Bearpaw Formation Research Articles

Scaphites of the “Nodosus Group” from the Upper Cretaceous (Campanian) of the Western Interior of North America

Scaphitid ammonites (scaphites) are common in the Upper Cretaceous Pierre Shale and Bearpaw Shale of the Western Interior of North America. We redescribe Hoploscaphites nodosus (Owen, 1852) and H. brevis (Meek, 1876) from the Baculites compressus–B. cuneatus zones of the upper Campanian. The types of both of these species were collected in the mid-19th century in what was then called Nebraska Territory, and included parts of present-day South Dakota, North Dakota, and Montana. Based on our present knowledge of the distribution of these species, the type material was probably collected from the B. compressus–B. cuneatus zones in the Pierre Shale at Sage Creek, a tributary of the Cheyenne River, Pennington County, South Dakota.Traditionally, the more robust, more coarsely ornamented scaphites (comprising the “nodosus group”) from the Pierre Shale and Bearpaw Shale were assigned to Jeletzkytes Riccardi, 1983, and the more slender, more finely ornamented scaphites were assigned to Hoploscaphites Nowak, 1911. However, our large collections of these scaphites from the Baculites compressus–B. cuneatus zones reveal a complete intergradation between the two morphological extremes, and for many specimens, the choice of genus is arbitrary. In addition, our studies of other biostratigraphic zones in the Pierre Shale and Bearpaw Shale reveal that cooccurring species of these two “genera” share more in common with each other than they do with congeneric species from other horizons. Furthermore, contrary to earlier assumptions, Jeletkytes is not endemic to the Western Interior Basin of North America and occurs, for example, in the U.S. Atlantic Coastal Plain and Europe. We thus provisionally treat Jeletzkytes as a junior subjective synonym of Hoploscaphites. This expanded definition of Hoploscaphites is consistent with present-day concepts of other scaphitid genera such as Discoscaphites Meek, 1876, and Trachyscaphites Cobban and Scott, 1964.In Hoploscaphites nodosus and H. brevis, the juvenile shell is planispirally coiled with a small umbilicus. The whorl section is initially depressed and becomes more compressed through ontogeny. The angle of the body chamber in juveniles is approximately two-thirds of a whorl. At the approach of maturity, the shell uncoils, forming a relatively long shaft and recurved hook. The ratio of whorl width to whorl height reaches a minimum value at midshaft. The apertural margin at maturity is constricted and terminates in a flared lip. Commonly, the last two or three septa, corresponding to the formation of the hook, are more closely spaced (approximated). These features indicate that the rate of growth decreased and eventually stopped at maturity (“morphogenetic countdown” associated with determinate growth). Both species of scaphites occur as dimorphs, which are referred to as macroconchs (presumably females) and microconchs (presumably males). In samples of specimens of the same species within a single concretion, macroconchs are approximately 20% larger than microconchs. In addition to size, dimorphs are distinguished by differences in shape, including the presence or absence of an umbilical bulge, the size of the umbilical diameter, the outline of the umbilical shoulder relative to that of the venter in side view, and the relative change in whorl height in passing from the mature phragmocone to the shaft of the body chamber.The holotype of Hoploscaphites nodosus, by monotypy, is UC 6381, the original of Scaphites nodosus Owen (1852: 581, pl. 8, fig. 4). Adults exhibit a range of variation in size, degree of compression, and coarseness of ornament. The exposed phragmocone occupies most of the coiled portion of the shell, and is approximately two-thirds of a whorl in angular length. Adults are large (LMAX averages 91.8 mm in macroconchs and 78.0 mm in microconchs) and ellipsoidal in side view, with a strongly recurved hook (apertural angle averages 73° in macroconchs). The ratio of whorl width to whorl heigh

Jaws of Late Cretaceous Placenticeratid Ammonites: How Preservation Affects the Interpretation of Morphology

Abstract We describe upper and lower jaws of Placenticeras Meek, 1876, from the Upper Cretaceous (upper Campanian) Bearpaw Shale and Pierre Shale of the Western Interior of North America and lower jaws of the related ammonite Metaplacenticeras Spath, 1926, from the Campanian Yasukawa Formation of Hokkaido, Japan. One lower jaw is preserved inside the body chamber of Placenticeras costatum Hyatt, 1903. The other jaws are isolated but are generally associated with fragments of placenticeratid shells. The jaws from North America are attributed to Placenticeras meeki Bohm, 1898, and P. costatum, while those from Japan are attributed to Metaplacenticeras subtilistriatum (Jimbo, 1894). All of the jaws are presumed to be from adults. The jaws of Placenticeras attain lengths of up to 95 mm. They are preserved as steinkerns with a thin film of black material, representing diagenetically altered chitin. X-ray diffraction analysis of samples of this material indicates that it consists of magnesium-rich calcite, pyri...

