Milk River Formation Research Articles

A new plastomenid trionychid (Testudines: Pan-Trionychidae) from Milk River Formation of southern Alberta (Cretaceous: Santonian)

The pre-Campanian trionychid fossil record in North America is composed of highly fragmentary specimens, which are often not identifiable beyond Pan-Trionychidae. Here, we describe a new species of plastomenid soft-shelled turtle based on a partial shell (ROM 56647) from the Santonian Milk River Formation of southern Alberta, estimated at approximately 84 Ma. This represents the oldest relatively complete shell of a trionychid from North America. Plastomenidae, a pan-trionychid clade known only from the fossil record, is classically characterized by the complete suturing of its posterior plastral bones along the midline, a crescent-shaped entoplastron, and enlarged costals VIII. ROM 56647 has a unique combination of plastomenid characters (i.e., mid-line contact of hypoplastra and xiphiplastra, anteroposteriorly long eighth costal) and apomorphies (an emarginate nuchal, enlarged tubercles on the carapace, a wide pygal notch with a straight anterior edge, and a fused hyo-hypoplastron) that allows us to identify it as a new taxon, Jimemys glaebosus gen. et sp. nov. Phylogenetic analysis places J. glaebosus within Plastomenidae as the sister taxon to a clade containing Plastomenus, Helopanoplia, and Hutchemys. This phylogenetic position implies that Aspideretoides foveatus Leidy, 1856, Atoposemys, and Gilmoremys, all of which are more basal within Plastomenidae, had ghost lineages extending at least to the Santonian. As the oldest pan-trionychid that is diagnostic to the species level in North America, J. glaebosus provides new insights into both the early evolution of trionychids in North America and the biodiversity of southern Alberta during a poorly sampled time in the Late Cretaceous of North America.

Morphometric analysis of the teeth and taxonomy of the enigmatic theropod Richardoestesia from the Upper Cretaceous of Uzbekistan

ABSTRACTRichardoestesia is an enigmatic theropod dinosaur originally described on the basis of a pair of dentaries (holotype of R. gilmorei Currie et al., 1990) and isolated distinctive teeth, which are abundant in the Upper Cretaceous of North America. Richardoestesia asiatica (Nesov, 1995) is represented by numerous isolated teeth from the Upper Cretaceous Khodzhakul, Bissekty, and Aitym formations of Uzbekistan. Richardoestesia asiatica shows the typical features of the genus, including small mesial and distal denticles (on average 28.4 and 34.5 denticles per 5 mm, respectively), rectangular or knob-like denticles, and an apically convex distal margin of the tooth crown. The sample of R. asiatica has a large proportion of straight teeth (morphotype A) and includes several types of recurved teeth related to their position in the jaws (morphotypes B–E). This species differs from R. gilmorei in the presence of straight teeth and fully serrated mesial carinae. Richardoestesia asiatica cannot be distinguished from R. isosceles Sankey, 2001, which we consider a nomen dubium due, in part, to the imperfect nature of its holotype. Discriminant function analysis (DFA) of dental measurements found no distinction between the samples from Khodzhakul and Bissekty formations. In DFA including North American samples, the morphospace of R. asiatica completely overlaps that of the sample from the Lance Formation (Maastrichtian) of Wyoming and only partly overlaps with that of the sample from the Milk River Formation (Santonian) of Alberta.

Erratum: Non-marine fishes of the late Santonian Milk River Formation of Alberta, Canada – evidence from vertebrate microfossil localities

n/a

Non-marine fish of the late Santonian Milk River Formation of Alberta, Canada – evidence from vertebrate microfossil localities

The diversity of fishes from the late Santonian Milk River Formation is investigated using a combined taxonomic/morphotype approach. Twenty-one taxa are present, including four elasmobranchs, seven basal actinopterygians, and of ten teleosts. The Milk River fish assemblage is more similar to assemblages from southern Utah than it is to the late Campanian assemblage of Alberta in the presence of the elasmobranch Lonchidion and a member of amiid subfamily Vidalamiinae, the relatively high abundance of the ostariophysan teleost U3/BvD, and the absence of sturgeon, Holostean A, Holostean B, and Coriops. This similarity is hypothesized to be the result of a northern shift in the distribution of these taxa during times of high global temperature, resulting in the presence of a “southern” faunal assemblage in Alberta during the late Santonian. In the relative abundance patterns of major groups of fish, the Milk River Formation assemblage is similar to late Campanian assemblages and different from those of late Maastrichtian in that amiids and lepsisoteids are of relatively low abundance. The abundance of acanthomorph teleosts in the Milk River Formation is similar to that of contemporaneous assemblages from Utah, which supports a pattern of increasing abundance of acanthomorphs from their first occurrence in non-marine vertebrate assemblages of the Western Interior in the Coniacian through to the end of the Cretaceous.

