Several sauropods seem tofollow a similar pattern, with Suuwassea and Kaatedocusrestricted to north-central Wyoming and Montana (Whitlocket al. 2018), whereas Brachiosaurus and Camarasaurus aremore widespread

studies indicate that stegosaurs from northern

exposures of the formation are different from those found fur-ther south

However, thereare some indications of geographical and, potentially, tempo-ral segregation among various animals

The diets of all Morrison vertebrates obviously dependedon an abundant and diverse flora and invertebrate fauna.Plant fossils are common elements preserved in the forma-tion (see also Chap. 6). Representatives of green algae,mosses, horsetails, many ferns, cycads, bennettitales, gink-goes (Ginkgo and Czekanowskia), and a diversity of conifersare well documented

Ornithischians include the small-bodied, likelyomnivorous Fruitadens, the armored ankylosaursMymoorapelta and Gargoyleosaurus, several plated stego-saurs including the famous Stegosaurus, possibly severalsmall, bipedal forms like Drinker and Othnielosaurus, andthe somewhat larger Dryosaurus and Camptosaurus.Theropod dinosaurs are relatively diverse, including large-bodied taxa like Ceratosaurus, Torvosaurus, Allosaurus, andSaurophaganax. Smaller theropods were the poorly knownMarshosaurus, Ornitholestes, Coelurus, the early tyranno-sauroid Stokesosaurus, and the bird-like troodontid Koparion.

They include Dystrophaeusfrom the stratigraphically oldest Tidwell Member along withHaplocanthosaurus from low stratigraphic levels. Otherknown sauropods have only been found in the upper layers.These sauropods include Camarasaurus and Brachiosaurusamong macronarians, and a number of diplodocoids(Suuwassea, Diplodocus, Barosaurus, Supersaurus,Kaatedocus, Galeamopus, Apatosaurus, Brontosaurus

Reconstruction of the paleoclimate of western NorthAmerica comes from several different proxies including fos-sil plants, ancient soils (paleosols) and sediments, and com-puter models. Interpretations of the prevailing climate duringMorrison times differ

The western half of the continent was an enormousfloodplain

A slight relative sea-level fallor an increase in the dissolved oxygen content of thebottom waters terminated anoxic conditions

Considerable reliefalong the unconformity

The overlying transgressive systems tract (TST) ischaracterized by retrogradational deposition, with strataonlapping the sequence boundary in a landward directionand downlapping the transgressive surface in a seawarddirection

The Lodgepole Formationis composed of three members

Contrarily to the ‘‘modern-land” flooding esti-mates, the ‘‘continental” flooding estimates (Fig. 3a) must becorrected relative to present-day flooding to translate them intorelative sea level estimates.

‘Continental” flooding is here defined as the difference betweenthe area of the continental crust (continental crust polygons mod-ified from Torsvik and Cocks, 2017) and the area of exposed landthrough time (Fig. 2b). This flooding refers to the inundation ofentire continental shelves and includes areas of for examplestretched continental crust (pre-breakup extension) along passivemargins. Computing the area of flooded continental shelvesrequires more detailed paleogeographic and plate tectonic knowl-edge of those areas

This can lead to confusion and faulty com-parisons as these estimates utilize different measurements, andmay be subjected differently to corrections for isostatic processes

From the Bathonian to the Oxfordian, the Sundance Seaway extendedas shallow epeiric sea a remarkable 2000 km southward from itsconnection to the open ocean near the southern border between theYukon Territory and Northwest Territories of Canada (Blakey 2013).This narrow seaway was bounded to the west by a volcanic arc, to the eastby the North American craton, and to the south by a prograding coastalplain draining from the Ancestral Rockies (Fig. 3; Imlay 1957; West 1985;Peterson 1994; Kvale et al. 2001). Recent paleogeographic reconstruc-tions place Wyoming at 35u to 40uN during the Middle Jurassic (Blakey2013), with a slow northward drift, although older reconstructions placeWyoming in a more southerly position, at 22u to 33uN (e.g., Kocurek andDott 1983). Previous sedimentologic studies indicate that the SundanceFormation records deposition in a variety of environments, includingshallow shelf (Wright 1973; Brenner and Davies 1974; DeJarnette andUtgaard 1986; Parcell and Williams 2005), tidal flat (Kvale et al. 2001),eolian (Stone and Vondra 1972; Kilibarda and Loope 1997), andestuarine