A NEW POLYCOTYLID PLESIOSAUR (REPTILIA: SAUROPTERYGIA) FROM THE UPPER CRETACEOUS BEARPAW FORMATION IN SASKATCHEWAN, CANADA

A new species of polycotylid, Dolichorhynchops herschelensis, is described based on a well-preserved skeleton from the Upper Cretaceous (Campanian–Maastrichtian) Bearpaw Formation of Herschel, Saskatchewan. Its small size as an adult, the sharp rise in front of the parietal crest, and the straight ilium with narrow dorsal end are diagnostic features of this taxon. This is the first polycotylid plesiosaur described from the Bearpaw Formation, deposited during the critical period when polycotylids were disappearing from the Western Interior of North America.

Magnetostratigraphic and palynostratigraphic correlation of late Campanian to early Maastrichtian strata of the Bearpaw and Horseshoe Canyon formations between the CPOG Strathmore corehole and the Red Deer Valley section, Alberta, Canada

Abstract The Horseshoe Canyon and Bearpaw formations in the Canadian Pacific Oil and Gas Strathmore (CPOG) cored well (7-12-25-25W4M) in southern Alberta were sampled for magnetostratigraphy and palynostratigraphy. The studied interval begins in the continental upper Horseshoe Canyon Formation (early Maastrichtian) and continues to the base of the marine Bearpaw Shale (late Campanian) encompassing 358 m. A total of 152 horizons were sampled for magnetostratigraphy, and 48 horizons for palynostratigraphy. Four magnetozones (31r, 32n, 32r and 33n), 26 magnetosubzones and 8 cryptozones, a subdivision of a subzone, were identified. Most of the subzones had been previously identified in the Red Deer Valley outcrops. Magnetozone 32n in the CPOG core is only 70 per cent as thick as in the Red Deer Valley. Most of this difference occurs within the interval of coals 1 to 9 of the Red Deer Valley. The palynomorphs provide important information on the ranges of a number of key species that allow comparisons to other equivalent age sites in southern Alberta. These ranges indicate that the top of the CPOG Strathmore well is considerably younger than the top of Castor well as documented in Lerbekmo et al. (2003). The palynological results support the conclusions arrived at during the paleomagnetostratigraphic studies. The paleoecological inferences show similar oscillating marine and nonmarine conditions within the core developed by previous studies of the cored interval between the top of the Dinosaur Park and basal Horseshoe Canyon formations (Wall et al., 1971; Hills & Levinson, 1975).

Diagenetic effects on the oxygen isotope composition of bones of dinosaurs and other vertebrates recovered from terrestrial and marine sediments

Assessing effects of diagenesis on oxygen isotope composition of bone is essential to its use in reconstructing habitats and lifestyles of ancient vertebrates. These effects are a matter of controversy, particularly in the case of extinct animals such as dinosaurs. To investigate the effects of diagenesis on isotopic composition of fossil bone, bone samples from both marine and terrestrial Campanian sediments from Alberta, Canada, have been analysed. The isotopic compositions of oxygen ( δ 18 O SMOW ) were determined in bones sampled from articulated skeletons of exclusively terrestrial animals recovered from the terrestrial Dinosaur Park Formation, and compared with bones from the marine Bearpaw Formation. The articulated skeleton of an exclusively terrestrial dinosaur (hadrosaur) found in marine sediments yielded similar δ 18 O values for both structural carbonate and phosphate fractions (mean δ 18 O SMOW values 22.6‰ and 16.9‰, respectively) in bone to marine reptiles (mosasaurs) recovered from the same locality (mean δ 18 O SMOW values 24.2‰ and 17.3‰, respectively). The isotopic composition of both skeletons recovered from marine sediments was significantly more positive than that of articulated hadrosaur skeletons recovered from contemporaneous terrestrial sediments (mean phosphate δ 18 O SMOW value 12.9‰), and outside the range of phosphate δ 18 O SMOW values previously reported for terrestrial dinosaur skeletons ( c . 9–14‰). These data suggest that the isotopic composition of oxygen in the phosphate and structural carbonate ions in the bone apatite was altered during diagenesis and can be used for neither palaeoclimate nor physiological reconstruction.

