Neal L. Larson

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

Abstract 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 height at midshaft averages 0.99 in macroconchs and 1.02 in microconchs. The whorl section is subquadrate/ovoid to reniform, with broadly rounded to flat flanks, and a broadly rounded venter. Ribs are straight to slightly flexuous and cross the venter with a weak adoral projection. There are 5–7.25 ribs/cm on the venter at midshaft in macroconchs and 6–8 ribs/cm on the venter at midshaft in microconchs. Umbilicolateral tubercles usually appear midway on the exposed phragmocone, but may already be present near the point of exposure. At midshaft, they occur at one-third to one-half whorl height and are relatively widely and evenly spaced, and usually extend to the aperture. Ventrolateral tubercles are generally present at the point of exposure. They are unevenly spaced on the exposed phragmocone, becoming more evenly spaced on the shaft. They attain their maximum size at midshaft, sometimes forming large, subspinose clavi that project out to the side. They become smaller and more closely spaced on the hook, and usually extend to the aperture. The suture is characterized by a broad, asymmetrically bifid first lateral saddle and narrow, symmetrically to asymmetrically bifid first lateral lobe. The holotype of Hoploscaphites brevis, by original designation and monotypy, is USNM 367, the original of Scaphites nodosus var. brevis Meek (1876: 428, pl. 25, fig. 1). Adults exhibit a wide range of variation in size, degree of compression, and coarseness of ornament. Adults are small to large (LMAX ranges from 29.5 to 101.5 mm in macroconchs and from 26.7 to 81.2 mm in microconchs), and rounded to ellipsoidal in lateral view. The body chamber consists of a relatively short shaft and a weakly recurved hook (apertural angle averages 59° in macroconchs). The ratio of whorl width to whorl height averages 0.73 at midshaft in both macroconchs and microconchs. The whorl section is subquadrate/ovoid, with fairly flat flanks, and a broadly rounded venter. Ribs are fine and flexuous and cross the venter with a weak adoral projection. There are 6–14 ribs/cm on the venter at midshaft in macroconchs and 8–18 ribs/cm on the venter at midshaft in microconchs. In most small specimens, umbilicolateral tubercles are absent on the phragmocone and, instead, the primary ribs are strong and adorally concave in this area. In large specimens, umbilicolateral tubercles usually appear midway or near the adoral end of the exposed phragmocone. They are small and relatively evenly spaced. In almost all specimens, umbilicolateral tubercles are present on the body chamber, and occur at one-fourth to one-third whorl height. They are more or less uniformly spaced with, occasionally, some approximation near the point of recurvature. In specimens that are rounded in lateral view, the umbilicolateral tubercles are arranged in a broad arc, which is one of the hallmarks of this species. Ventrolateral tubercles are stronger than umbilicolateral tubercles, and are usually present on the phragmocone starting anywhere from the point of exposure to the adoral end of the phragmocone. In general, they are unevenly spaced on the phragmocone, commonly occurring in pairs or clusters, becoming more evenly spaced on the shaft. As in H. nodosus, they attain their maximum size at midshaft. The ventrolateral tubercles usually die out near the point of recurvature, but if not, they become smaller, more bullate, and more closely spaced toward the aperture. The suture is the same as that in H. nodosus. Hoploscaphites landesi Riccardi, 1983, is a junior subjective synonym of H. brevis. The holotype is a small, compressed, finely ornamented microconch of H. brevis. It grades into larger, more robust specimens, with coarser ornament. We thereby expand the definition of H. brevis to include a wide range of variation in adult size, and argue that such variation reflects variation in the age (and size) at which individuals reach maturity. Establishment of separate species for different size specimens, given that all other aspects of their morphology (shell shape, whorl cross section, pattern of ribs, distribution of umbilicolateral and ventrolateral tubercles, and suture) are the same, seems unwarrented. Hoploscaphites nodosus and H. brevis are widespread in the Baculites compressus–B. cuneatus zones in the Western Interior of North America, which represent a time interval of approximately 580 ky (Cobban et al., 2006). They are also present in parts of the underlying Didymoceras cheyennense Zone and the overlying B. reesidei Zone, but their exact distribution in these zones is not yet known. Hoploscaphites nodosus and H. brevis occur in the Bearpaw Shale in Alberta and Saskatchewan, the Bearpaw Shale and Pierre Shale in Montana, and the Pierre Shale in North Dakota, South Dakota, Wyoming, Colorado, and Kansas. They occur in nearshore deposits such as the unnamed shale member of the Pierre Shale in Grand County, Colorado, in offshore deposits such as the DeGrey Member of the Pierre Shale in Buffalo County, South Dakota, and in cold methane seeps in the Pierre Shale in Custer County, South Dakota. They are absent in the U.S. Gulf and Atlantic Coastal plains and northern Europe, although similar forms are present in both these areas in strata above and below the biostratigraphic interval containing the ranges of H. nodosus and H. brevis. Thus, although H. nodosus and H. brevis are endemic to the Western Interior of North America, they are part of a more broadly distributed clade that is also present in the U.S. Gulf and Atlantic Coastal plains and northern Europe. As adults, these scaphites probably lived just above the sea bottom. They preferred oxygenated water, as indicated by the fact that they are generally associated with a diverse molluscan community. Habitat depths are estimated at less than 100 m, based on studies of the mechanical properties of the septa and siphuncle (Tsujita and Westermann, 1998). The high angle of orientation of the aperture a