Dinosaur eggshells from the Santonian Milk River Formation of Alberta, Canada

The North American fossil record of dinosaur eggshells for the Cretaceous is primarily restricted to formations of the middle (Albian–Cenomanian) and uppermost (Campanian–Maastrichtian) stages, with a large gap in the record for intermediate stages. Here we describe a dinosaur eggshell assemblage from a formation that represents an intermediate and poorly fossiliferous stage of the Upper Cretaceous, the Santonian Milk River Formation of southern Alberta, Canada. The Milk River eggshell assemblage contains five eggshell taxa: Continuoolithus, Porituberoolithus, Prismatoolithus, Spheroolithus, and Triprismatoolithus. These ootaxa are most similar to those reported from younger Campanian–Maastrichtian formations of the northern Western Interior than they are to ootaxa reported from older middle Cretaceous formations (i.e., predominantly Macroelongatoolithus). Characteristics of the Milk River ootaxa indicate that they are ascribable to at least one ornithopod and four small theropod species. The taxonomic affinity of the eggshell assemblage is consistent with the dinosaur fauna known based on isolated teeth and fragmentary skeletal remains from the formation, although most ornithischians and large theropods are not represented by eggshell. Relative to the Milk River Formation eggshell, similar oospecies occurring in younger Cretaceous deposits tend to be somewhat thicker, which may reflect an increase in body size of various dinosaur lineages during the Late Cretaceous.

A case study showing the value of multioffset synthetic seismograms in seismic data interpretation

A baseline 3D3C seismic survey was acquired in May 2014 at a Field Research Station in Southern Alberta, Canada, which is the site of experimental [Formula: see text] injection into an Upper Cretaceous sandstone at approximately 300 m depth. We have created synthetic seismograms from sonic and density logs to identify reflectors seen on the processed seismic data. The high-amplitude positive response (peak) at the top of the Upper Cretaceous Milk River Formation sandstone on the normal incidence PP synthetic seismogram does not match the response seen on the migrated PP seismic data, which is a very low amplitude peak. For such a high impedance, low Poisson’s ratio sandstone, the Zoeppritz equations predict a high-amplitude reflection coefficient at zero offset, then a decrease in amplitude, and even a change in polarity with increasing source-receiver offset. To match the stacked seismic data better, we have created offset synthetic seismograms using P- and S-wave sonic logs and density logs. The character of the top Milk River reflection on the seismic data stacked using all offset traces resembles that observed on the stacked offset synthetic seismogram, which is a similar low-amplitude peak. The character of the top Milk River reflection on the seismic data stacked using only near-offset traces to 250 m looks like that seen on the normal incidence synthetic seismogram.

A unified hydrogeological conceptual model of the Milk River transboundary aquifer, traversing Alberta (Canada) and Montana (USA)

A conceptual model of the transboundary Milk River Aquifer (MRA), extending across the Canada–USA border, was developed based on literature, focused fieldwork and a three-dimensional geological model. The MRA corresponds to the Virgelle Member of the Milk River Formation (Eagle Formation in Montana, USA) and it is an important groundwater resource over a large area (25,000 km2). The Virgelle outcrops near the international border and along the Sweet Grass Arch in Montana. The down-gradient limit of the MRA is the unconformity separating the Virgelle from the gas-bearing sandy shale of the Alderson Member. The MRA is confined above by the Pakowki/Claggett Formations aquitards and below by the Colorado Group aquitard. The MRA contains higher transmissivity areas resulting in preferential flowpaths, confirmed by natural geochemical tracers. Tritium and 14C delineate restricted recharge areas along the outcrops on both sides of the international border. Drastic decreases in horizontal hydraulic gradients indicate that the Milk River intercepts a large proportion of groundwater flowing to the north from the recharge area. Downgradient of the Milk River, groundwater movement is slow, as shown by 36Cl residence times exceeding 1 Ma. These slow velocities imply that groundwater discharge downgradient of the Milk River is via vertical leakage through the Colorado Group and upward along buried valleys, which act as drains and correspond to artesian areas. When confined, the MRA contains a fossil groundwater resource, not significantly renewed by modern recharge. Groundwater exploitation thus far exceeds recharge, a situation requiring properly managed MRA groundwater depletion.

