Belly River Group Research Articles

Stratigraphic architecture of the Belly River Group (Campanian, Cretaceous) in the plains of southern Alberta: Revisions and updates to an existing model and implications for correlating dinosaur-rich strata.

The Upper Cretaceous (Campanian) Belly River Group (BRG) of southern Alberta has a complex internal stratigraphic architecture derived from differential geometries of its component formations that resulted from regionalized tectonic influences and shifting source areas. A full understanding of BRG architecture has been compromised heretofore by a limited understanding of subsurface data in southwestern- and southeastern-most Alberta. In this study outcrop exposures throughout southern Alberta are tied to reference well logs and subsurface cross-sections allowing a more precise understanding of BRG architecture and how it relates to well-known vertebrate fossil producing areas. Modifications to an existing stratigraphic model of the BRG show that the Oldman and the Dinosaur Park formations have reciprocal north-to-south wedge-shaped geometries and a diachronous contact that become prominently expressed south of Twp 12. The updated model also demonstrates that the Oldman Formation thickens stratigraphically up-section to the south, and that the Foremost-Oldman contact is, essentially, a datum across much of southern Alberta. Identification of the Oldman Formation in the subsurface remains based on its relatively high gamma-ray response in mudstone successions, but it is also recognized that many of its sandstones exhibit relatively low gamma-ray responses like those in underlying and overlying formations. Nomenclature and subdivisions of the Oldman Formation are revised to accommodate this updated understanding, and modifications are also made to the definition of the Judith River-Belly River discontinuity, a newly recognized surface that marks the onset of accommodation and eustatic rise in sea-level in the northern Western Interior Basin at ~76.3 Ma.

The “Judith River−Belly River problem” revisited (Montana-Alberta-Saskatchewan): New perspectives on the correlation of Campanian dinosaur-bearing strata based on a revised stratigraphic model updated with CA-ID-TIMS U-Pb geochronology

Terrestrial strata of the Judith River−Belly River wedge, widely exposed in the plains of north-central Montana, southern Alberta, and southwestern Saskatchewan, were pivotal in early stratigraphic investigations of the Western Interior of North America and are renowned to this day for their spectacular preservation of Late Cretaceous fossils, most notably dinosaurs. Correlation of the Judith River Formation in Montana with the Foremost, Oldman, and Dinosaur Park Formations (= Belly River Group) in Canada has been challenging for a variety of reasons, including lithostratigraphic complexities, legacy bentonite ages of limited comparability, and distinctly different stratigraphic models on opposite sides of the international border. An updated model calibrated with U-Pb zircon ages provides an improved framework for stratigraphic analysis. New geochronology indicates that the Oldman−Dinosaur Park discontinuity in Dinosaur Provincial Park correlates in age with the mid-Judith discontinuity in the Judith River Formation in Montana, which is interpreted as an expansion surface linked to a major pulse of accommodation and onset of the Bearpaw transgression at ca. 76.3 Ma. The regionally expressed shift in alluvial facies marking the mid-Judith discontinuity can be traced in well logs from Montana to southern Canada, where it loses distinction and transitions to a subsurface signature typical of the Oldman−Dinosaur Park discontinuity, which in turn can be traced north to Dinosaur Provincial Park and beyond. Across this expanse, both discontinuities parallel the Eagle/Milk River shoulder at approximately the same stratigraphic height, confirming their chronostratigraphic significance. These findings have clear implications for regional correlation and the evolution of alluvial depositional systems in a foreland basin setting, and they afford an opportunity to evaluate existing interpretations and advance understanding of the stratigraphy and paleontology of the Judith River−Belly River wedge. The term “Judith River−Belly River discontinuity” should be used henceforth to refer to the chronostratigraphically significant stratal discontinuity that subdivides the Judith River−Belly River wedge throughout the plains of north-central Montana, southern Alberta, and southwestern Saskatchewan.

