Baculites Research Articles

Methane seeps as refugia during ash falls in the Late Cretaceous Western Interior Seaway of North America

Abstract Methane seeps host rich biotic communities, forming patchy yet highly productive ecosystems across the global ocean. Persistent hydrocarbon emissions fuel chemosynthetic food webs at seeps. Methane seeps were abundant in the Western Interior Seaway of North America during the Late Cretaceous. This area also experienced intermittent ash falls, which negatively impacted the marine fauna. We propose that methane seeps acted as refugia during these environmental perturbations. We report a laterally continuous bentonite within the upper Campanian Baculites compressus Zone of the Pierre Shale in southwestern South Dakota (USA) that fortuitously cuts across a methane seep deposit. We compare the macroinvertebrate record below and above the bentonite at seep and non-seep sites. Our results reveal that the paleocommunity (measured by abundance and diversity) was largely unaffected by the ash fall at the seep site, whereas it was significantly altered at the non-seep site. Thus, methane seeps in the Western Interior Seaway may have provided refuges or served as oases in the aftermath of severe environmental perturbations.

Vertical escape tactics and movement potential of orthoconic cephalopods.

Measuring locomotion tactics available to ancient sea animals can link functional morphology with evolution and ecology over geologic timescales. Externally-shelled cephalopods are particularly important for their central roles in marine trophic exchanges, but most fossil taxa lack sufficient modern analogues for comparison. In particular, phylogenetically diverse cephalopods produced orthoconic conchs (straight shells) repeatedly through time. Persistent re-evolution of this morphotype suggests that it possesses adaptive value. Practical lateral propulsion is ruled out as an adaptive driver among orthoconic cephalopods due to the stable, vertical orientations of taxa lacking sufficient counterweights. However, this constraint grants the possibility of rapid (or at least efficient) vertical propulsion. We experiment with this form of movement using 3D-printed models of Baculites compressus, weighted to mimic hydrostatic properties inferred by virtual models. Furthermore, model buoyancy was manipulated to impart simulated thrust within four independent scenarios (Nautilus-like cruising thrust; a similar thrust scaled by the mantle cavity of Sepia; sustained peak Nautilus-like thrust; and passive, slightly negative buoyancy). Each model was monitored underwater with two submerged cameras as they rose/fell over ~2 m, and their kinematics were computed with 3D motion tracking. Our results demonstrate that orthocones require very low input thrust for high output in movement and velocity. With Nautilus-like peak thrust, the model reaches velocities of 1.2 m/s (2.1 body lengths per second) within one second starting from a static initial condition. While cephalopods with orthoconic conchs likely assumed a variety of life habits, these experiments illuminate some first-order constraints. Low hydrodynamic drag inferred by vertical displacement suggests that vertical migration would incur very low metabolic cost. While these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges. The current experiments suggest that orthocones sacrifice horizontal mobility and maneuverability in exchange for highly streamlined, vertically-stable, upwardly-motile conchs.

Large Scaphitid Ammonites (Hoploscaphites) from the Upper Cretaceous (Upper Campanian–Lower Maastrichtian) of North America: Endless Variation on a Single Theme

We describe three species of large scaphitid ammonites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (upper Campanian–lower Maastrichtian) of the Western Interior of North America. Each species occurs as two dimorphs, referred to as macroconch and microconch. All three species share a similar pattern of ornamentation consisting of long, thin, nonbifurcating ribs on the adoral part of the phragmocone, suggesting that they constitute a single monophyletic clade. Macroconchs of Hoploscaphites crassus (Coryell and Salmon, 1934) are characterized by a globose whorl section, with closely spaced ventrolateral tubercles on the body chamber, usually persisting to the aperture. Macroconchs of Hoploscaphites plenus (Meek and Hayden, 1860) differ from those of H. crassus in having a more subquadrate whorl section with flatter flanks, and fewer, larger, and more widely spaced ventrolateral tubercles. Macroconchs of Hoploscaphites peterseni, n. sp., closely resemble those of H. crassus, but differ in being nearly circular in side view with a more compressed whorl section. All three species lived at approximately the same time in the same general area and depositional environment. They are abundant in the Baculites baculus Zone but also occasionally occur in the B. eliasi Zone and possibly lower part of the B. grandis Zone. They are present in the Pierre Shale of east-central Montana and east-central Wyoming, the Lewis Shale of south-central Wyoming, and the Bearpaw Shale of northeast Montana. It is possible that these three species represent subspecies within a single species or a “flock” of very closely related species, similar to the “species flocks” observed in modern cichlid fishes.