paleolatitude likely would have led to prevailing winds from the west,with the Bighorn Basin lying in an arid zone when at the southern end ofthese paleolatitudes and in a humid, temperate zone when at the northernend of these paleolatitudes. In this scenario, with a volcanic arc to thewest, prevailing winds would likely have generated a rain shadow over theregion. The orientation of the seaway perpendicular to the prevailingwinds would likely have also limited the fetch for waves.

In shape and areal dimensions, the Sundance Seaway was comparableto the modern Red Sea, but with a widened terminus; however, theshallow marine facies and the thinness of the Sundance Formationindicate that the Sundance Sea was far shallower

facies interpreted as shallow marine

The Bathonian to Oxfordian (168–157 Ma) Sundance Formation isapproximately 100 m thick and records cyclic deposition in a mixedsiliciclastic-carbonate system. The Sundance occurs over a broad regionstretching from South Dakota to Idaho and from Colorado to Montana(Parcell and Williams 2005), and it is particularly well exposed inhogbacks on the western slopes of the Bighorn Mountains (Fig. 1). TheSundance Formation overlies the Bajocian Gypsum Spring Formationand Piper Formation,

The most prominent suggests that both terrestrial diver-sity and sampling should be higher when sea level is lowand non-marine surface area is greatest

Some workers have proposedthat increased terrestrial surface area, resulting from thereduction in continental flooding associated with lowersea level, might lead to genuinely higher terrestrial biodi-versity [17 – 19], as well as greater accumulation ofterrestrial sediments [4,7], and thus more opportunitiesto sample palaeodiversity (electronic supplementarymaterial, figure S1). Other authors have suggested thathigh sea level and increased continental flooding mightlead to increased environmental heterogeneity and ende-mism in the terrestrial realm, generating higherbiodiversity [20 –22], with sampling also potentiallyincreasing owing to the enhanced preservation of terres-trial fossils in shallow marine and coastal environments

The Ranchester is interpreted as a shallow, off-shore marine facies characterized by periods ofrestricted e n v i r o n m e n t a l c o n d i t i o n s . It r e p r e s e n t s adiminution of the supply of clay and silt into the areaof deposition followed by deposition of regressive sanddeposits during Quadrant and Tensleep time

Manchester Limestone Member— The Ranchester isa poorly exposed, variable sequence of cherty dolomiteand limestone, red shale and siltstone, and sandstone thatranges from about 20 to 80 feet in thickness in the Beartooth Mountains.

Both its lower and upper boundariesare transitional

The dark shale and limestone facies that occursin the lower part of the member at sections 5 and 6 mayrepresent a local partly non-marine sediment

The Horseshoe Shale Member is interpreted as a shallow, off-shore, predominantly marine facies of the transgressing Amsden sea

Horseshoe Shale Member — This m e m b e r consistsprincipally of red siltstone and shale and discontinuousthin beds of fine-grained quartz sandstone. Thin beds ofvaricolored, argillaceous, fine-grained limestone occur atsome localities (sections 9, 13, and 14), and at others (sections 5 and 6), the sequence includes plant-bearing darklimestone and shale. The Horseshoe ranges from about40 to 125 feet in thickness in the Beartooth Mountains.Although fossils are extremely rare in the HorseshoeShale Member, conodonts of Chesterian age (identifiedby J. W. Huddle) were collected at four localities

transcontinental arch

Darwin Sandstone Member—The Darwin is a discontinuous unit that consists principally of unfossiliferous,fine- to medium-grained, ordinarily cross-bedded quartzsandstone. It includes some red siltstone and shale inthe lower half at the north end of the Beartooth Mountains(sections 13 and 14). As in central Wyoming, the Darwinis absent at many localities (sections 5, 6, and 10) or represented only as sandstone deposits in sinkholes or solution cavities below the top of the Madison (section 1).Where present (sections 9, 13, and 14), it is about 30 to50 feet thick