Problematic putative pachycephalosaurids: Synchrotron µCT imaging shines new light on the anatomy and taxonomic validity of Gravitholus albertae from the Belly River Group (Campanian) of Alberta, Canada

The taxonomic validity of the holotype and sole specimen of the pachycephalosaurid Gravitholus albertae (TMP 1972.027.0001) from the Belly River Group (Alberta, Canada), remains unresolved forty years after its first description. The diagnosis for this species is tenuous at best and extensive cranial fusion has prevented a thorough description and taxonomic referral of TMP 1972.027.0001. We used synchrotron µCT imaging to identify fused sutures and segment the individual elements that comprise TMP 1972.027.0001. This allowed for a detailed description of the specimen in a more thorough comparative framework with other known pachycephalosaurid specimens. Using new observations of contacts between cranial elements, the morphological distinction of TMP 1972.027.0001 from other Belly River Group pachycephalosaurids was tested with bivariate and multivariate morphometric analyses. TMP 1972.027.0001 is morphologically consistent as an end-stage semaphorant of Stegoceras validum. Furthermore, we find no taxonomically significant morphometric distinctions between Gravitholus albertae, Hanssuesia sternbergi, and Stegoceras validum, and propose the former two are synonymous with the later. Large Stegoceras validum frontoparietals show statistically significant dimorphism in the thickness of the frontonasal boss, which is not apparent amongst juvenile and subadult specimens. Pathologies consistent with intraspecific combat (“headbutting”) appear restricted to frontoparietal domes with proportionally taller frontonasal bosses, and suggests that the two morphs represent sexual dimorphs, rather than separate species. Foraminacephale brevis and Stegoceras validum are the only named pachycephalosaurids recognised in the Dinosaur Park Formation. The stratigraphic and temporal range of Stegoceras validum is extended into the underlying Oldman Formation. Pachycephalosaurid diversity in the Campanian is reduced as a result of these revised taxonomic hypotheses. A revised phylogenetic character matrix, recognising taxonomic synonymies and ontogenetically dependent character states results in a largely unresolved Pachycephalosauria.

The oldest occurrence of brachylophosaurin hadrosaurids in Canada

Hadrosaurids are a diverse and widely distributed group of ornithischian dinosaurs that are particularly well represented in the upper Campanian Dinosaur Park Formation of the Belly River Group of Alberta. However, the origin of this hadrosaurid diversity in Alberta is poorly understood, as the lower Campanian terrestrial deposits of the underlying Oldman and Foremost formations of the group have produced comparatively few body fossils. Here we provide the first description of a partially articulated hadrosaurid and hadrosaurid material from a bonebed from the Foremost Formation and refer it to the brachylophosaurin Probrachylophosaurus sp. indet. The material represents the oldest occurrence of Brachylophosaurini in Alberta and the oldest known hadrosaurid diagnostic to the genus level from Canada. In Alberta, Hadrosaurinae display a distinct pattern of replacement with the tribes Brachylophosaurini and Kritosaurini being confined to the lower to middle Campanian strata (below the marine Bearpaw Formation) and replaced above the Bearpaw Formation by members of Saurolophini (Prosaurolophus, Saurolophus) and Edmontosaurini (Edmontosaurus), with the latter clade persisting to the end of the Maastrichtian. Although the worldwide stratigraphic distribution of the Hadrosaurinae is complex, this pattern generally holds true for northern Laramidian hadrosaurine tribes, suggesting that their pattern of evolution and replacement may be driven by some common underlying factor such as an environmental response to fluctuations in the margins of the Western Interior Seaway due to sea level change.

Evolution of the Western Interior Seaway in west-central Alberta (late Campanian, Canada): Implications for hydrocarbon exploration

