New findings of the ammonite Rhaeboceras , including a new species, from the Pierre Shale of eastern Montana

Grier

Cochran

et al. 2018

LET

Self Cite

The nautilid Eutrephoceras dekayi (Morton 1834) is relatively abundant in the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east-central Montana. We analysed the morphology, size frequency distribution, and isotope composition of a large collection of 220 well-preserved specimens including hatchlings, juveniles and adults. The newly hatched shell is approximately 14 mm in diameter with a body chamber one-third whorl in angular length terminating in the nepionic constriction. Internally, the embryonic shell contains five septa. Juveniles are abundant and comprise twothirds of the sample whereas sub-adults, defined by the incipient flattening of the venter, are rare. Adults comprise approximately one-third of the sample and average 100 mm in diameter. The co-occurrence of newly hatched shells, small juveniles and adults suggests that the eggs were laid in the same area in which the hatchlings developed. Based on the excellent preservation of the juveniles, we conclude that they did not float into the area after death, but lived in the region, implying that this area served as a nursery for young animals. The calculated temperatures of the embryonic shells are similar to those of the post-embryonic shells and generally range from 16 to 18°C. Upon hatching, the nautilids probably followed a demersal mode of life and lived in well-oxygenated water ≤50 m deep. An examination of lethal injuries (puncture holes) suggests that all ontogenetic stages were equally vulnerable to predation. The proximity of the site to the Sheridan Delta suggests that the specimens were smothered by sudden pulses of sediment transported into the area by major storms.

“…This area has been previously described by Bishop (, ), Clement (), Grier et al . (, ), Grier & Grier (), Walaszczyk et al . () and Landman et al .…”

Section: Geological Settingmentioning

confidence: 98%

Nautilid nurseries: hatchlings and juveniles of Eutrephoceras dekayi from the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east‐central Montana

Grier

Cochran

et al. 2018

LET

Self Cite

“…1a). This area has been described previously by Bishop (1967Bishop ( , 1973, Clement (1986), Grier and Grier (1998), and Grier et al (1992Grier et al ( , 2007. At the time of deposition of the concretion, the shoreline of the Western Interior Seaway was approximately 80 km to the south along the margin of the Sheridan Delta (Reiskind 1975).…”

Section: Geologic Settingmentioning

confidence: 73%

3-D orientation and distribution of ammonites in a concretion from the Upper Cretaceous Pierre Shale of Montana

Grier²,

Grier

et al. 2015

Swiss J Palaeontol

Self Cite

One of the most common modes of preservation of ammonites in the Upper Cretaceous US Western Interior is in concretions. We examine an accumulation of ammonites from a single concretion in the lower Maastrichtian Pierre Shale of eastern Montana. The concretion is an oblate spheroid 50 cm in length and 26 cm in diameter, with its long axis parallel to the substrate. It contains approximately 90 ammonite specimens representing three species of Hoploscaphites including adults and juveniles. The concretion also contains other fauna, primarily bivalves and gastropods. A total of 33 ammonites, mostly adults, are concentrated in a cluster that spans 71 % of the length of the concretion (called the ''sculpture''). 3-D measurements of the ammonites in the sculpture reveal that (1) the shells dip at all angles, with a significant trend toward more horizontal from west to east; (2) the shells dip with a highly significant bias toward the east, suggesting a current from that direction; and (3) a highly significant number of the shells that are non-vertical face with their left side up. Most of the shells show lethal damage as indicated by missing pieces of body chamber. After settling to the bottom, the shells may have piled up against each other, creating a sediment trap. Other organisms such as scavenging gastropods may have been attracted to the site to feed on the stranded ammonite carcasses. The chambers of the ammonite phragmocones and even some of the body chambers are empty, suggesting relatively rapid burial. Oxygen and carbon isotopic analyses of the ammonite shells reveal that they preserve their original isotopic signature. The values of d 13 C of the carbonate cement in the concretionary matrix are much lighter than those in the ammonite shells and imply that cementation of the concretion occurred in association with the decomposition of organic matter.

“…Cretaceous section on the anticline has been described by Calvert (1910), Coryell and Salmon (1934), Erdmann et al (1936), DeWolf and West (1939), Gries (1954), Robinson et al (1959), Bishop (1967Bishop ( , 1973, Gill and Cobban (1973), Clement (1986), Grier et al (1992Grier et al ( , 2007, Grier and Grier (1998), Walaszczyk et al (2001), Vuke et al (2003), Linn (2010), and Landman et al (2015aLandman et al ( , 2018b. As a result of renewed study of this section in the last several years, we present a modified version of the stratigraphic column originally published by Bishop (1967Bishop ( , 1973.…”

Section: Baculites Clinolobatus Baculites Grandismentioning

confidence: 94%

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

Bulletin of the American Museum of Natural History

Kennedy

Larson³

et al. 2019

Self Cite

453

Two species of scaphitid ammonites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (lower Maastrichtian) of the Western Interior of North America are described. Hoploscaph ites 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 crip tonodosus (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.