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

Abstract

FRONTISPIECE. Breaks along the Yellowstone River (background) exposing the Upper Cretaceous Pierre Shale on the Cedar Creek Anticline, Dawson County, Montana. This is one of the classic sites first explored by Ferdinand V. Hayden during the 1850s, and the source of some of the largest species of Hoploscaphites in North America. Photo by T. Linn. BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO.

Geographic and temporal morphological stasis in the latest Cretaceous ammonoidDiscoscaphites irisfrom the U.S. Gulf and Atlantic Coastal Plains

“…Previous studies of morphological change in both juvenile and adult scaphitids have noted a consistent pattern of increasing whorl compression in specimens and species from nearshore versus offshore environments—also using grain-size as a proxy (e.g., Jacobs et al 1994; Klein and Landman 2019; Landman et al 2020; Witts et al 2020). Hydrodynamics suggest that a streamlined morphology is more suited to higher-energy conditions in shallow, nearshore settings and that whorl compression is a particularly plastic trait in ammonoids.…”

“…The point at which the hook curves backward is called the point of recurvature. Dimorphism is present in D. iris , as in all scaphitid ammonoids and many modern cephalopods, and is interpreted as sexual in nature (Landman and Waage 1993; Davis et al 1996; Landman et al 2010). The dimorphs are referred to as the macroconch, presumably the female, and the microconch, presumably the male.…”

“…It is important to note that these data represent variation among mature adults rather than ontogenetic variation (Landman et al 2008). This is because the mature stage of scaphitid ammonoids is well defined by the uncoiling of the body chamber (e.g., Landman and Waage 1993; Landman et al 2010); whorl compression ratios taken at different points on the shell provide some information on ontogenetic changes.…”

“…Shifts in the distribution, richness, and abundance of various groups of marine plankton occur alongside these climate changes before the K/ Pg boundary (Olsson 2001;Thibault 2016;Vellekoop et al 2019), while morphological changes (dwarfing) and increased fragmentation in some planktonic foraminifera hint at stressful conditions in the oceans (MacLeod et al 2000;Keller and Abramovich 2009;Henehan et al 2016;Gilabert et al 2021). The influence of these environmental changes on ammonoid species has been debated, with some workers suggesting widespread global decline of ammonoids leading into the K/Pg (e.g., Stinnesbeck et al 2012) and others supporting species' health and even increased abundance during the latest Cretaceous (Landman et al 2014(Landman et al , 2015Witts et al 2015Witts et al , 2018Witts et al , 2021.…”

“…1) using collections from multiple sites. Scaphitid ammonoids are well preserved, widely distributed, and locally abundant in Late Cretaceous shallow-marine sediments across the Northern Hemisphere (e.g., Landman and Waage 1993; Machalski 2005; Landman et al 2010, 2014). Discoscaphites in particular is one of the last common ammonoid genera found at the end of the Cretaceous (see review by Landman et al 2015), and in North America, numerous species occur in the Maastrichtian of the ACP, GCP, and the Western Interior Seaway (WIS) (Fig.…”