This study presents the first integrated, high-resolution stratigraphic analysis of a large area of the Cretaceous Western Interior Basin in Alberta (western Canada), providing new tools to discriminate sedimentary processes and stratigraphic patterns of transgressive-regressive (T-R) cycles in correlative marine and non-marine domains. We integrate gamma ray well-log analysis, measured sections, and paleontological data to determine sediment accumulation and distribution during the second-order T-R cycle of the late Campanian Western Interior Seaway over a previously unstudied area encompassing approximately 97,000 km2 of the Alberta foreland basin. The Bearpaw Formation, historically regarded as the product of a single transgression, is shown to include two T-R cycles, whose timing is constrained by new chronostratigraphic data that provides an unprecedented resolution (~200 kyr) for the Cretaceous of western North America. Seven reference stratigraphic markers were mapped across the study area from the marine deposits into the fluvial domains. 3D-modelled stratigraphic surfaces and stratigraphic intervals resulted in isopach maps for consecutive systems tracts, allowing detailed interpretations of their architecture and patterns of sediment accumulation. Our analysis provides paleogeographic maps for the Western Interior Seaway, focusing primarily on the evidence of the paleo-coastlines during the documented cycles. The distribution of fine-grained, primarily marine, sediments resulted in an effective seal for hydrocarbon accumulation in the Belly River Group. Further oil migration upsection, within the Edmonton group, was prevented by the occurrence of these sealing units. Data support the interpretation that eustasy provided the main control on the evolution of the Western Interior Seaway during the late Campanian.

Stratigraphy and depositional environments of the Belly River Group (Campanian) in southwestern Saskatchewan, Canada

Stratigraphy and depositional environments of the Belly River Group (Campanian) in southwestern Saskatchewan, Canada

CANADA'S FIRST KNOWN DINOSAURS: PALAEONTOLOGY AND COLLECTING HISTORY OF UPPER CRETACEOUS VERTEBRATES IN SOUTHERN ALBERTA AND SASKATCHEWAN, 1874–1889

The early collecting history of dinosaurs and other fossil vertebrates in Western Canada during the 1870s and 1880s is poorly documented. Initial finds were made by the British North American Boundary Commission and the Geological Survey of Canada in modern Saskatchewan and Alberta but, beyond a few well-publicized examples, little is known about precisely what was found and where. Much of the collected material is now housed in the collections of the Canadian Museum of Nature in Gatineau, Quebec, and a recent survey of these historic finds allows for the first comprehensive narrative regarding their identity and procurement. The collection is heavily biased towards vertebral centra and phalanges, reflective of both taphonomic and collecting biases. Given current understanding of Upper Cretaceous assemblages of North America, ornithomimids and small theropods are overrepresented, whereas ceratopsids and ankylosaurs are underrepresented. Fossils from the Belly River Group are best represented, after repeated visits to the areas of present-day Dinosaur Provincial Park and Ross Coulee near Irvine, Alberta. Taxonomic identification of the material has yielded numerous first Canadian occurrences, in addition to some first global occurrences. The latter include the first ever occurrences of Caenagnathidae (1884) and Thescelosauridae (1889). The Upper Cretaceous fossil record of Western Canada is among the richest in the world, and has been thoroughly studied over the last century. These fossils have informed our understanding of dinosaur behaviour, taphonomy, ecology, diversity dynamics, and extinction, among other aspects. But, like the animals themselves, the story of Canada's dinosaur-hunting legacy had humble beginnings—a story that has not been fully revealed before now.

Paleocommunity mixing increases with marine transgression in Dinosaur Park Formation (Upper Cretaceous) vertebrate microfossil assemblages

AbstractVertebrate microfossil assemblages in a stratigraphic sequence often yield similar assortments of taxa but at different relative abundances, potentially indicative of marginal paleocommunity changes driven by paleoenvironmental change over time. For example, stratigraphically younger assemblages in the Dinosaur Park Formation (DPF) yield proportionally more aquatic taxa, consistent with marine transgression. However, individual deposits may have received specimens from multiple source paleocommunities over time, limiting our ability to confidently identify ecologically significant, paleocommunity differences through direct assemblage comparisons. We adapted a three-source, two-tracer Bayesian mixing model to quantify proportional contributions from different source habitats to DPF microfossil assemblages. Prior information about the compositions of separate, relatively unmixed terrestrial, freshwater, and marine assemblages from the Belly River Group allowed us to define expected taxon percent abundances for the end-member habitats likely contributing specimens to the mixed deposits. We compared the mixed assemblage and end-member distributions using 21 different combinations of vertebrate taxa. Chondrichthyan, dinosaur, and amphibian occurrence patterns ultimately allowed us to parse the contributions from the potential sources to 14 of the 15 mixed assemblages. The results confirmed a significant decline in terrestrial contributions at younger DPF sites, driven primarily by increased freshwater specimen inputs—not incursions from the adjacent marine paleocommunity. A rising base level likely increased lateral channel migration and the prevalence of freshwater habitats on the landscape, factors that contributed to increased paleocommunity mixing at younger channel deposit sites. Bayesian methods can account for source-mixing bias, which may be common in assemblages associated with major paleoenvironmental changes.