Bull Ridge Member of Mission Canyon Limestone —This member also does not vary greatly from equivalentbeds to the south. However, the solution zone at the baseof the member is somewhat thinner (10 to 25 feet thick),although the maximum thickness of the member (120 feet)is greater than it is in the Wyoming Province. Moreover,several thin terrigenous intervals occur above the uppersolution zone at most localities. T h e thickness of themember is quite variable (10 to 120 feet) owing to thevarying depth of post-Madison

Cliffy limestone member of Mission Canyon Lime-stone— This member is essentially the same as its expression in the Wyoming Province, differing chiefly in itsslightly greater thickness. It consists of cliff-forming limestone overlying a basal solution breccia (Figure 9). Thesolution breccia (lower solution zone) is about 25 to 50

The lower limestonem e m b e r is i n t e r p r e t e d as a s h a l l o w - w a t e r s e d i m e n tdeposited in a generally less restricted environment thanits equivalent to the south

the upper beds are in part shattered and brecciated likethose of the cherty dolomite member of the WyomingProvince. Stromatolites occur in some of the limestonebeds. Fossils are generally rare except for their occurrencein the lower oolitic beds, which contain a fauna of Osageanage similar to, but less diversified than, that of the underlying Woodhurst Limestone Member

Lower limestone member of Mission Canyon Lime-stone— The lowest unit of the Mission Canyon Limestoneis a cherty sequence of limestone and dolomite about 280to 325 feet thick that is the time-stratigraphic equivalentof the cherty dolomite member to the south. This unitbegins with thick, commonly cross-bedded, crinoidal oos-parite strata that stand in sharp contrast to the thinnerbeds of the underlying Lodgepole Limestone. The upperpart of the member consists predominantly of interbeddedfine-grained limestone and dolomite. At some localities

Woodhurst Limestone Member of Lodgepole Lime-stone — The Woodhurst in this area (Figure 8) is verysimilar to its expression in the Wyoming Province, differing by its slightly increased t e r r i g e n o u s c o n t e n t andslightly increased thickness (340 to 378 feet

represents an environment of somewhat deeper water than thatof t h e lower d o l o m i t e m e m b e r .

Identifiable fossils are rare, consisting of a fewsmall solitary corals, brachiopods, and bryozoans. Cono-donts from this unit belong to the Siphonodella crenultaZone, which is of Kinderhookian age.

Paine Shale Member of Lodgepole Limestone — ThePaine in this area consists of about 110 to 155 feet ofpredominantly thin-bedded, dark, fine-grained, verycherty limestone (Figure 7). Most of the limestone bedsappear to be micrite or pelletal limestone, but coarsegrained bioclastic debris is also present, particularly in thelower 20 to 30 feet

Cottonwood Canyon Member of Lodgepole Limestone— This member is greatly reduced from its expressionin t h e s o u t h e r n part of t h e Beartooth M o u n t a i n s . Itincludes only 2 to 3 feet of silty, dark, conodont-bearingshale and mudstone noted at three localities. It was notrecorded by Richards (1957) in the Shell Mountain section. The Cottonwood Canyon Member is slightly youngerin this province than in the Wyoming Province; its cono-dont fauna ranges from highest Siphonodella sandbergi-S.duplicata Zone to Lower Siphonodella crenulta Zone

Like the cliffy limestone member it represents a periodof lagoonal and carbonate bank deposition. The terrigenous beds of the u p p e r solution zone may reflect earlypulses of the regional uplift that occurred prior to deposition of the overlying Amsden Formation

T h e Bull Ridge contains a limited fauna of corals (Syr-ingopora, Diphyphyllum, Canadiphyllum?, a n dVesiculophyllum), brachiopods, and foraminifers of earlyMeramecian age