Palaeoecological analysis of a methane seep deposit from the Upper Cretaceous (Maastrichtian) of the U.S. Western Interior

Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east-central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well-preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non-seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non-seep sites.

Rediscovery of the holotype of the extinct cephalopod Baculites ovatus Say, 1820 after nearly two centuries

Thomas Say (1787–1834) based his description of the ammonite species Baculites ovata (= B. ovatus) on a single specimen in the collection of his childhood friend, the Quaker naturalist Reuben Haines III (1786–1831). However, the specimen's whereabouts faded from memory after Haines and Say both died unexpectedly in the early 1830s. The holotype specimen has been missing for more than 180 years, and was thought to be lost or destroyed until 2017, when I relocated it at Haines's ancestral home (Wyck) in Philadelphia, Pennsylvania, where his collection was preserved by his descendants in its original wooden cabinet. Herein, I present the first photographic images, an illustration of its suture line, and quantitative measurements for use by systematic paleontologists.

Description of Two Species of Hoploscaphites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (Lower Maastrichtian) of the U.S. Western Interior

Two species of scaphitid ammonites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (lower Maastrichtian) of the Western Interior of North America are described. Hoploscaphites macer, n. sp., is medium size, with coarse ribs on the phragmocone, which become finer on the body chamber, and closely spaced ventrolateral tubercles. It occurs in the upper part of the Baculites baculus Zone and lower part of the overlying B. grandis Zone in the Pierre Shale in Montana, Wyoming, and Colorado, and in the Bearpaw Shale in Montana. Hoploscaphites criptonodosus (Riccardi, 1983) is larger and more coarsely ornamented, including one or two rows of lateral tubercles on the flanks of the phragmocone. It occurs in the upper part of the Baculites baculus Zone and overlying B. grandis Zone in the Pierre Shale in Montana, Wyoming, Colorado, and possibly South Dakota, and in the Bearpaw Shale in Montana and Saskatchewan, Canada. Both species form part of an evolving lineage of Hoploscaphites that first appears in the Western Interior of North America in the middle Campanian.

Campanian–early Eocene cephalopods from Kharga Oasis, Western Desert, Egypt

The Campanian–lower Eocene sedimentary succession in the Kharga Oasis yields rare cephalopods that have so far received little attention. Eight cephalopod species; six nautiloids and two ammonites, are identified in the study area. The nautiloids are referred to five genera in three families. All nautiloid species are recorded from the Paleocene and Eocene rocks, two of which are described as new, as follows: Cimomia kurkurensis nov. sp. and Deltoidonautilus hassani nov. sp. The two ammonite species are Libycoceras ismaelis (von Zittel, 1884) and Baculites ovatus Say, 1820, representing the families Sphenodiscidae and Baculitidae, respectively. Baculites anceps of Quaas (1902, non Lamarck, 1822) from the Maastrichtian of the Western Desert of Egypt is here assigned to Baculites ovatus. The palaeobiogeography of these species is studied in detail.

Description of juvenile specimens of Prosaurolophus maximus (Hadrosauridae: Saurolophinae) from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada, reveals ontogenetic changes in crest morphology