Revised geochronology, correlation, and dinosaur stratigraphic ranges of the Santonian-Maastrichtian (Late Cretaceous) formations of the Western Interior of North America.

Interbasinal stratigraphic correlation provides the foundation for all consequent continental-scale geological and paleontological analyses. Correlation requires synthesis of lithostratigraphic, biostratigraphic and geochronologic data, and must be periodically updated to accord with advances in dating techniques, changing standards for radiometric dates, new stratigraphic concepts, hypotheses, fossil specimens, and field data. Outdated or incorrect correlation exposes geological and paleontological analyses to potential error. The current work presents a high-resolution stratigraphic chart for terrestrial Late Cretaceous units of North America, combining published chronostratigraphic, lithostratigraphic, and biostratigraphic data. 40Ar / 39Ar radiometric dates are newly recalibrated to both current standard and decay constant pairings. Revisions to the stratigraphic placement of most units are slight, but important changes are made to the proposed correlations of the Aguja and Javelina formations, Texas, and recalibration corrections in particular affect the relative age positions of the Belly River Group, Alberta; Judith River Formation, Montana; Kaiparowits Formation, Utah; and Fruitland and Kirtland formations, New Mexico. The stratigraphic ranges of selected clades of dinosaur species are plotted on the chronostratigraphic framework, with some clades comprising short-duration species that do not overlap stratigraphically with preceding or succeeding forms. This is the expected pattern that is produced by an anagenetic mode of evolution, suggesting that true branching (speciation) events were rare and may have geographic significance. The recent hypothesis of intracontinental latitudinal provinciality of dinosaurs is shown to be affected by previous stratigraphic miscorrelation. Rapid stepwise acquisition of display characters in many dinosaur clades, in particular chasmosaurine ceratopsids, suggests that they may be useful for high resolution biostratigraphy.

First North American occurrences of Qiupalong (Theropoda: Ornithomimidae) and the palaeobiogeography of derived ornithomimids

Ornithomimid material from the Belly River Group (Campanian) of Alberta, Canada is described as sharing characters with Qiupalong henanensis from the Qiupa Formation of Henan Province, China. Derived characters and character combinations of the pubis and astragalocalcaneum were previously used to diagnose Q. henanensis and support the referral of this material to Qiupalong sp., representing the first known occurrences of Qiupalong outside of China. Qiupalong is the sixth ornithomimid taxon to be reported from the Dinosaur Park Formation and the first ornithomimid genus with a transcontinental distribution. The Alberta material represents the oldest known occurrences of Qiupalong, and a reconsideration of character evidence suggests that this genus is phylogenetically nested within other North American ornithomimids. A North American origin for Qiupalong and subsequent dispersal to Asia is proposed.

Structural architecture and glacitectonic evolution of the Mud Buttes cupola hill complex, southern Alberta, Canada

This paper presents the results of a detailed multidisciplinary study of the deformed bedrock and overlying Quaternary sediments exposed at the Mud Buttes in southern Alberta, Canada. This large, arcuate cupola hill is composed of intensely folded and thrust sandstones, siltstones and mudstones of the Cretaceous Belly River Group. Glacitectonism responsible for the development of this internally complex landform occurred at the margin of the newly defined Prospect Valley lobe of the Laurentide Ice Sheet. Analysis of the deformation structures reveals that construction of this landform occurred in response to at least two phases of south-directed ice sheet advance separated by a period of retreat. The first phase led to the formation of a forward propagating imbricate thrust stack leading to polyphase deformation of the Belly River Group. D1 thrusting led to the detachment of thrust-bound slices of bedrock which were accreted to the base of the developing imbricate stack. This process resulted in the structurally higher and older thrust-slices being progressively “back-rotated” (tilted), accompanied by D2 thrusting and folding. Further thrusting during D3 was restricted to the core of the Mud Buttes as the deforming sequence accommodated further compression imposed by the advancing ice. Minor oscillations of the ice margin led to localised brittle-ductile shearing (D4) of the bedrock immediately adjacent to the ice contact part of the thrust stack. The second phase of ice advance led to the accretion of a relatively simple thrust and folded sequence seen the northern side of Mud Buttes. The resulting composite thrust moraine was subsequently overridden by ice advancing from the NNW to form a dome-like cupola-hill. This readvance of the Prospect Valley lobe led to the formation of a thin carapace of Quaternary sediments mantling the Mud Buttes which include glacitectonite, till and an organic-rich clay-silt (?palaeosol).