Diversity, Morphology, and Phylogeny of Coleoid Cephalopods from the Upper Cretaceous Plattenkalks of Lebanon–Part II: Teudopseina

Abstract Morphologic analyses of a large quantity of teudopseid coleoids from the Upper Cretaceous (Cenomanian) of Lebanon has yielded a much higher diversity than previously assumed and revealed numerous extraordinarily well-preserved soft-part characters. The Teudopseina is represented by three families (only the Muensterellidae are still unknown in the Lebanon Plattenkalks). The Teudopsidae is represented by one species (Teudopsinia haasi), the Trachyteuthididae by five species (Trachyteuthis bacchiai n. sp., Glyphiteuthis libanotica, Gl. abisaadiorum, Gl. freijii n. sp., and Glyphidopsis waagei n. gen. n. sp.), and the Palaeololiginidae by one species (Rachiteuthis donovani). Gl. freijii n. sp. represents the first record of a coleoid cephalopod from Nâmmoura. A detailed comparison with Jurassic gladii shows that the Teudopseina is a very homogenous group that can be easily differentiated from other Mesozoic, gladius-bearing groups (Prototeuthidina, Loligosepiina). Despite comparatively large gaps in the fossil record, each of the Late Cretaceous gladii can be readily associated with Jurassic precursors. Particularly, the first record of two pairs of fins in Glyphiteuthis exemplifies that in addition to gladius similarities, similarities in soft-parts exist as well. A previously undescribed specimen of Rachiteuthis exhibits unusually well-preserved gills. In general, the soft-part morphology of the Teudopseina clearly indicates octobrachiate affinities (i.e., absence of tentacles, circular suckers, cirri, two pairs of fins, octopod-like eye capsules); no existing evidence supports a decabrachiate relationship.

Methane seeps as ammonite habitats in the U.S. Western Interior Seaway revealed by isotopic analyses of well-preserved shell material

Methane seep deposits are common in the Upper Cretaceous Pierre Shale of the U.S. Western Interior. They contain a rich fauna including ammonites, bivalves, gastropods, sponges, corals, echinoids, crinoids, and fish. In an effort to understand the role of ammonites in these ecosystems, we examined a seep from the upper Campanian Didymoceras cheyennense Zone in Custer County, South Dakota, that contains molluscs with well-preserved shell material permitting isotopic analyses. Values of δ13C of the micritic limestone at the seep range from −46.94‰ to −11.49‰, confirming the influence of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. The ammonites also consistently display light values of δ13C ranging from −13.71‰ to 0.68‰. These values are generally lighter than those in nonseep specimens from age-equivalent rocks elsewhere in the basin (–1.75‰ to 3.42‰). In a single specimen of Baculites corrugatus from the seep, light δ13C values occur throughout ontogeny. These data suggest that ammonites incorporated isotopically light methane-derived carbon in their shells and lived near the vent fluids and methane-oxidizing bacteria. Both juvenile and adult specimens are present, implying that these ammonites spent their entire lives at the seep and formed an integral part of an interwoven community. The values of 87Sr/86Sr in the limestone and well-preserved fossils at the seep (0.707690–0.707728) are higher than that of the open ocean at this time (0.707659). These elevated values suggest that the seep fluids were imprinted with a radiogenic Sr signature, perhaps derived from equilibration with granitic deposits at depth during the initial uplift of the Black Hills.