ABSTRACTThree juvenile specimens of Prosaurolophus maximus, represented by articulated to disarticulated skeletons, are the smallest known individuals for the taxon. Cranial anatomy of the juvenile specimens indicates that diagnostic characters of P. maximus are ontogenetically variable. In the smallest individual, the crest and deeply excavated fossa at the caudal margin of the circumnarial depression are poorly developed or absent. The crest approaches adult-like morphology in large juveniles, whereas crest robusticity and the deep excavation of the circumnarial depression occur only in subadult and adult individuals. The shape of the caudal margin of the circumnarial depression is consistent between juvenile and adult individuals, potentially making this feature a reliable character for taxonomic identification at younger ontogenetic stages. The crest of P. maximus grows isometrically during ontogeny, unlike the positive allometric growth of lambeosaurine hadrosaur crests, suggesting that this taxon may have had soft tissue structures associated with the narial-crest region, rather than the bony crest itself, selected for sexual display. Recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n, the juvenile specimens are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation (Alberta) and contemporaneous with most specimens from the Two Medicine Formation (Montana), extending the temporal range of the taxon to 75.7–74 Ma. The occurrence of P. maximus in well-drained terrestrial deposits of the Dinosaur Park and Two Medicine formations and marine sediments of the Bearpaw Formation indicates that this taxon inhabited various paleoenvironments in western North America.

Isotope sclerochronology of ammonites (Baculites Compressus) from methane seep and non-seep sites in the Late Cretaceous Western Interior Seaway, USA: Implications for ammonite habitat and mode of life

Ammonites, as well as other fauna, were common in methane seeps of the Late Cretaceous Western Interior Seaway (WIS) of North America. Biogeochemical processes at the seeps, in particular the anaerobic oxidation of methane, produced a dissolved inorganic carbon reservoir with a low δ13C, manifested in the carbon isotope composition of the inorganic calcium carbonate concretions associated with the seeps and recorded in well-preserved shells of ammonites documented at the sites. Detailed sclerochronological sampling of six well-preserved specimens of Baculites compressus collected at seep sites in the Pierre Shale of South Dakota reveals three patterns that can be explained by reference to two specimens of the same species collected at age-equivalent non-seep sites. Three of the specimens exhibit uniformly low values of δ13C that are significantly different (unpaired t-test, p

On the paleoecology of the index ammonites Baculites (Lamarck, 1799) and Scaphites (Parkinson, 1811) as revealed by a study of relatively rare epizoans

On the paleoecology of the index ammonites Baculites (Lamarck, 1799) and Scaphites (Parkinson, 1811) as revealed by a study of relatively rare epizoans

40Ar/39Ar date of a bentonite associated with a methane seep deposit in the upper Campanian Baculites compressus Zone, Pierre Shale, South Dakota

40Ar/39Ar dating of a bentonite associated with a cold methane seep deposit in the upper Campanian Baculites compressus Zone of the Pierre Shale in South Dakota yields an age of 73.79 ± 0.36 Ma. This is in close agreement with the previously published age of 74.05 ± 0.39 Ma for this zone (Obradovich, 1993) and nearly identical to an unpublished age of 73.70 ± 0.13 Ma for the same zone, both samples of which are from a more proximal locality near the base of the Bearpaw Shale in Montana, as recalculated relative to a Fish Canyon standard age of 28.201 Ma. We also report dates for two bentonites from the upper Campanian Exiteloceras jenneyi and Didymoceras stevensoni zones.

Foraminifera from the upper Campanian Pierre Shale methane cold-seeps, South Dakota

Investigations into ancient benthic foraminiferal populations are limited, particularly in the Late Cretaceous methane cold-seeps of the Western Interior Seaway. Poorly lithified shale samples (400 cc) from 8 methane cold-seeps and one coeval non-seep locality from the upper Campanian Pierre Shale in South Dakota were disaggregated, sieved, and examined for specimens (>150 μm). Seeps yielded between 100 and 148 specimens per locality. The foraminiferal populations found at seeps and the single non-seep locality contain many of the same species. The most abundant benthic genera found at late Campanian seeps in South Dakota include Guardryina, Haplophragmoides, Trochammina, buliminids (Buliminella and Praebulimina), Lagena, and Gavelinella. Planktic foraminiferal assemblages are much less diverse and dominated by Globigerinelloides. Only two seeps and the non-seep locality contain biserial species, these localities fall in the Baculites compressus zone. The foraminifera from both seeps and the non-seep have Shannon-Weiner (ln) and Fisher's diversity indices that show that the foraminiferal populations are of moderate diversity with a small peak in seeps from the Baculites compressus Zone. Agglutinate to calcareous ratios are consistent for nearly all seep localities, however, one seep locality (AMNH loc. 3528a) and the non-seep sample render a slightly higher percentage of agglutinated species, these localities are within the Baculites compressus Zone. We conclude that, like their modern counterparts: 1) there is no evidence of unique physiological differences between seep and non-seep foraminifera, 2) the species found in seep environments are most likely more tolerant of seep conditions, and 3) there are no seep obligate species.