Palaeoenvironmental drivers of vertebrate community composition in the Belly River Group (Campanian) of Alberta, Canada, with implications for dinosaur biogeography.

BackgroundThe Belly River Group of southern Alberta is one of the best-sampled Late Cretaceous terrestrial faunal assemblages in the world. This system provides a high-resolution biostratigraphic record of terrestrial vertebrate diversity and faunal turnover, and it has considerable potential to be a model system for testing hypotheses of dinosaur palaeoecological dynamics, including important aspects of palaeoecommunity structure, trophic interactions, and responses to environmental change. Vertebrate fossil microsites (assemblages of small bones and teeth concentrated together over a relatively short time and thought to be representative of community composition) offer an unparalleled dataset to better test these hypotheses by ameliorating problems of sample size, geography, and chronostratigraphic control that hamper other palaeoecological analyses. Here, we assembled a comprehensive relative abundance dataset of microsites sampled from the entire Belly River Group and performed a series of analyses to test the influence of environmental factors on site and taxon clustering, and assess the stability of faunal assemblages both temporally and spatially. We also test the long-held idea that populations of large dinosaur taxa were particularly sensitive to small-scale environmental gradients, such as the paralic (coastal) to alluvial (inland) regimes present within the time-equivalent depositional basin of the upper Oldman and lower Dinosaur Park Formations.ResultsPalaeoenvironment (i.e. reconstructed environmental conditions, related to relative amount of alluvial, fluvial, and coastal influence in associated sedimentary strata) was found to be strongly associated with clustering of sites by relative-abundance faunal assemblages, particularly in relation to changes in faunal assemblage composition and marine-terrestrial environmental transitions. Palaeogeography/palaeolandscape were moderately associated to site relative abundance assemblage clustering, with depositional setting and time (i.e. vertical position within stratigraphic unit) more weakly associated. Interestingly, while vertebrate relative abundance assemblages as a whole were strongly correlated with these marine-terrestrial transitions, the dinosaur fauna does not appear to be particularly sensitive to them.ConclusionsThis analysis confirms that depositional setting (i.e. the sediment type/sorting and associated characteristics) has little effect on faunal assemblage composition, in contrast to the effect of changes in the broader palaeoenvironment (e.g. upper vs. lower coastal plain, etc.), with marine-terrestrial transitions driving temporal faunal dynamics within the Belly River Group. The similarity of the dinosaur faunal assemblages between the time-equivalent portions of the Dinosaur Park Formation and Oldman Formation suggests that either these palaeoenvironments are more similar than characterized in the literature, or that the dinosaurs are less sensitive to variation in palaeoenvironment than has often been suggested. A lack of sensitivity to subtle environmental gradients casts doubt on these forces acting as a driver of putative endemism of dinosaur populations in the Late Cretaceous of North America.Electronic supplementary materialThe online version of this article (doi:10.1186/s12898-016-0106-8) contains supplementary material, which is available to authorized users.

