synonym of P. maximus.CaenagnathusC. sternbergiKnown from the articular region of a lower jaw, catalogued as MOR 1107Sometimes considered a synonym of ChirostenotesDaspletosaurusD. horneriBonebedGorgosaurusG. libratus? 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. 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. Stebinger was the first to identify the Two Medicine Formation and formally described the first fossils in a scientific paper published in 1914. Gilmore returned to the Formation in 1928 and 1935. During this time frame only three species were named and of these only Styracosaurus ovatus and Edmontonia rugosidens are still regarded as valid. Barnum Brown prospected the formation in 1933, but found nothing significant. Both of their research were interrupted by World War II. In 1977 Trexler reports finding hadrosaur remains west of Choteau, Montana. During the next year baby hadrosaurs were discovered. In 1979 Horner and Makela referred these hadrosaur bones to Maiasaura peeblesorum. The announcement attracted renewed scientific interest to the formation and many new kinds of dinosaurs were discovered. More nesting sites were discovered later, including the Devil's Coulee site yielding Hypacrosaurus stebingeri in southern Alberta in 1987. The loosely consolidated fine grain sediments composing the formation allow for fast plant growth in badland areas, limiting the number of exposed outcrops. Paleosols, fluvial deposits and bentonitic layers are common in the Two Medicine Formation. The Two Medicine Formation spans from 80 to 74 Ma, nearly the entire length of the Campanian stage. The formation has been dated using 40Ar/39Ar dating at volcanic ash layers located 10 metres (33 ft) below the top and 105 metres (344 ft) above the base. The deposition of the formation may be diachronous. The Lower Two Medicine dates to late Santonian to early Campanian times. The Upper Two Medicine dates to middle-late Campanian times. 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 Canada–US border, the Two Medicine Formation correlates to the Belly River Group in southwest Alberta, and the Pakowki Formation eastward. The Two Medicine overlies the Virgelle Sandstone, which formed from the beach sands exposed on northern and western shores of the receding Colorado Sea. 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. 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. This portion is stratigraphically equivalent to the Judith River Formation and Judith River Group. The sediments are mainly bentonitic siltstones and mudstones with 'occasional sandstone lenses.' These sediments are though to be the remains of a coastal plain 'far removed' from the interior sea. The upper portion is about one half of the formation. Its sediments are similar to the middle portions but punctuated by extensive red beds and caliche horizons. The uppermost 80 m were deposited after the inundation of the Judith River equivalent sediments by the Bearpaw Sea. They are thought to have been deposited in only 500,000 years. Bentonitic ash is common in the Two Medicine. To the south extrusive volcanic activity occurred in association with the Boulder Batholith collectively called the Elkhorn Volcanics. Most of the vertebrate fossils are preserved by CaCo3 permineralization. This type of preservation preserves high levels of detail, even down to the microscopic level. However, it also leaves specimens vulnerable to weathering when exposed to the surface. 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. Some of the dinosaurs from the formation have been speculated to have shown signs of drought related death. A more upland environment existed in the south of the Two Medicine Formation. Streams had a northeasterly flow away from these southwestern uplands. The southern part of the Two medicine formation grades into brackish water silstone/sandstone series called the Horsethief Formation. The sediments of the Horsethief represent shallower water deposits than the Bearpaw Shale adding further evidence of higher elevation areas existing in the south.