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 Böhm, 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, pyrite, and amorphous material (organic compounds). The upper jaw is approximately the same length as the lower jaw and is U-shaped, with narrow wings that converge anteriorly to a dome-shaped hood. The lower jaw is composed of two lamellae. The outer lamella is broad and consists of two wings terminating in a bilobate posterior margin. The inner lamella is one-half the length of the outer lamella. The two lamellae are separated except in the apical region and along the sides. The junction between the lamellae appears as a U or V-shaped outline on the anterior portion on the ventral surface of the jaw. This junction is especially conspicuous in specimens in which part of the inner lamella has eroded away. In crushed specimens, the lower jaw is subquadrate in shape. In specimens that retain some or all of their original curvature, the central portion is gently convex and the sides bend steeply dorsally. The rostrum projects slightly anteriorly and dorsally and there is a thickened rim of chitin along the anterior margin where the two lamellae are doubled over. A small indentation appears at the apical end and, in most specimens, develops into a midline slit that extends posteriorly 10–15 mm. However, as shown in well-preserved specimens and based on comparisons with the jaws of closely related ammonites, this slit represents the remnants of a narrow ridge on the ventral side of the inner lamella. This ridge is surrounded by an elongate boss of thickened chitin, which corresponds to a depression on the dorsal side. The ventral surface of the outer lamella bears a midline ridge with a central groove, which essentially forms a continuation of the ridge on the inner lamella. The ventral surface of the outer lamella is ornamented with thin, radial striations and irregular broad undulations paralleling the posterior margin. The posterior end is generally incomplete, probably as a result of predation or postmortem degradation, and the lateral margins are commonly creased, indicating postmortem plastic deformation. The lower jaws of Metaplacenticeras subtilistriatum are much smaller than those of Placenticeras but are otherwise similar in morphology. However, they retain pieces of a very thin, fibrous outer layer comprising two plates. X-ray diffraction analysis of samples of this layer indicates that it consists of calcite enriched in magnesium. Each plate covers the ventral surface of one of the wings and terminates at the midline ridge. Based on the close affinity of Metaplacenticeras and Placenticeras, and in comparison with published descriptions of placenticeratid jaws from elsewhere, we hypothesize that similar plates covered the lower jaws of all placenticeratids, although these plates have not been found in any Placenticeras material from North America. The thin nature and fibrous microstructure of this layer would have made it susceptible to mechanical breakage and chemical dissolution. Furthermore, jaws are internal structures embedded in the buccal bulb. The micro-environment within this bulb may have promoted dissolution of the outer calcitic layer of the lower jaw. The presence of a pair of calcitic plates (aptychi) and a midline ridge with a central groove on the outer lamella of the lower jaw are unique features of the lower jaws of the Aptychophora Engeser and Keupp, 2002. Although differences in preservation obscure this similarity, the lower jaws of placenticeratids conform to the description of aptychus-type jaws. However, unlike the thick calcitic aptychi of other Ammonitina, the thin calcitic aptychi of placenticeratids probably did not function as opercula and would have served simply to strengthen the lower jaw. The jaws of placenticeratids were probably designed for biting and cutting food rather than for passively collecting and straining plankton. Other data about the habitat and mode of life of placenticeratids are consistent with this interpretation. These ammonites probably inhabited surface waters and were capable of pursuing and attacking sluggish prey. An ecological analog of placenticeratids may be the modern ocean sunfish Mola mola (Linnaeus, 1758), which inhabits surface waters and feeds on gelatinous zooplankton.

Lakotacrinus brezinai n. gen. n. sp., a new stalked crinoid from cold methane seeps in the Upper Cretaceous (Campanian) Pierre Shale, South Dakota, United States

Abstract. Despite a rich and varied record, Mesozoic stalked crinoids are relatively rare in the Western Interior Seaway of North America compared to those found in Northern Europe. A unique example of Mesozoic stalked crinoid is described from cold methane seeps (hydrocarbon seep mounds also called “tepee buttes”) from the Upper Cretaceous (upper Campanian) of the Northern Great Plains of the United States; the first crinoids to be described from such an environment. The Late Cretaceous Western Interior Seaway has never before yielded any identifiable stalked crinoid remains. Nevertheless, there have been significant studies on both free living and stalked crinoids from other locations in the Upper Cretaceous of North America that provide a good basis for comparison. Lakotacrinus brezinai n. gen. n. sp. is characterized by a tapering homeomorphic column with through-going tubuli, lacking any attachment disc. The arms are unbranched and pinnulate, with muscular and syzygial articulations. The unique morphology of the column justifies the establishment of Lakotacrinidae new family. A new suborder Lakotacrinina n. subord., is also proposed as there exists no corresponding taxon within the Articulata that can accommodate all the characteristics of this new genus. This new crinoid shares many features with other members of the articulates, including bathycrinids, bourgueticrinids and guillecrinids within the Order Comatulida, as currently defined in the revised Treatise of Invertebrate Paleontology. Reconstructing the entire crinoid using hundreds of semi-articulated and disarticulated (well preserved) fossils, reveals a unique paleoecology and functional morphology specifically adapted to living within this hydrocarbon seep environment.

