70-17 Distribution of the Late Cretaceous ammonite Hoploscaphites in the Western Interior of North America

Session: Paleontology, Biogeography/Biostratigraphy & Phylogenetic/Morphological Patterns (Posters)

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Abstract:

The Late Cretaceous ammonite Hoploscaphites is well represented in the Upper Cretaceous Western Interior of North America. Approximately 40 species lived in the basin starting in the middle Campanian and extending to the late Maastrichtian. During this time interval, the Seaway shrank and expanded, reflecting changes in sea level and tectonic activity, with consequent shifts in the position of the shoreline, but never dried up. We plotted the geographic distribution of Hoploscaphites through this time interval based on 897 occurrences from 310 localities (USGS, AMNH, YPM, BHI, etc). Of course, such maps illustrate only a fraction of the areal extent of each species inasmuch as the locality data reflect the limited extent of outcrops, various taphonomic factors, and collection bias. We used several programs, chiefly GEOlocate, Googe Maps, and Google Earth following the MaNIS/HerpNet/ORNIS georeferencing guidelines. TRS data were treated in GEOLocate, and when this approach did not work, we geocoded in a GIS framework.  The TRS locality information was converted to Decimal Degree coordinates. We plotted the localities on shoreline maps of the Seaway published in Cobban et al. (1994), Slattery et al. (2015), and information provided by K.C. McKinney (pers. comm., 2024). Hoploscaphites first appears in the Seaway at the start of the middle Campanian (corresponding to the Baculites obtusus Zone). Most of the species are shared in common with West Greenland and parts of Europe. Hoploscaphites attains its maximum extent in the late, but not latest Campanian (corresponding to the B. reesidei and B. jenseni zones) stretching from New Mexico to Alberta and Saskatchewan, reflecting the elevated sea levels at this time. Most of the species are shared in common with parts of Europe. In the latest Campanian (corresponding to the B. eliasi Zone), the distribution is equally broad, but all of the species are endemic to the Seaway or shared in common with the Gulf and/or Atlantic Coastal Plains. This pattern persists into the Maastrichtian, but the distribution of Hoploscaphites shrinks in concert with the retreat of the Seaway.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6880

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