Bull Ridge Member — The highest member in theMadison Limestone consists of cliffy, medium- to thick-b e d d e d cherty limestone that overlies a t h i n - b e d d e ddolomitic siltstone and shale interval (upper solutionzone) which is brecciated at some localities. The memberis 85 to 100 feet thick in the Beartooth segment of theWyoming Province; the solution zone at its base is about30 feet thick. Like the cliffy limestone member, the limestone beds of the Bull Ridge consist of bioclastic debrisin a fine-grained matrix. Brecciation of the limestonesrelated to post-Madison karst development is a commonfeature, and red sand, silt, and clay from the overlyingAmsden cycle is present in sinkholes and solution cavities

The cliffy limestone member represents a short period of evaporite deposition in a lagoonalenvironment followed by carbonate deposition on shallowoff-shore marine banks

At the base of the cliff-forming unit is a solution breccia(Figure 5) about 15 to 35 feet thick that r e p r e s e n t s aleached interval of evaporites, carbonates, and terrigenoussediments (lower solution zone

mainly crinoidal)in a micrite matrix. A moderately diversified fauna ofbrachiopods (mainly large spiriferoids), corals (Syr-ingopora, Vesiculophyllum, Homalophyllites), a n dforaminifers indicates an Osagean age.

Cliffy limestone member— This member consists predominantly of cherty, medium- to thick-bedded limestoneabout 160 to 175 feet thick that forms prominent cliffsa n d flatirons t h r o u g h o u t t h e area s t u d i e d . At somelocalities many of the limestone beds are partly or completely dolomitized. The limestone consists principallyof commonly oolitic bioclastic debris

The cherty dolomite member appearsto represent a restricted marine facies, perhaps lagoonalin origin. Extensive brecciation and shattering may bethe result of leaching of thin evaporite beds throughoutthe sequence. The age of the member is Osagean by virtueof its position above and below beds dated paleontologi-cally as Osagean

Fossilsare rare; the fauna consists of a few scattered solitary coralsand brachiopods

Cherty dolomite member — Above the Woodhurst isa cherty carbonate sequence about 185 to 225 feet thickthat consists mostly of dolomite and dolomitic limestoneand subordinate limestone beds

T h e Woodhurst is interpretedas an alternation of shallow-water off-shore marine limestone and shoal-water limestone periodically interruptedby minor influxes of terrigenous sediment

In the Beartooth Mountains, the Madison Group represents an essentially uninterrupted sequence of sedimentation that began in Kinderhookian time and continuedinto the early Meramecian. This sequence is boundedbelow by an unconformity that separates it from the underlying Three Forks Formation of Late Devonian age andis terminated by an unconformity that separates it fromthe overlying Amsden Formation whose lowest beds weredeposited during Meramecian or Chesterian time

Cottonwood Canyon Member—This member consistsof interbedded quartzose shale and mudstone and quartzsiltstone from about 40 to 50 feet thick. It was studiedat only two localities (sections 1 and 5)

at section 5 it consists of cherty, thinb e d d e d dolomitic limestone and dolomite (Figure 3).Megafossils are rather rare in the dolomite member; thefauna consists mainly of corals (Vesiculophyllum, Syrin-gopora, Cyathaxonia, Zaphrentites?), small brachiopods,and conodonts of Kinderhookian age (Siphonodella cre-nulta Zone). These fossils are more abundant at the morenorthern localities. The lower dolomite member appearsto represent a dolomitized shoal-water sediment that wasdeposited in clear waters that d e e p e n e d northward.

T h e Woodhurst contains an abundant faunathat consists principally of corals, brachiopods, and gastropods; foraminifers and conodonts are also present. Burrowing and bioturbation by benthonic organisms is a dis-

tinctive feature. The fauna includes the corals Rylstonia,Michelinia, Lithostrotionella, Vesiculophyllu,Homalophyllites, and Zaphrentites mainly indicative ofOsagean age