Three-dimensional unified geological model of the Milk River Transboundary Aquifer (Alberta, Canada – Montana, USA)

The Milk River Transboundary Aquifer (Canada–USA) has been so intensively used over the 20th century that concerns have risen about the durability of this resource since the mid-1950s. This aquifer actually corresponds to the middle Virgelle Member of the Upper Cretaceous Milk River Formation (called Eagle Formation in Montana). To assess the conditions needed for a sustainable use of the aquifer, a comprehensive and unified portrait of the aquifer is needed across its international boundary. The stratigraphic framework and geometry of geological units on both sides of the international border were thus unified in a 50 000 km2three-dimensional (3D) geological model. The Virgelle Member is 0–60 m thick and it subcrops near the border and along both sides of the Sweetgrass Arch. It dips away from the subcrop areas in a semi-radial pattern. The Medicine Hat gas field hosted by the Alderson Member (Alberta), which is separated from the other members by a regional unconformity, and the Tiger Ridge gas field near the Bears Paw Mountains (Montana) limit the extent of the aquifer. The unified 3D geological model forms the necessary basis for conceptual and numerical hydrogeological models of the Milk River Aquifer.

A new tribotherian (Mammalia, Boreosphenida) from the late Santonian to early Campanian upper Milk River Formation, Alberta

A new tribotherian mammal, Tirotherium aptum gen. et sp. nov., is described from the late Santonian to early Campanian upper Milk River Formation of Verdigris Coulee, southern Alberta, Canada. The new mammal is known only from isolated teeth, five upper and three lower molars. The upper molars represent two or possibly three pre-ultimate loci and are marked by reduction and loss of the stylar shelf anteriorly, loss of the stylocone, a paracone that is larger than the metacone, weakly developed conules, a low, small protocone, and specialized postvallum single-rank shear. The lower molars probably represent two pre-ultimate loci and are characterized by an anteriorly positioned paraconid, trenchant paracristid, small, posterolingual metaconid, a distal metacristid, broadly open trigonid angle, and a short, basined talonid in which the hypoconulid is closer to the entoconid than to the hypoconid. The molars of Tirotherium most closely resemble those of Picopsis Fox, 1980, a tribotherian that also occurs in the upper Milk River Formation, but the molars of Tirotherium are significantly larger than those of Picopsis. Nonetheless, Tirotherium aptum is best classified in the Picopsidae, a boreosphenidan family of tiny mammalian faunivores of uncertain relationships to other tribotherians, and displaying a unique mosaic of primitive and derived characters.

New leptoceratopsids from the Upper Cretaceous of Alberta, Canada

Two new leptoceratopsid neoceratopsians are described based on partial dentaries collected from the Dinosaur Park (Campanian) and Milk River (Santonian) formations of Alberta. The new Campanian taxon has a unique dentary tooth shape not shared by other leptoceratopsid taxa, which has implications for the evolution of the Leptoceratopsidae. The Santonian specimen represents the oldest known leptoceratopsid (∼83 Ma), and probably represents the smallest adult-sized ceratopsian known from North America.

Upper Cretaceous sulcate pollen from the Timerdyakh Formation, Vilui Basin (Siberia)