Cranial variation and systematics ofForaminacephale brevisgen. nov. and the diversity of pachycephalosaurid dinosaurs (Ornithischia: Cerapoda) in the Belly River Group of Alberta, Canada

Studies of large-scale diversity changes and patterns of evolution can be adversely affected by a lack of understanding of alpha-taxonomy and systematics, and pachycephalosaur dinosaurs have significantly contributed to this problem. This is primarily a result of the relatively incomplete nature of the pachycephalosaur fossil record and the lack of understanding of specific and ontogenetic variation within the group. The taxon previously known as ‘Prenocephale’ brevis has been particularly problematic and has been placed in several different genera in addition to being synonymized with the species Stegoceras validum multiple times. Here we evaluate the validity of this taxon based on old and new material and using multiple, independent lines of evidence. We identify and describe the first peripheral skull elements, as well as the first juvenile and adult specimens, assignable to this taxon. These provide information on key parts of the morphology that have previously been lacking. Comparative anatomy, bivariate and multivariate morphometrics, osteohistology, and systematic analyses were used to test the hypothesis that this material is representative of a distinct taxon and to evaluate the differences between the other pachycephalosaur species of the closely related Belly River Group assemblage. No support was found for the assignment of ‘P.’ brevis to Stegoceras or Prenocephale, and thus we have erected a new genus, Foraminacephale gen. nov., to contain this species. Our data support the conclusion that the Foraminacephale brevis material represents an ontogenetic growth series of a single species distinct from St. validum. Evaluation of the two other members of the Belly River Group assemblage (Hanssuesia sternbergi and Colepiocephale lambei) was hindered by the small number of known specimens, but our results suggest that specimens currently referred to H. sternbergi may represent at least two species. We find that both ontogeny and osteohistology can vary greatly amongst even closely related species. It is therefore critical that multiple, independent lines of evidence be used to establish the ontogenetic trajectories of species prior to delineation, and that results of these analyses are used to produce robust and phylogenetically informative characters for use in phylogenetic analyses.

Regional T-R sequence stratigraphy and lithostratigraphy of the Bearpaw Formation (Upper Campanian), west-central and southwestern Alberta plains

Abstract In west-central and southwestern Alberta, the lower and upper tongues of the marine mudstone-dominated Bearpaw Formation have a complex lateral relationship with marginal marine and non-marine strata of the Horseshoe Canyon and Blood Reserve formations. Wireline log, core and outcrop data permit the establishment of a regional transgressive-regressive (T-R) sequence stratigraphic framework for the upper Campanian Bearpaw strata. This provides a context within which lithostratigraphic boundaries of the Bearpaw tongues with laterally equivalent and overlying strata can be more rigorously mapped. Maximum flooding (MFS) and transgressive (TS) surfaces were defined on the basis of stratal stacking patterns and inferred lapout relationships wherever possible. Lower Bearpaw tongue and laterally equivalent strata are assigned to four regionally mappable T-R sequences. In the study area the top of the underlying Belly River Group is placed at the MFS in the lowermost of these sequences. In the overlying lower Bearpaw sequences, the TS is generally considered to coincide with the MFS within the limits of wireline log resolution, and the succession consists mainly of stacked regressive systems tracts which usually show a near-parallel internal stratal geometry. Upper Bearpaw tongue and laterally equivalent strata are assigned to three regionally mappable T-R sequences. The basal upper Bearpaw sequence has a well-developed transgressive systems tract consisting of backstepping, individually regressive parasequences, and southeast-prograding clinoform geometry is clear in each of the upper Bearpaw tongue regressive systems tracts. The lithostratigraphic upper boundaries of the lower and upper Bearpaw tongues are time-transgressive facies contacts that step up-section to the southeast (paleoseaward) through the respective regressive systems tracts. The base of the upper Bearpaw tongue steps up-section from the basal TS of the lowermost sequence in the southeast, through the overlying transgressive systems tract, to coincide with the maximum flooding surface to the northwest (paleolandward).

