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

Abstract

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.

Evolutionary stasis, ecophenotypy and environmental controls on ammonite morphology in the Late Cretaceous (Maastrichtian) Western Interior Seaway, USA

“…Trends in shell size, shape and compression have been documented previously in more restricted studies of scaphitids and other ammonite groups from the WIS (Landman ; Landman & Waage , b; Yacobucci , ; Landman et al . , , ), in some cases showing correlation with changing environmental parameters (Jacobs et al . ; Klein & Landman ).…”

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

“…Hoploscaphites crassus and H. plenus also occur in the B. eliasi and B. baculus zones of the Bearpaw Shale of Saskatchewan and Alberta (Riccardi, 1983). Three other species of Hoploscaphites were previously described from the same general area: H. macer Landman et al, 2019, andH. criptonodosus Riccardi (1983) from the upper part of the B. baculus and lower part of the B. grandis zones, and H. sargklofak Landman et al, 2015a, from the B. grandis Zone.…”

“…However, the depth at each locality depends on the particular time slice under consideration. For example, the depth of the WIS on the Cedar Creek Anticline in east-central Montana during the deposition of the upper part of the Baculites baculus Zone was probably less than 50 m (Landman et al, 2019).…”

“…Litho-and biostratigraphy: The lithoand biostratigraphy of the Upper Cretaceous part of the Cedar Creek Anticline has been recently reviewed by Landman et al (2018aLandman et al ( , 2019 and Grier et al (2019). The Pierre Shale consists of light to dark gray, fissile, silty or sandy shale 60-65 m thick, interbedded with bentonites and bentonitic shale.…”

“…In fact, the rarity or near-absence of B. eliasi, and B. baculus Meek and Hayden, 1861, from the overlying zone, has led to uncertainty in FIGURE 4. Stratigraphic section of the Pierre Shale and Fox Hills Formation near Glendive, Dawson County, Montana, on the northwestern end of the Cedar Creek Anticline, slightly modified from the latest iteration (Landman et al, 2019) to include the presence of cold methane seep deposits. The ranges of six species of Hoploscaphites are illustrated (solid vertical lines = known occurrences; dashed vertical lines = likely occurrences).…”