One inaperturate and 16 monosulcate pollen types are described from the latest Campanian to earliest Maastrichtian sediments of the Vilui basin, Siberia, using both light and scanning electron microscopy, and assigned systematically when possible to modern families or subfamilies. Despite their scant occurrence, the pollen grains show considerable diversity. Two new genera have been erected. Lasioideaecidites with two species, Lasioideaecidites hessei and Lasioideaecidites bogneri, represents the earliest record of the subfamily Lasioideae (Araceae). Aristolochiacidites with one species, Aristolochiacidites viluiensis, is assigned to the subfamily Aristolochioideae (Aristolochiaceae) and represents the first fossil pollen record of the family. A new species of Liliacidites, Liliacidites goldblattii, is closely similar to pollen of Isophysioideae (Iridaceae) and/or Doryanthaceae and is the first fossil evidence of the clade that includes these closely related families. A further nine new fossil species are described (Clavatipollenites timerdyakhensis, Liliacidites palaeofritillaria, Retimonocolpites longosucatus, R. microreticulatus, R. microrugulatus, R. lysichitonoides, R. chapmaniae, Monosulcites parvus and Arecipites tyungensis) and assigned to Chloranthaceae, Araceae (Orontioideae, ?Pothoideae), Liliaceae, ?Hypoxidaceae and Arecaceae. The Hydatellaceae (Nymphaeales) may also be present in the palynoflora of the Vilui Basin, represented by pollen similar to M. rivularis Braman from the Santonian to Campanian (Upper Cretaceous) Milk River Formation, Canada.

A new articulated freshwater fish (Clupeomorpha, Ellimmichthyiformes) from the Horseshoe Canyon Formation, Maastrichtian, of Alberta, CanadaThis article is one of a series of papers published in this Special Issue on the themeAlbertosaurus.

Horseshoeichthys armaserratus , gen. et sp. nov., (Clupeomorpha: Ellimmichthyiformes: Sorbinichthyidae) is described from the Horseshoe Canyon Formation (Maastrichtian), Albertosaurus bonebed locality, Alberta, Canada. Horseshoeichthys armaserratus is classified as an ellimmichthyiform based on the following characters: the presence of a sixth infraorbital with a sensory canal that leads to the fifth infraorbital, absence of a supraorbital bone, subrectangular predorsal scutes, parietals in contact with each other at the midline, and two supramaxillae. The specimen is classified in the Sorbinichthyidae Family as it has abdominal ribs articulating in pits on the centra, posterior spines on predorsal scutes, and absence of a median spine on predorsal scutes. A new genus and species is proposed based on the presence of (anteriorly) Y-shaped mesethmoid, supraorbital, subrectangular predorsal scutes with coarse, rounded serrae on the posterior margin and a large anterior projection, scales with serrae on the circuli, and two postcleithra. This specimen represents the first freshwater ellimmichthyiform from the Upper Cretaceous of North America and the highest paleolatitude (59°N) occurrence known for the family. Furthermore, the dentary and centra have distinctive morphologies that are matched by specimens in microvertebrate localities from three underlying formations, including the Milk River Formation (Santonian), which indicates at least a 14 million year history for this lineage.

Diversity and variation of theropod dinosaur teeth from the uppermost Santonian Milk River Formation (Upper Cretaceous), Alberta: a quantitative method supporting identification of the oldest dinosaur tooth assemblage in Canada

The Santonian Deadhorse Coulee Member of the Milk River Formation preserves the oldest dinosaur body fossils found in Alberta. However, vertebrate remains consist almost exclusively of isolated elements and microvertebrate assemblages. Here, 1572 relatively complete shed non-avian theropod teeth from 20 localities in the Deadhorse Coulee Member are measured and analyzed to assess species diversity. Teeth are referred to or similar to Tyrannosaurinae indet., cf. Richardoestesia gilmorei , cf. Richardoestesia isosceles , Dromaeosauridae indet., Dromaeosaurinae indet., Velociraptorinae indet., and cf. Paronychodon lacustris . For the taxa identified, the large sample size allows for the assessment of their range of variation and accurate identification, without the benefit of comparable material of this age. Multivariate statistics, including a principal component analysis and a canonical variate analysis, provide reasonable separation of all taxa, although better results are achieved by separate analyses based on qualitative observations of denticle shape. The best results of the canonical variate analysis identified 96.0% of specimens correctly. This corroborates the qualitative identification of specimens and illustrates a valid way of evaluating diversity in areas and formations from which no described jaw material is known.