Terminonatator ponteixensis, a new elasmosaur (Reptilia; Sauropterygia) from the Upper Cretaceous of Saskatchewan

ABSTRACT A new elasmosaur, Terminonatator ponteixensis, gen. et sp. nov., from the Upper Campanian Bearpaw Formation of southwestern Saskatchewan, Canada, is one of the youngest plesiosaurs from the Western Interior, and one of the few plesiosaurs for which detailed osteology of both skull and postcrania is available. The skull includes the upper side of the inner ear cavity and a median portion of the roof of the braincase. This taxon is distinguished from other elasmosaurs by a combination of several osteological characters variable among elasmosaurs, as well as the unique feature of a high and pointed coronoid process. Anomalous ossification is seen in the fused or pronged gastralia, development of tubercules in metacarpale, and healed fracture of the limb. The dorsal portion of the braincase is compared with other plesiosaurs, showing a variation in plesiosaur braincase morphology. Identity of carpal and tarsal elements is examined based on recent debates on the homology of mesopodials.

Magnetostratigraphic and palynostratigraphic correlation of late Campanian strata of the Bearpaw and Horseshoe Canyon formations of the RCA Castor corehole to the Red Deer Valley, Alberta

ABSTRACT The Research Council of Alberta stratigraphic corehole near Castor, Alberta (RCA Castor 13-34-37-13W4) was cored to a depth of 181.7 m (596 ft.) in 1967. The corehole was spudded in the basal Horseshoe Canyon Formation and continued into the top of the Dinosaur Park Formation. Fifty-six samples were studied for palynology and 44 stratigraphic levels were sampled for magnetostratigraphy. An additional 3 outcrop samples were taken for magnetostratigraphy near Castor to supplement an interval of no recovery in the core. The recovered palynomorphs and their relationship to the magnetostratigraphy support regional correlations developed previously for the Red Deer Valley to Cypress Hills area of southern Alberta. The close magnetostratigraphic correlation between the Castor core and Red Deer Valley sections has enabled the recognition of polarity subchrons 32n.5n to 33n.3n in the Castor core. A hiatus of some 0.3 m.y. occurs below the Horseshoe Canyon Formation in the Castor core. The base of the Horseshoe Canyon Formation is shown to be a little younger in the Castor area than in the Red Deer Valley (32n.6n vs. 32r.1r), suggesting progradation from the west rather than northwest. The Second Castor Sandstone correlates to the Dorothy Sandstone of the Red Deer Valley. End_Page 70-------------------------

The Dorothy bentonite: an extraordinary case of secondary thickening in a late Campanian volcanic ash fall in central Alberta

The late Campanian (~73 Ma) Dorothy bentonite outcrops in the marine Bearpaw Shale for 20 km along the Red Deer River valley east of Drumheller, Alberta, and is up to 13.5 m thick. An isopach map based upon 230 sub surface and surface thickness measurements illustrates an elongated southwest–northeast lobe with maximum dimensions of about 300 km by 50 km. The volume is ~57 km3 distributed within an area of ~11 000 km2. This bentonite is the altered product of what is believed to be a short-lived Plincan-type eruption from part of the Howell Creek Instrusives in southeastern British Columbia. Fortuitous preservation of the original ash in an up to 2.5 m thick calcite-cemented tuffaceous zone near the middle of the bentonite shows the original ash to have been >99% glass shards and pumice. The remainder of the ash is a crystal component consisting mostly of plagioclase and biotite. The silica content of the isotropic glass shards of about 77%, and a refractive index of 1.503 ± 0.001, suggest a magma of rhyolitic composition.