A New Species of Hoploscaphites (Ammonoidea: Ancyloceratina) from Cold Methane Seeps in the Upper Cretaceous of the U.S. Western Interior

ABSTRACT We describe Hoploscaphites gilberti, n. sp. (Ammonoidea: Ancyloceratina), from the Upper Cretaceous (middle-upper Campanian) Pierre Shale spanning the zones of Baculites scotti and Didymoceras nebrascense in Colorado, Wyoming, Montana, and South Dakota. This species is strongly dimorphic and is characterized by a compressed whorl section, with a rounded to elongate outline in lateral view. The apertural angle is approximately 50° in macroconchs. The body chamber is ornamented with fine flexuous ribs, umbilicolateral bullae, and ventrolateral tubercles. Hoploscaphites gilberti, n. sp., most closely resembles H. gilli Cobban and Jeletzky, 1965, but differs from this species in several important features: (1) the flanks of the body chamber are nearly subparallel rather than steeply convergent toward the venter, (2) the ventrolateral tubercles are larger and more numerous, and (3) the ribs are more widely spaced. Hoploscaphites gilberti, n. sp., is abundant at “tepee buttes” in the Pierre Shale, which are now interpreted as cold methane seeps.

The Jaw Apparatus of the Late Cretaceous Ammonite Didymoceras

Abstract We report on well-preserved upper and lower jaws found inside the body chambers of two specimens of Didymoceras nebrascense (Meek and Hayden, 1856) from the Upper Cretaceous Pierre Shale of the USA. The finds are described and compared to existing material, and their possible functions are discussed.

A New Species of Scaphitid Ammonite from The Lower Maastrichtian of the Western Interior of North America, with Close Affinities to Hoploscaphites constrictus Sowerby, 1817

ABSTRACT We describe a new species of scaphitid ammonite from the Upper Cretaceous (lower Maastrichtian) of North America. Hoploscaphites sargklofak, n. sp., is endemic to the U.S. Western Interior, but closely resembles H. constrictus Sowerby, 1817, from the Maastrichtian of northern Europe.

Inquilinism of a Baculite by a Dynomenid Crab from the Upper Cretaceous of South Dakota

ABSTRACT We describe a small crab inside the phragmocone of a heteromorph cephalopod Baculites sp. smooth from the Gammon Ferruginous Member (lower Campanian) of the Pierre Shale in Butte County, South Dakota. The crab Ferricorda kimberlyae (Bishop, 1987) is well preserved with its carapace and pereiopods parallel to and between two septa of the phragmocone. Because of its superb preservation, the specimen is unlikely to have been washed into the phragmocone. The crab probably retreated into the phragmocone chamber to avoid predation or to molt and was subsequently buried by an influx of rapid sedimentation. This is the first instance of inquilinism by a crab in a heteromorph ammonite. Despite the rarity of such fossils, the occurrence of crabs inside ammonite shells was probably not uncommon on sea floors during the Mesozoic. Morphological details of the specimen reveal that Ferricorda is a dynomenid crab.

A new species of the asteroid genus Betelgeusia (Echinodermata) from methane seep settings, Late Cretaceous of South Dakota

Abstract Betelgeusia brezinai new species (Radiasteridae, Paxillosida, Asteroidea) is described from diversely fossiliferous Upper Cretaceous methane seep deposits of South Dakota. Asteroids are rare at modern chemosynthetic settings, although a hydrothermal vent occurrence is known, and two possible fossil methane seep occurrences have been reported. The Radiasteridae is important to the interpretation of crown-group asteroid phylogeny. Two extant genera are assigned to the family: Radiaster is known from relatively few but geographically widely dispersed largely deeper-water settings, and Gephyreaster is uncommon over a range of depths in the North Pacific Ocean. Jurassic and Cretaceous radiasterids have been described from geographically widely separated localities. In morphological-based phylogenetic analyses, the Radiasteridae has been assigned to the order Paxillosida, and Gephyreaster is similarly placed in a molecular evaluation; Radiaster has not yet been treated in a molecular study. In molecular treatment, an approximately traditional Paxillosida is a sister taxon to a significant part of the traditional Valvatida. Comparative morphology of Mesozoic and extant asteroids enables a hypothesis for a stemward, Mesozoic paxillosidan.