Santonian to ?earliest Campanian (Late Cretaceous) fungi from the Milk River Formation, Southern Alberta, Canada

During a detailed palynological study of the Milk River Formation in southern Alberta, Canada, a diverse and well‐preserved assemblage of fungi including dispersed fungal spores and isolated fragments of microthyriaceous fruiting bodies were encountered and these are described. Based on ammonites, magnetostratigraphy, and palynomorphs, these strata are dated as latest Santonian to ?earliest Campanian (Late Cretaceous). This palynomorph assemblage represents one of the few documented occurrences of fungi of this age. Eighteen genera of fungal spores and three of microthyriaceous thyriothecia, comprising 45 species are recorded. The fungal assemblage is characterized by species of toruloid spores in chains, and cylindrical spores belonging to Dicellaesporites, Diporicellaesporites, Pluricellaesporites, Reduviasporonites, and Scolecosporites that mainly occur as saprophytes today. Helicoid spores belonging to Involutisporonites and Paragranatisporites are also represented and these generally occur in marshy and swamp‐like conditions in an open environment mainly characterized by taxodiaceous forest. Microthyriaceous thyriothecia are represented by several specimens of Asterothyrites menonii, Phragmothyrites eocaenicus, and Trichothyrites sp. Although the assemblage consists of diverse fungal taxa, the dispersed spores and fruiting structures are not abundant, and only a few scattered specimens for many species were recorded. One reason for the low frequency may have been the unfavorable paleoenvironmental conditions which prevailed in these marine to nearshore terrestrial depositional settings. In spite of the low abundances, it is thought that documentation of the fungal occurrences in these strata will be of importance in future biostratigraphic and evolutionary studies of fungi. Many of the fungal species encountered are distinctive and probably restricted stratigraphically, being different from those recorded from the better documented Paleogene/Neogene assemblages, and may prove to be biostratigraphically useful.

SANTONIAN TO ?EARLIEST CAMPANIAN (LATE CRETACEOUS) FUNGI FROM THE MILK RIVER FORMATION, SOUTHERN ALBERTA, CANADA

Abstract During a detailed palynological study of the Milk River Formation in southern Alberta, Canada, a diverse and well-preserved assemblage of fungi including dispersed fungal spores and isolated fragments of microthyriaceous fruiting bodies were encountered and these are described. Based on ammonites, magnetostratigraphy, and palynomorphs, these strata are dated as latest Santonian to ?earliest Campanian (Late Cretaceous). This palynomorph assemblage represents one of the few documented occurrences of fungi of this age. Eighteen genera of fungal spores and three of microthyriaceous thyriothecia, comprising 45 species are recorded. The fungal assemblage is characterized by species of toruloid spores in chains, and cylindrical spores belonging to Dicellaesporites, Diporicellaesporites, Pluricellaesporites, Reduviasporonites, and Scolecosporites that mainly occur as saprophytes today. Helicoid spores belonging to Involutisporonites and Paragranatisporites are also represented and these generally occur i...

Permeability anisotropy and heterogeneity of a sandstone reservoir analogue: An estuarine to shoreface depositional system in the Virgelle Member, Milk River Formation, Writing-on-Stone Provincial Park, southern Alberta

Abstract In this study we investigate the patterns of permeability anisotropy and multi-directional heterogeneity of permeability structure based on a large number of lithofacies-specific horizontal and vertical cylindrical plugs from Upper Santonian Virgelle Member sandstones at Writing-on-Stone Provincial Park, southern Alberta. Diagnostic lithofacies of sandstone-rich, upward-coarsening siliciclastic marine shorefaces and overlying estuarine coastal plain are very common in the geological record. Hence, corresponding permeability distributions are thought to be a good analogue for broadly similar lithofacies and marginal marine successions known to make-up important petroleum reservoirs in Alberta and worldwide (e.g. Belly River Formation, Misoa Formation, Brent Group). The use of permeability distribution types and vertical-to-horizontal permeability ratios (kv/kh) derived in this study can be readily transferred and tested on reservoir models in the subsurface. Highlighted among the results is the relative homogeneity of vertical permeability (kv) distributions that may arise from the baffling effect of laminar, current-induced fabrics acting as filter. Cross-laminar fluid flow in such facies is expected to form a more uniform and efficient displacement front, bypassing fewer pores relative to a heterogeneous, 'fingering' layer-parallel flow profile. Overall, the permeability anisotropy in major lithofacies is low, with mean ratios kv/kh of 0.7–1, consistent with the absence of thick, continuous clay laminae, and only minor baffling by thin, discontinuous carbonaceous laminae or bands of mud chips. The ratio kv/kh of upper shoreface and channel lithofacies varies from 0.25 to > 2, but these lithofacies are all, to some degree, laminated at a scale of 1–30 mm that is smaller than the size of plugs. Hence, the magnitude of kv/kh is obscured due to preferred orientation of laminae and the interlaminar contrast in horizontal permeability (kh). In the Virgelle Member and laminated reservoir sandstones in general, the plug-scale kv/kh ratio can only rarely be considered truly representative of permeability anisotropy, and improved definition of subvertical, cross-layer flow requires distinction of anisotropy at a smaller scale.

