Ammonite habitat revealed via isotopic composition and comparisons with co-occurring benthic and planktonic organisms

Abstract

Ammonites are among the best-known fossils of the Phanerozoic, yet their habitat is poorly understood. Three common ammonite families (Baculitidae, Scaphitidae, and Sphenodiscidae) co-occur with wellpreserved planktonic and benthic organisms at the type locality of the upper Maastrichtian Owl Creek Formation, offering an excellent opportunity to constrain their depth habitats through isotopic comparisons among taxa. Based on sedimentary evidence and the micro-and macrofauna at this site, we infer that the 9-m-thick sequence was deposited at a paleodepth of 70-150 m. Taxa present throughout the sequence include a diverse assemblage of ammonites, bivalves, and gastropods, abundant benthic foraminifera, and rare planktonic foraminifera. No stratigraphic trends are observed in the isotopic data of any taxon, and thus all of the data from each taxon are considered as replicates. Oxygen isotope-based temperature estimates from the baculites and scaphites overlap with those of the benthos and are distinct from those of the plankton. In contrast, sphenodiscid temperature estimates span a range that includes estimates of the planktonic foraminifera and of the warmer half of the benthic values. These results suggest baculites and scaphites lived close to the seafloor, whereas sphenodiscids sometimes inhabited the upper water column and/or lived closer to shore. In fact, the rarity and poorer preservation of the sphenodiscids relative to the baculites and scaphites suggests that the sphenodiscid shells may have only reached the Owl Creek locality by drifting seaward after death.A mmonites have constituted a primary data source for the fields of evolution, paleoceanography, biostratigraphy, and paleoecology for more than a century; their ubiquity, diversity, occurrence in a wide variety of marine environments, and readily preservable shell account for their utility in both paleontological and geological studies. Ammonites have been used extensively in studies of heterochrony because their shells preserve distinct ontogenetic changes that can be tracked in evolving lineages (1, 2); they are valued in paleoceanographic research because, like most mollusks, they are inferred to have precipitated their aragonitic shells in isotopic equilibrium with the surrounding seawater (3, 4). Thus, shell chemistry may record temperature, via oxygen isotopes (δ 18 O) (5), and water mass properties, such as strontium isotopes ( 87 Sr/ 86 Sr), which are used to estimate numerical age (6). Ammonites are also a textbook example of an index fossil; besides being abundant and widespread, they evolved rapidly, making them the dominant Mesozoic tool for relative dating and correlation of shallow water strata. For example, the 35-My-long stratigraphic record of Upper Cretaceous deposits in the US Western Interior Seaway (WIS) has been partitioned into 66 ammonite zones (7). Finally, ammonites underwent a spectacular extinction at the close of the Mesozoic. Explanations for why the ammonites, which were flourishing immediately before the Cretaceous-P...

Late Cretaceous marine arthropods relied on terrestrial organic matter as a food source: Geochemical evidence from the Coon Creek Lagerstätte in the Mississippi Embayment

“…Although we cannot further constrain the Coon Creek Formation to a specific depositional setting or depth, such as a back-bar environment (Ebersole, 2016) or subtidal-intertidal zone (Bishop, 2016), our isotopic results do not refute these studies, and they can be used to bolster future interpretations of the depositional environment based upon sedimentology. Our decapod carbon isotope data add to previous inorganic isotopic analyses of molluskan fauna and foraminifera in this region (Keller et al, 2006;Lowenstam & Epstein, 1954;Sessa et al, 2015;Weiner et al, 1976;Zakharov et al, 2006Zakharov et al, , 2014 and with these studies provide a basic framework for understanding carbon cycling in the nearshore of the Late Cretaceous Mississippi Embayment. The presence of biogenic isotope results in Coon Creek decapods suggests that unaltered biogenic organic isotopic signals are also probably preserved in associated bivalves and ammonites, and thus these taxa may represent viable substrates for future isotope analyses.…”

“…Our results provide strong geochemical support for recent interpretations of the Coon Creek Formation as a shallow, nearshore marine setting and, in doing so, help constrain the problematic environment of the northern Mississippi Embayment during the late Campanian-early Maastrichtian. Along with recent multiproxy isotope studies of Upper Cretaceous taxa in this region (e.g., Sessa et al, 2015), the combined organismal and sedimentary geochemical analysis presented here is a promising approach for understanding ecological responses to major climatic shifts in the Mississippi Embayment during the Late Cretaceous. More broadly, the increasingly regular discoveries of unaltered or vestigial chitin of geologic age suggest that biogenic isotopic signals from fossil arthropods have the potential to refine our understanding of a wide range of environments and foraging ecologies of organisms in deep time.…”

Evidence for a composite organic–inorganic fabric of belemnite rostra: Implications for palaeoceanography and palaeoecology

“…Characteristic geochemical patterns and fabrics in biogenic carbonate hardparts have been used to test for example molluscs (e.g., Cochran et al, 2003, Sessa et al, 2015, Immenhauser et al, 2016, brachiopods (Parkinson et al, 2005), or foraminifera (e.g., Huber and Hodell, 1996;Kozdon et al, A C C E P T E D M A N U S C R I P T…”

Geochemical evidence (C and Sr isotopes) for methane seeps as ammonite habitats in the Late Cretaceous (Campanian) Western Interior Seaway

“…Non-seep data for these groups are limited in the D. cheyennense Zone, so we have tabulated results from samples collected in South Dakota from two additional ammonite Zones (Baculites compressus and Hoploscaphites nebrascensis Zones). We also include results from the Discoscaphites iris Zone on the Gulf Coastal Plain because of the large number of samples and the very high quality of preservation (Sessa et al 2015). For the Cochran et al (2010) and Dennis et al (2013) d Cochran et al (2003) and Dennis et al (2013); excludes brackish samples e Sessa et al (2015) ** Indicates difference with seep ammonites is statistically significant (unpaired t test) at indicated P value (Table 1).…”