Do marine faunas track lithofacies? Faunal dynamics in the Upper Cretaceous Pierre Shale, Western Interior, USA

Most studies examining faunal assemblages use their sedimentary context as a critical element in constraining and reconstructing their underlying environmental controls. This has resulted in the assumption that an absence of lithofacies change in a section should be reflected in a lack of environmental variation. This inference, however, has been placed into question by evidence that marine species are influenced by a broader range of environmental dynamics than just change in lithofacies. In this study, we examine the sensitivity of marine faunas to broadly defined environmental change within lithologically homogenous strata by examining concretionary fossil assemblages of the Baculites eliasi through B. clinolobatus biozones in monotonous, clay-rich strata of the Campanian-Maastrichtian Pierre Shale in Wyoming. We recognize five biofacies, which reflect different environmental conditions related to benthic oxygenation, substrate firmness, and water depth. Analyses of abundance patterns, raw species richness trends, and life-habit patterns display recurrent switching, upsection, between low- and high-diversity intervals. Our data reveal that samples with lower diversity show a strong relationship with intervals when water conditions were deepest, whereas higher diversity samples are associated with periods when shallow-water conditions prevailed in the study area. The distribution of taxa and diversity of the assemblages most likely reflect migrating oxygen- and substrate-controlled biofacies that were responding to changes in depth. This study shows that substantial changes in biofacies, diversity, and life habits can arise in response to variations in water depth with limited to no apparent change in lithofacies supporting the hypothesis that fossil taxa are much more sensitive indicators of environmental change than lithofacies.

Ammonite dating of latest Cretaceous mosasaurid reptiles (Squamata, Mosasauroidea) from Jordan—preliminary observations

Jagt, J.W.M., Jagt-Yazykova, E.A., Kaddumi, H.F. & Lindgren, J., April 2017. Ammonite dating of latest Cretaceous mosasaurid reptiles (Squamata, Mosasauroidea) from Jordan—preliminary observations. Alcheringa 42, 587-596. ISSN 0311-5518 Newly collected ammonoid material from the uppermost Cretaceous portion of the Muwaqqar Chalk Marl Formation exposed some 30 km southeast of the Qasr Al’Harrana area (east-central Jordan) includes medium-sized baculitids (Baculites ovatus auctorum, non Say), the sphenodiscid Libycoceras acutodorsatus (Noetling) and the pachydiscids Menuites fresvillensis (Seunes) and Pachydiscus (Pachydiscus) dossantosi (Maury). Of the two last named taxa, the former is a good marker species for the upper Maastrichtian, with records from Europe, central Chile, South India, Baluchistan (Pakistan), Australia, Madagascar and South Africa. The latter is known from the United Arab Emirates/Oman border area, from strata of (late) early to early late Maastrichtian age, as well as from more poorly constrained Maastrichtian levels in Brazil and Nigeria. A comparison with ammonoid assemblages from the Maastrichtian type area (southeast Netherlands/northeast Belgium) suggests correlation of the Muwaqqar Chalk Marl Formation with the middle/upper Maastricht Formation (Emael and Nekum members, ca 66.5–66.1 Ma) and the upper part of the coeval Kunrade Formation. However, associated ‘tegulated’ inoceramids of the Tenuipteria argentea group from the Muwaqqar Chalk Marl Formation favour equivalence with a higher level of the Maastrichtian type area, i.e., the Meerssen Member. From the upper Maastricht Formation and the equivalent upper part of the Kunrade Formation, the following mosasaur genera are currently known: Mosasaurus, Prognathodon, Plioplatecarpus and Carinodens. Interestingly, coeval strata of the Muwaqqar Chalk Marl Formation in east-central Jordan have yielded remains of a largely comparable suite comprising Prognathodon, Mosasaurus, Carinodens and an unnamed, highly derived plioplatecarpine. John W.M. Jagt* [ john.jagt@maastricht.nl ], Natuurhistorisch Museum Maastricht, de Bosquetplein 6–7, 6211 kJ Maastricht, the Netherlands; Elena A. Jagt-Yazykova [ eyazykova@uni.opole.pl ], Opole University, Department of Biosystematics, Oleska 22, 45-052 Opole, Poland; Hani F. Kaddumi [ blossom_trail@yahoo.com ], Eternal River Museum of Natural History, Maroof Al’Rusafi Street, PO Box 11395, Amman 11123, Jordan; Johan Lindgren [ johan.lindgren@geol.lu.se ], Lund University, Department of Geology, Sölvegatan 12, 223 62 Lund, Sweden.