Santonian (late Cretaceous) mammals from the John Henry Member of the Straight Cliffs Formation, Grand Staircase-Escalante National Monument, Utah

ABSTRACT The first North American mammals of definite Santonian age are described from the John Henry Member of the Straight Cliffs Formation of the Grand Staircase-Escalante National Monument of southern Utah. The fauna includes the oldest documented record of the multituberculates Mesodma (Mesodma sp., cf. M. minor and Mesodma sp.), the Cimolomyidae (?Cimolomys sp.), the tribotheres Potamotelses sp., Picopsis sp., and the marsupial Varalphadon sp. Also present in the fauna is the multituberculate Cimolodon foxi (formerly only known from the Judithian), other cimolodontids (Cimolodon spp., ?Cimolodon sp.), Cedaromys sp., cf. C. hutchisoni, Cedaromys sp., the symmetrodont Spalacotheridium sp., and the marsupials Alphadon sp., cf. A. halleyi, and a stagodontid(?). The fauna has close affinities with the fauna of the Milk River Formation of Alberta, Canada, and may indicate that the Milk River fauna is of latest Santonian age rather than early Campanian.

A new stratigraphic framework for the Upper Colorado Group (Cretaceous) in southern Alberta and southwestern Saskatchewan, Canada

ABSTRACT Extensive marine shales and shallow-marine sandstones of the Cretaceous Upper Colorado Group represent one of the last thick, informally named, stratigraphic intervals in the southern part of the Western Canada Sedimentary Basin. Two regionally-mappable formations in southern Alberta and southwestern Saskatchewan are introduced: The Carlile Formation (Turonian) overlying the Second White Specks Formation, and the Niobrara Formation (Coniacian-Santonian) underlying the Milk River Formation. Both names are extensions of lithologically-similar and laterally-equivalent strata in adjacent parts of the Interior Seaway in Canada and the United States. The boundary between the Carlile and Niobrara formations is recognized at a distinct zone (3 to 15 m thick) of 17 bentonites in the lower part of the Niobrara Formation, whereof two are argon-argon dated to 89.19 (± 0.51) and 89.40 (± 0.31). Regional variations in lithology, petrophysics and geochemistry of the two new formations make it possible to further subdivide these into formal subunits. The Carlile Formation is subdivided into the informal lower, middle and upper units. The Niobrara Formation is formally subdivided into three mappable members in ascending order: the shaly, non-calcareous Verger Member, the sandstone-rich Medicine Hat Member, and the shaly calcareous First White Specks Member. An outcrop reference section of the Carlile Formation is designated at Deer Creek (east of West Butte) in the Sweetgrass Hills of north-central Montana. The single core cut from the Carlile Formation in southern Alberta and southwestern Saskatchewan is located at 13-20-17-7W4 and includes twelve metres of the upper part of the formation. It is used as a reference section for the boundary between the Carlile and Niobrara formations. A reference core of the Niobrara Formation is located at 4-16-22-15W4, which also is designated as the type section of the Verger, Medicine Hat and First White Specks members. An outcrop section at the Ghost River Dam Spillway, west of Calgary, serves as the outcrop reference section for the Medicine Hat Member and is correlated to the subsurface using wireline logs and foraminiferal biostratigraphy. End_Page 304------------------------

Depositional environments and stratigraphic architecture of the Late Cretaceous Milk River and Eagle formations, southern Alberta and north-central Montana: relationships to shallow biogenic gas