Stratigraphy and structure of Pleistocene collapse in the Regina Low, Saskatchewan, Canada

The Regina Low is a collapse structure, formed as a result of dissolution of salt from the Middle Devonian Prairie Evaporite Formation. In this study, collapse has affected the Upper Cretaceous Pierre Shale and the Ardkenneth and Snakebite members of the Bearpaw Formation of the Montana Group; the Mennon, Dundurn, and Warman formations of the Sutherland Group; and the Floral and Battleford formations of the Saskatoon Group. A structural closure of 125–175 m approximates the thickness of the Middle Devonian Prairie Evaporite Formation. In the Early Pleistocene, about 54 m of collapse took place in eastern Regina accounting for the preservation of Snakebite Member. Major collapses of about 58 and 86 m took place in northern Regina during deposition of Middle Pleistocene, pre-Illinoian Dundurn Formation. The final major collapse of about 127 m took place in northeastern Regina between deposition of the lower and upper tills of the Floral Formation. Eighty-seven metres of this collapse took place during deposition of the Late Pleistocene, interglacial, Sangamon Pasqua Member of the Floral Formation. The Pleistocene fill in the Regina Low collapse structure suggests that collapse took place when dissolution of salt from the Prairie Evaporite Formation was accelerated by high hydraulic gradients created by the surcharge pressures of the glaciers.

Magnetostratigraphic and biostratigraphic correlation of late Campanian and Maastrichtian marine and continental strata from the Red Deer Valley to the Cypress Hills, Alberta, Canada

Two stratigraphic sequences, Red Deer Valley and Cypress Hills sections in Alberta, spanning the late Campanian to Paleocene are magnetostratigraphically and biostratigraphically characterized. The Red Deer Valley section had previously been studied, but additional material has allowed the extension of the magnetozonation downwards into the Bearpaw Formation. Higher sample resolution has allowed the identification of a number of magnetosubzones in 32n, 32r, and 33n. Four reversed magnetosubzones have been identified in magnetozone 32n in addition to the one presently recognized in geomagnetic polarity time scales. One normal subzone in 32r and one reversed subzone in 33n have also been recorded. The 32r–33n boundary is shown to occur in the Baculites cuneatus Zone. The Campanian-Maastrichtian boundary is indicated to be at or near the Baculites eliasi – Baculites baculus boundary at the top of 32n. Comparisons of the biostratigraphy between the two areas indicate that palynomorph ranges differ from data published to date, suggesting that readjustments to the previously published zonation are needed. Correlations between the two areas are possible using the magnetostratigraphic and palynological zones and allow comparisons of depositional rates.

Stratigraphy and structure of a Late Wisconsinan salt collapse in the Saskatoon Low, south of Saskatoon, Saskatchewan, Canada: an update

The Saskatoon Low is a collapse structure that formed as a result of dissolution of salt from the Middle Devonian Prairie Evaporite Formation. In this study, the collapse has affected the Upper Cretaceous Lea Park, Judith River, and Bearpaw formations of the Montana Group; the Early and Middle Pleistocene Mennon, Dundurn, and Warman formations of the Sutherland Group; and the Late Pleistocene Floral, Battleford, and Haultain formations of the Saskatoon Group. Locally, the collapse is about 180 m, which is about equal to the thickness of the salt. The first phase of collapse took place after deposition of the Ardkenneth Member of the Bearpaw Formation and before glaciation or during a pre-Illinoian glaciation. The second phase of collapse occurred during the Battleford glaciation (Late Wisconsinan). Prior to deposition of the Battleford Formation, the Saskatoon Low was glacially eroded, removing the Sutherland Group and the Floral Formation. After the glacial erosion, up to 110 m of soft till of the Battleford Formation and up to 77 m of deltaic sand, silt, and clay of the Haultain Formation were deposited in the Saskatoon Low. Lastly, the South Saskatchewan River eroded up to about 40 m into the deltaic sediment and tills before up to about 15 m of Pike Lake Formation was deposited. The Haultain and Pike Lake formations are new stratigraphic units.

Were limpets or mosasaurs responsible for the perforations in the ammonite Placenticeras?