Ammonoids from the Maastrichtian (Late Cretaceous) at El Zancudo, Nuevo Laredo, Tamaulipas, Mexico

A small assemblage of Maastrichtian ammonoids was collected at El Zancudo, south of Nuevo Laredo, Mexico, on the border with Texas. They are preserved as goethitic internal moulds of the innermost whorls. Nine taxa are described in abbreviated form. Eight taxa have been recorded from northeastern Mexico before: Baculites ovatus, Brahmaites (Anabrahmaites) vishnu (Forbes, 1846), Diplomoceras cylindraceum, Fresvillia sp. indet., Gaudryceras kayei, Hypophylloceras (Neophylloceras) cf. H. (N.) surya, Phyllopachyceras forbesianum, and Tetragonites superstes. The genus Discoscaphites was recorded and described for the first time in this region. The assemblage indicates an early late Maastrichtian age and is compared to other ammonoid assemblages from Texas and northeastern Mexico. It provides an important level marker for long-distance correlation into the deltaic sediments of northeastern Mexico, known as the Difunta Group.

The climate of the Late Cretaceous: New insights from the application of the carbonate clumped isotope thermometer to Western Interior Seaway macrofossil

We apply the carbonate clumped isotope thermometer (Δ47) to macrofossils from the Baculites compressus (∼73.5Ma) and the Hoploscaphites nebrascensis (∼67Ma) ammonite zones of the Western Interior Seaway (WIS) of North America, and nearby coeval terrestrial and open marine environments. The carbonate clumped isotope thermometer is based on a single-phase isotope exchange equilibrium that promotes the ‘clumping’ of two heavy isotopes together within a single carbonate molecule as temperature decreases. Due to the thermometer's isotopic independence from water, coupled measurements of Δ47 and the bulk oxygen isotopic composition of a carbonate (δ18Oc) enable the reconstruction of both paleotemperature and the isotopic composition of the water in which the organisms grew. Before applying the technique to the aragonite shells of fossil marine organisms (mostly ammonites, but also some gastropods, bivalves, and one belemnite), we measure the clumped isotopic composition of modern nautilus and cuttlefish, two of the nearest living relatives to the Cretaceous ammonites. Modern cephalopods exhibit disequilibrium isotope effects with respect to Δ47, but not δ18Oc, therefore a simple correctional scheme is applied to the Late Cretaceous macrofossil data before reconstructing paleotemperatures. Diagenesis is also assessed by visual preservation and previously measured Sr concentrations (Cochran et al., 2003). Temperatures reconstructed for the Late Cretaceous Western Interior Seaway range from 16.4±3.5°C for an offshore Interior Seaway environment from the H. nebrascensis zone to 24.2±0.4°C for the B. compressus ammonite zone. The seaway itself has an isotopic composition of approximately −1‰ (relative to VSMOW), the expectation for an ice-free global ocean average, while a nearby freshwater environment has an isotopic composition approaching −20‰. We compare the attributes of the reconstructed climate to predictions based on Late Cretaceous climate models and previous reconstructions of the seaway, and also assess the sensitivity of our results to the modern cephalopods correction by comparisons to suitable modern analogs. Finally, our clumped isotope data are consistent with cooling between the Late Campanian and Maastrichtian, as also seen in benthic foraminfera δ18O.