ABSTRACT The Milk River and Eagle formations in southern Alberta and north-central Montana form the core of the first Upper Cretaceous clastic wedge in this part of the Western Interior Foreland Basin. The wedge is a host for shallow biogenic gas in both Canada and the USA. Facies analysis in outcrop and cores shows two overall marine progradational successions with markedly differing depositional environments. The lower, Virgelle succession, is dominated by swaley and hummocky cross-stratification and represents a storm-dominated shoreface deposit. The upper, Upper Eagle succession, is dominated by a high mudstone content and has tidal indicators. Importantly, no abundant swaley or hummocky cross-stratifications can be found within the Upper Eagle succession, indicating deposition under dominantly fair-weather conditions in a restricted, probably deltaic environment. Down-dip correlation with 2760 wireline logs reveals that each succession consists of a series of three progradational allomembers separated by minor transgressions. The three Virgelle allomembers step consecutively farther basinward and were terminated by a regional transgression. The three Upper Eagle allomembers also step seaward in a shingled manner and were partly drowned before being terminated by the regional Pakowki/Claggett transgression. Production data indicates that only the Upper Eagle A, B and, when present, C allomembers are productive in the Bearpaw Mountains area. All Virgelle allomembers commonly test wet, probably due to a large amount of inter-connectivity among the coastal sandstones of the allomembers, which enabled the migration of hydrocarbons up structural dip. Unconventional reservoirs such as the Alderson Member of the southern Alberta Milk River Gas Pool are time equivalent to the Upper Eagle allomembers. Because deltaic sedimentation with little or no storm activity prevailed during Upper Eagle time, the Alderson fine-grained rocks could be the result of a plume of sediment of deltaic origin (pro-delta) being deposited on the shelf during prevailing fair-weather conditions. End_Page 155------------------------

Revision of the dinosaur Stegoceras Lambe (Ornithischia, Pachycephalosauridae)

ABSTRACT Stegoceras (sensu lato) has been the recipient of a number of species. Most have been placed into synonymy with S. validum, some have been transferred to other genera (i.e., Gravitholus, Ornatotholus), whereas others have been interpreted as being sexual dimorphs. A parsimonious phylogenetic analysis using 49 characters, which include 9 new cranial characters based on pachycephalosaur frontoparietal domes, now permits a revision of the genus Stegoceras. This analysis concludes that Stegoceras validum (sensu stricto) is a primitive, incipiently-domed pachycephalosaur that is characterized by a well-developed squamosal shelf and open supratemporal fossae. It is the sister taxon to the fully-domed Pachycephalosaurinae. The taxon Ornatotholus browni has a parietal and displays features identical to S. validum and is therefore a subjective junior synonym. Specimens consisting of flat, paired frontals are immature individuals of S. validum. The holotype of “Stegoceras” lambei, and all morphotypes of this species, are unique in the construction of the posterior squamosal region and are placed in a new genus, Colepiocephale. Colepiocephale lambei is known solely from the Foremost Formation and is the oldest diagnostic pachycephalosaur from North America. An incomplete, and indeterminate, skull from the upper part of the Milk River Formation (upper Santonian) has the distinction of being the oldest known North American pachycephalosaur. “S.” sternbergi lacks a posterior squamosal shelf and has squamosals directed laterally and is placed in a new genus, Hanssuesia, as H. sternbergi. Although the taxon Gravitholus albertae displays some characters (i.e., retention of the posterior squamosals separated by a distinct median parietal) and is similar to H. sternbergi in some respects, the holotype is too incomplete for any definitive diagnosis and thus it is considered a nomen dubium. Sphaerotholus is a subjective junior of Prenocephale and the species Sphaerotholus buchholtzae is a subjective junior synonym of Prenocephale edmontonensis. The taxa Prenocephale brevis, P. edmontonensis, and P. goodwini, form a monophyletic clade with monotypic Asian taxa Prenocephale prenes and Tylocephale gilmorei as an unresolved sister group. They are united by the possession of a distinct row of nodes on the squamosal and parietale. Tylocephale, if valid, is interpreted as the sister taxon to the Prenocephale clade. Stenotholus kohleri is formally recognized as a junior synonym of Stygimoloch spinifer. Based on the presence of hypertrophied nodes Stygimoloch spinifier and Pachycephalosaurus wyomingensis are united in the clade Pachycephalosaurini new taxon. Small-to-medium size Late Cretaceous pachycephalosaurs were rather diverse, especially during the late Campanian (ludithian), and many species coexisted during Campanian and Maastrictian times. Ancestry and directionality of dispersal (in part) of the North American and Asian taxa remains uncertain and certainly antedates Campanian time.