We present evidence defending the hypothesis that specimens of the ammonite Placenticeras from the Upper Cretaceous Bearpaw Formation of southern Alberta were preyed upon by mosasaurs. Perforations linked to diagenetic modification of limpet home scars are anomalous features that are rare in comparison to true tooth marks. Point loading of the ammonite phragmocone wall by mosasaur teeth produced distinct perforations without the development of long, angular fractures. This was due to low pressure and rate of loading permitted by the unusually loose articulation of the lower jaw of mosasaurs, combined with the nearly uniform internal support of the shell wall by complexly fluted septa and their sutures. In contrast, the body-chamber, which lacked septal support (except at its posterior end), tended to shatter. Remnants of the body-chamber preserve distinctive notches that resemble tooth marks and sometimes show evidence of healing. Thorough reassessment of the perforations and associated features indicates that the large majority of perforations in shells of Placenticeras, at least those from southern Alberta, are best explained by predatory activities of mosasaurs. The currently popular hypothesis that all perforations preserved in shells of Cretaceous ammonites were produced by sediment loading and collapse of limpet home scars fails to explain many diagnostic features of the perforations. These include the shape, siting and arrangements of perforations as well as the equal puncturing of all size classes of alleged limpet home scars. The limpet hypothesis also fails to consider structural differences between shells of ammonoids and Nautilus and erroneously assumes point loading by sediment compaction. Furthermore, the extreme rarity of limpets that had the potential of excavating home scars in Placenticeras from the Upper Cretaceous Bearpaw Formation of Alberta, Canada, cannot be attributed to preservational bias and is at odds with the large number of perforated specimens known from this formation. The mosasaur-bite hypothesis, on the other hand, accommodates all of these features.

The significance of multiple causes and coincidence in the geological record: from clam clusters to Cretaceous catastrophe

Specific causes of unusual events recorded in the geological record are commonly difficult to distinguish and isolate; in some instances, event strata contain features that cannot be explained by a single causal mechanism. Unicausal hypotheses, when applied to complex problems, can lead to the misidentification, misinterpretation, and force-fitting of observations ("great expectations syndrome"). The close timing or temporal overlap of significant events, although statistically improbable on short time scales, becomes possible on long time scales. Event coincidence may occur on a wide range of scales, from local to global. On the local scale, a multiple-event interpretation is offered for both the concentration and clustering of bivalves at specific levels within the Upper Cretaceous Bearpaw Formation of southern Alberta. For this example, the relative timing of fluctuations in benthic substrate texture, oxygen concentration, abundance of planktotrophic larvae, and degree of sea-floor scouring was crucial to the formation and preservation of shell concentrations. On the sharply contrasted global scale, the implications of multiple events warrant much closer consideration than they have received hitherto in terms of major proposed causes for the Cretaceous–Tertiary (K–T) mass-extinction event: bolide impact, sea-level change, climatic change, and flood-basalt volcanism. By considering the predictable effects of these synchronous factors, both individually and in combination, a multiple-cause explanation of the K–T mass extinction emerges as entirely plausible. Certainly it needs to be considered in all future investigations of this important issue.

Magnetostratigraphy of the Canadian Continental Drilling Program Cretaceous-Tertiary (K-T) Boundary Project core holes, western Canada

The Canadian Continental Drilling Program (CCDP) Cretaceous-Tertiary Boundary Project cored three holes, to 160, 188, and 170 m in southern Alberta, Saskatchewan, and Manitoba, respectively, to intersect the Cretaceous-Tertiary boundary. One hundred and seventy to 200 samples from 50 to 60 levels were taken from each of these unoriented cores for magnetostratigraphy. Samples were subjected to both alternating field (AF) and thermal cleaning. AF demagnetization in steps of 5 mT up to 30 mT showed that samples from all three cores carried only a low-coercivity viscous overprint, which was largely removed at 10 mT. Thus, three samples from each depth were routinely demagnetized at 10 mT or 100°C. The magnetic inclination of the sample giving the smallest measuring error was chosen to indicate the polarity at that depth. With the aid of palynologic control, magnetochrons 29n to 30r inclusively, in the Ravenscrag, Frenchman, Battle, Whitemud, and Eastend formations, were identified in the Elkwater core hole, western Cypress Hills, Alberta (49°36'N, 110°18'W). The Cretaceous-Tertiary boundary is missing at the Frenchman-Ravenscrag contact. The Wood Mountain core hole (49°05'N, 106°16'W) spans the interval from magnetozone 28n to 30r in the Ravenscrag, Frenchman, Battle, Whitemud, Eastend, and Bearpaw formations. The Cretaceous-Tertiary boundary is present in 29r. The Turtle Mountain core (49°01'N, 100°33'W) has a number of significant disconformities; only magnetozones 28n, 28r, 29n, 30n, and 32n or 33n are present in the Turtle Mountain, Boissevain, and Pierre formations. At the Cretaceous-Tertiary contact near the base of the Turtle Mountain Formation, there is a hiatus of 0.6 × 106 to 0.8 × 106 years. At the Campanian-Maastrichtian contact, between the Odanah and Coulter members of the Pierre Formation, there is a time gap of at least 3 × 106 years but probably about 6 × 106 years.

Upper Cretaceous - lower Tertiary lithostratigraphic relationships of three cores from Alberta, Saskatchewan, and Manitoba, Canada

Cores from three stratigraphic test holes provide a relatively complete section from the lower Maastrichtian into the Paleocene. The hole drilled in the Cypress Hills of Alberta recovered core from the Ravenscrag Formation to the top of the Eastend Formation and demonstrated that most of the stratigraphy above the Battle Formation is assignable to the Ravenscrag Formation. The hole drilled in the Wood Mountain area of Saskatchewan recovered core from the Ravenscrag Formation to the Bearpaw Formation. This core helps explain the difficulty that has been encountered subdividing the interval between the Bearpaw and Ravenscrag formations into separate formations and provides a cored record of all the Ravenscrag coals within the Wood Mountain area. The core hole drilled at Turtle Mountain, Manitoba, recovered core from the Turtle Mountain Formation to the Pierre Formation. It demonstrates the difficulty in separating the Peace Garden and Goodlands members of the Turtle Mountain Formation, is useful as a principal reference section for the Coulter Member of the Pierre Formation, documents a disconformity within the Pierre Formation, and crosses the obscure contact between the Coulter and Odanah members.

A new specimen of Plioplatecarpus (Mosasauridae) from the lower Maastrichtian of Alberta: comments on allometry, functional morphology, and paleoecology

An incomplete specimen of the plioplatecarpine mosasaur Plioplatecarpus from the lower Maastrichtian locality of Scabby Butte (Saint Mary River Formation) closely resembles the slightly older Plioplatecarpus primaevus from Saskatchewan (Bearpaw Formation). However, quantitative and qualitative differences are observed: the vertebrae of the Scabby Butte specimen are more gracile and, although of similar shape and proportions, are relatively smaller. Since these differences cannot be readily explained by allometry or sexual dimorphism, the hypothesis that this specimen represents a new species is provisionally accepted. However, in the absence of diagnostic cranial features, a new species is not named. The morphology of the well-preserved forelimb indicates that the animal cannot be reconstructed as a subaqueous flyer, but probably used the forelimbs for paddling. The associated matrix indicates preservation in overbank deposits in a deltaic system exhibiting no evidence of open marine environments. This suggests that this mosasaur was able to exploit estuarine and freshwater environments.

Magnetostratigraphy of the Canadian Continental Drilling Program Cretaceous-Tertiary (K-T) Boundary Project core holes, western Canada

The Canadian Continental Drilling Program (CCDP) Cretaceous-Tertiary Boundary Project cored three holes, to 160, 188, and 170 m in southern Alberta, Saskatchewan, and Manitoba, respectively, to intersect the Cretaceous-Tertiary boundary. One hundred and seventy to 200 samples from 50 to 60 levels were taken from each of these unoriented cores for magnetostratigraphy. Samples were subjected to both alternating field (AF) and thermal cleaning. AF demagnetization in steps of 5 mT up to 30 mT showed that samples from all three cores carried only a low-coercivity viscous overprint, which was largely removed at 10 mT. Thus, three samples from each depth were routinely demagnetized at 10 mT or 100°C. The magnetic inclination of the sample giving the smallest measuring error was chosen to indicate the polarity at that depth. With the aid of palynologic control, magnetochrons 29n to 30r inclusively, in the Ravenscrag, Frenchman, Battle, Whitemud, and Eastend formations, were identified in the Elkwater core hole, western Cypress Hills, Alberta (49°36'N, 110°18'W). The Cretaceous-Tertiary boundary is missing at the Frenchman-Ravenscrag contact. The Wood Mountain core hole (49°05'N, 106°16'W) spans the interval from magnetozone 28n to 30r in the Ravenscrag, Frenchman, Battle, Whitemud, Eastend, and Bearpaw formations. The Cretaceous-Tertiary boundary is present in 29r. The Turtle Mountain core (49°01'N, 100°33'W) has a number of significant disconformities; only magnetozones 28n, 28r, 29n, 30n, and 32n or 33n are present in the Turtle Mountain, Boissevain, and Pierre formations. At the Cretaceous-Tertiary contact near the base of the Turtle Mountain Formation, there is a hiatus of 0.6 × 106 to 0.8 × 106 years. At the Campanian-Maastrichtian contact, between the Odanah and Coulter members of the Pierre Formation, there is a time gap of at least 3 × 106 years but probably about 6 × 106 years.

Ammonoid habitats and habits in the Western Interior Seaway: a case study from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada

Multidisciplinary research on the habitats and life-habits of ammonoids of the Upper Cretaceous Bearpaw Formation is based on hydrostatics and hydrodynamic aspects of shell shapes, occurrence in lithofacies, evidence of predation, and stable-isotopic data. The large and well-streamlined, oxyconic Placenticeras, with large chitinous anaptychus, had the weakest shells known, and suffered mosasaur tooth perforations at minimal ambient pressure. Anomalously light δ 18O signatures preserved in their aragonitic shells suggest that they grew in brachyhaline water. Juveniles and some adults of orthoconic Baculites had a vertical life orientation and primarily resided in the lower to middle levels of the water column, flourishing during oxygenation events. Some adults, however, may have changed to a mainly horizontal swimming habit in mid-water. Attitudinal change is indicated by relative shortening of the body-chamber during late ontogeny. Neutral buoyancy at maturity could have been achieved only with a partially liquid-filled phragmocone, implying the exceptional use of cameral liquid as an apical counterweight. Juvenile scaphitids were planktic or sluggishly nektic, as implied by sphaeroconic and discoconic immature whorls, respectively. The adults had different habits, as indicated by their strongly modified body-chambers, which gave high stability suited to vertical swimming. Adult Hoploscaphites and most Jeletzkytes, with open-hooked (scaphitoconic) body-chambers, were demersal and lived close to the sea floor during oxygenation events, whereas adult Rhaeboceras, with an elliptical, contracted body-chamber (elliptosphaerocone), lived in mid-water.

Alleged mosasaur bite marks on Late Cretaceous ammonites are limpet (patellogastropod) home scars

New data contradict the mosasaur-bite hypothesis for the origin of holes seen in Placenticeras ammonites from the Late Cretaceous Pierre Shale and Bearpaw Formation of the western interior of North America. Observations of a limpet-infested Placenticeras ammonite and of several Placenticeras specimens with radular scratch marks reveal that the limpets dwelled on floating ammonites, and their homing activity produced circular depressions. When altered during diagenesis, the depressions mimic tooth punctures. Crushing experiments on fresh Nautilus shells using a mosasaur robot show that mosasaur bites could not have produced holes resembling those in the fossils. Furthermore, sectioning of “bitten” ammonites reveals that sepia are sometimes intact under the holes, an observation irreconcilable with penetration by a tooth. We present an alternative interpretation that the alleged “bite marks” in Placenticeras ammonites are really limpet home scars that were altered after burial. While predation of mosasaurs on ammonites remains a possibility, it would have to be demonstrated by other criteria. Our findings do not support mosasaur-ammonoid coevolution in the Late Cretaceous.

The Dirt Hills structure: an ice-thrust feature in southern Saskatchewan, Canada

The Dirt Hills structure is expressed at the surface as a broad, arcuate moraine, 7.5 km wide, 120 m high, and 40 km long. The structure is composed mainly of bedrock and drift consisting of three slabs stacked to form a single block 215 m thick. The shear zones that bound the slabs are mainly in clays of the Bearpaw Formation. The Dirt Hills structure originated during the last deglaciation when the glacier, under compressive flow, readvanced up the Missouri Coteau escarpment and stacked the subglacial slabs of bedrock and drift. The subparallel, arcuate ice-thrust ridges are formed presumably by imbricate thrust faults originating from the uppermost shear zone.