A vertebrate microsite from a marine-terrestrial transition in the Foremost Formation (Campanian) of Alberta, Canada, and the use of faunal assemblage data as a paleoenvironmental indicator

Vertebrate microfossil assemblages contain abundant fossil material of small and large bodied animals recruited from the local paleocommunity that can provide important information for reconstructing regional paleoecology. The Foremost Formation is the oldest unit of the Belly River Group, and records the transition from the fully marine shales of the Pakowki Formation to the non-marine strata of the relatively well-sampled Oldman and Dinosaur Park formations. Although the Foremost Formation has relatively limited exposure and vertebrate fossil material is not abundant, it does contain some important early records of major vertebrate clades from Laramidia (e.g. ceratopsids, pachycephalosaurids).Here we document a new microfossil locality from the upper part of the formation that shows a transitional assemblage containing an abundance of terrestrial forms such as dinosaurs and other sauropsids, while still showing considerable marine influence. The proportion of marine taxa at this site is reduced relative to most sites known from the Foremost Formation, though much higher than more terrestrial sites of the overlying Oldman Formation. In addition to filling a gap in our understanding of this paleoenvironmental transition, this new site preserves two particularly significant specimens: a large, complete, cephalic spine of the hybodont shark Hybodus, and jaw fragments from the ratfish Elasmodus. This represents the first known complete cephalic spine of a hybodont shark from the Cretaceous of Alberta. The ratfish jaw fragments represent the first published record of this group from the Foremost Formation, further increasing our knowledge of chondrichthyan diversity in the Western Interior Seaway. Comparisons of this site to other microsites from the Campanian of Alberta demonstrate the utility of certain key groups, such as lissamphibians and chondrichthyans, in determining paleoenvironments from Mesozoic and Cenozoic vertebrate microsite assemblage data.

TAPHONOMY OF A MONODOMINANTCENTROSAURUS APERTUS(DINOSAURIA: CERATOPSIA) BONEBED FROM THE UPPER OLDMAN FORMATION OF SOUTHEASTERN ALBERTA

Abstract The horned dinosaur Centrosaurus apertus from the Belly River Group (Campanian) is represented by multiple articulated skulls and skeletons, and is particularly notable for its occurrence in dozens of large-scale monodominant bonebeds, which have been found in the Dinosaur Park Formation across southern Alberta and Saskatchewan. Here we present a detailed taphonomic analysis of the first large-scale Centrosaurus apertus bonebed (McPheeters bonebed) from the Oldman Formation of southeastern Alberta. The McPheeters bonebed rivals the richest bonebeds in the Dinosaur Park Formation in terms of bone density and size, and the complete disarticulation of elements. The bonebed occurs in an overbank facies and is dominated by small bone clasts, suggesting that only low energy water current contributed to the formation of the bonebed before its final burial event. Patterns of taphonomic modification suggest that bones experienced little weathering, breakage, or scavenging. In turn, these conclusions are c...

Review of Stagodontidae (Mammalia, Marsupialia) from the Judithian (Late Cretaceous) Belly River Group of southeastern Alberta, Canada

Although stagodontid marsupials are among the most distinctive mammals of Late Cretaceous age in North America, there remain significant gaps in knowledge of their dental anatomy, particularly that of the stratigraphically oldest genus, Eodelphis Matthew, 1916. We report here on stagodontid specimens from the Judithian Belly River Group of southeastern Alberta, Canada, that document what was until now previously unknown parts of the upper and lower premolar dentition of Eodelphis browni Matthew, 1916 and Eodelphis cutleri (Smith Woodward, 1916). The new information further confirms the distinctiveness of both nominal species of Eodelphis, and bolsters previous hypotheses suggesting a close phylogenetic relationship between E. cutleri and the advanced stagodontid Didelphodon Marsh, 1889. Although the new specimens from Alberta suggest that the problematic holotype of “Boreodon matutinus” Lambe, 1902 is an upper third premolar, a referral to Eodelphis cannot be made with confidence and the name should continue to be treated as a nomen dubium. Lastly, we report the first occurrence of Didelphodon in strata of the Belly River Group, extending the geological age of the genus into the Judithian, co-eval with species of Eodelphis, and suggesting a significant prior and as yet undiscovered evolutionary history.