Dinoflagellate cysts from the upper Campanian Pierre Shale and Bearpaw Shale of the U.S. Western Interior

The Upper Cretaceous Pierre Shale and Bearpaw Shale contain rich assemblages of dinoflagellates. We analyzed nine samples from the Baculites compressus – B. cuneatus Zones in Montana, South Dakota, and Colorado. According to the ammonite zonation of the Western Interior of North America, these zones represent the middle part of the upper Campanian. Dinoflagellate cysts include Alterbidinium acutulum, Cordosphaeridium fibrospinosum, Hystrichodinium pulchrum, Isabelidinium cooksoniae, Laciniadinium firmum , members of the Microdinium group Odontochitina operculata, Oligosphaeridium pulcherrimum, Palaeohystrichophora infusorioides, Palaeoperidinium pyrophorum, Phelodinium tricuspe , members of the Spiniferites group, Spongodinium delitiense , and Xenascus ceratioides . Variation in species composition among samples may indicate slight differences in environment and/or age. The environment and proximity of samples to the paleoshoreline was evaluated based on the ratio of marine to terrestrial palynomorphs and the relative abundance of dinoflagellate cysts versus other organic-walled remains. Our results suggest that the samples from Colorado were deposited in a nearshore environment, the samples from southwestern South Dakota and Montana in an offshore environment, and the sample from eastern South Dakota in an offshore environment far from shore. These results agree with previous interpretations about the position of the shoreline based on the distribution of litho- and biofacies.

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

Migration pathways of the late Campanian and Maastrichtian shallow facies ammonite Sphenodiscus in North America

Records of the ammonites Sphenodiscus lobatus and S phenodiscus pleurisepta from the latest Cretaceous Gulf of Mexico region, the Western Interior Seaway and the Atlantic coast have repeatedly been used as a stratigraphic index for a Maastrichtian age. In recent years, however, high-resolution biostratigraphy on north-eastern Mexican occurrences and its combination with other correlation tools, such as sequence stratigraphy, as well as a refined position of the Campanian–Maastrichtian boundary, led to a different interpretation regarding the age of the earliest Sphenodiscus. The wealth of data allows us to reconstruct their routes of dispersal over the North American continent. Migration appears to have been directed from South to North and is thus opposed to that of Baculites ovatus. Dispersal of all three species is clearly related to changes in the Maastrichtian sea level.

Hemipelagic cephalopods from the Maastrichtian (late Cretaceous) Parras Basin at La Parra, Coahuila, Mexico, and their implications for the correlation of the lower Difunta Group

Abstract Few biostratigraphic data exist from the Parras and La Popa basins, mainly due to the absence of index fossils. This paper describes 19 ammonoid species from 15 genera and 1 nautilid from La Parra, southeastern Coahuila, Mexico. The assemblage consists of Tethyan [(Baculites ovatus, Brahmaites (Anabrahmaites) vishnu, Fresvillia constricta, Hauericeras rembda, Pachydiscus (P.) ex gr. neubergicus, Solenoceras reesidei, Tetragonites cf. superstes], cosmopolitan (Anagaudryceras politissimum, Desmophyllites diphylloides, Diplomoceras cylindraceum, Gaudryceras kayei, Phyllopachyceras forbesianum, Pseudophyllites indra), and cold water taxa [Fresvillia teres, Hypophylloceras (Neophylloceras) surya, H. (N.) hetonaiense, Pachydiscus (P.) cf. egertoni]. Eutrephoceras sp. and Menuites juv. sp. were not determined to species level. A similar assemblage was recently described from the coeval Mendez Formation at Cerralvo, Nuevo Leon. Species endemic to North America, particularly the Western Interior Seaway, are absent at La Parra. The ammonoid assemblage and associated planktonic foraminifers allow for precise biostratigraphic assignation to the early Maastrichtian planktonic foraminiferal zone CF 5, and thus provide an important marker level for correlation of the lower Difunta Group. The new biostratigraphic data presented herein allow for the first time precise dating of the Canon del Tule Formation of the Difunta Group. Their combination with existing sequence- and magnetostratigraphic data improve the correlation of the lower Difunta Group with time-equivalent lithostratigraphic units such as the Cardenas Formation in Mexico. They also provide new insight into ammonoid migration patterns induced by sea-level changes. Baculites ovatus migrated into the La Popa Basin as a result of the sea-level highstand documented at La Parra.