Origins of dinosaur bonebeds in the Cretaceous of Alberta, Canada

Upper Cretaceous dinosaur bonebeds are common in Alberta, Canada, and have attracted continuous scientific attention since the 1960s. Since its inception, the Royal Tyrrell Museum of Palaeontology has documented the presence of hundreds of these sites and has been involved directly in the scientific study of many tens. Because many of these bonebeds have been used to address questions about the paleobiology and paleoecology of dinosaurs, questions have arisen about bonebed origins and preservation in the Cretaceous of Alberta. This study of 260 bonebeds delineates broad paleoenvironmental settings and associations, and taphonomic signatures of assemblages as a first step in assessing patterns of dinosaur bonebed origins in the Upper Cretaceous of Alberta. Bonebeds are known predominantly from the Belly River Group and the Horseshoe Canyon, lower St. Mary River, Wapiti, and Scollard formations. In these units, bonebeds are mostly associated with river channel and alluvial wetland settings that were influenced by a subtropical to warm-temperate, monsoonal climate. Most bonebeds formed in response to flooding events capable of killing dinosaurs, reworking and modifying skeletal remains, and burying taphocoenoses. The “coastal-plain-flooding hypothesis,” proposed in 2005, suggested that many bonebeds in the Dinosaur Park Formation formed in response to the effects of recurring coastal-plain floods that submerged vast areas of ancient southern Alberta on a seasonal basis. It remains the best mechanism to explain how many of the bonebeds were formed and preserved at Dinosaur Provincial Park, and here, is proposed as the mechanism that best explains bonebed origins in other Upper Cretaceous formations across central and southern Alberta.

Dinosaur trackways from the Upper Cretaceous Oldman and Dinosaur Park formations (Belly River Group) of southern Alberta, Canada, reveal novel ichnofossil preservation style

Dinosaur tracksites recently discovered in exposures of the Belly River Group in the Milk River Natural Area (MRNA) and Dinosaur Provincial Park (DPP) of southern Alberta represent a novel type of ichnofossils. The tracks, all referable to hadrosaurs, occur as sideritic or calcareous concretions protruding above fine-grained deposits and are here termed concretionary tracks. Detailed sedimentological, petrographic, and geochemical analyses reveal that, although the MRNA and DPP tracks are of different mineralogical compositions (calcium carbonate versus siderite, respectively), they display similar internal structures (microscopic convoluted laminations) and occur in depositional settings indicative of wet paleoenvironments, where the ground was soft and water saturated. These characteristics suggest that concretionary tracks are footprint casts that formed as groundwater rich in dissolved carbonates flooded depressions left in the soft substrate. As the ponded water evaporated, minerals began to precipitate and mix with clastic material transported into the depressions, settling as finely laminated mud within the tracks and filling them either completely or partially. The geochemical composition of the precipitate would depend on the prevalent groundwater conditions (e.g., pH, dissolved carbonate and sulphate concrentrations). Cementation of the tracks occurred relatively soon after burial (<100 years), possibly in response to microbial activity and saturation by mineral-rich groundwater, and modern erosion exposed the concretionary tracks by removing the softer host unit. Recognition of this novel type of ichnofossils suggests dinosaur tracks may be more common than previously thought. Unfortunately, concretionary tracks tend to break apart rapidly when the encasing and underlying substrate erodes away, altering their diagnostic shape and rendering them indistinguishable from nonichnogenic concretions. As such, concretionary tracks may be transient ichnofossils in the badlands, explaining why they are rarely recognized.

The Bow City structure, southern Alberta, Canada: The deep roots of a complex impact structure?

Geological and geophysical evidence is presented for a newly discovered, probable remnant complex impact structure. The structure, located near Bow City, southern Alberta, has no obvious morphological expression at surface. The geometry of the structure in the shallow subsurface, mapped using downhole geophysical well logs, is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region. Detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region. Field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure. Evidence of deformation along the eastern margin of the central uplift includes thrust faulting, folding, and steeply dipping bedding. Normal faults were mapped along the northern margin of the annular region. Isopach maps reveal that structural thickening and thinning were accommodated primarily within the Belly River Group. Evidence from legacy 2-D seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure, including a series of listric normal faults in the annular region and complex faulting within the central uplift. The absence of any ejecta blanket, breccia, suevite, or melt sheet (based on available data) is consistent with the Bow City structure being the remnant of a deeply eroded, complex impact structure. Accordingly, the Bow City structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks.