291 Cephalopods Through Time: Insights into Evolution, Ecology, and Environmental Reconstruction

This session explores cephalopod biology, ecology, and evolution, highlighting their role in paleoenvironmental reconstruction. We welcome analytical and computational research that advances understanding of these organisms, providing new perspectives on their evolution and ecological significance.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 304B

Towards reconstructing Cretaceous paleoecologies: paired oxygen and carbon isotope analyses of Western Interior Seaway fossiliferous concretions

Paper number: 291-7
Program: Technical Sessions
Time: 3:25 PM – 3:40 PM

Authors

Abstract

The Cretaceous Western Interior Seaway contains fossils of abundant molluscan fauna, but interpreting the ecology and biology of these extinct organisms can be difficult, particularly those like ammonites with no modern analogues. Fossiliferous carbonate concretions from the Western Interior Seaway provide exceptional preservation of diverse molluscan assemblages and probably represent rapid formation events (<10 years), potentially offering near-instantaneous ecosystem snapshots. The preservation of shell material within concretions allows for analysis of oxygen isotopes, with implications for paleotemperature, and carbon isotopes, a less frequently interpreted but significant tool for examining intrinsic biology.

We analyzed paired δ13C and δ18O from 101 specimens across eight concretions, including ammonites (Baculites, Hoploscaphites), nautiloids, and benthic mollusks (bivalves, inoceramids, and gastropods). Specimens were filtered using Preservation Index criteria (PI ≥3) and analyzed using standard mass spectrometry techniques. Results demonstrate that specimens of the same species within concretions are generally isotopically comparable to each other. δ18O ranges are consistently smaller within concretions than between concretions, implying that concretions record discrete environmental conditions from others. Systematic inter-morphotype relationships persist across concretions, particularly within δ13C values: bivalves consistently exhibit more positive δ13C than cephalopods (∆δ13C ~2-5‰), probably indicative of lower metabolic rates. δ18O relationships are less consistent than carbon, but scaphites show consistently more negative δ18O than baculites (~1‰, equivalent to ~4°C warmer temperatures), suggesting distinct water column habitats. These findings suggest concretions may be reliable targets for paleoenvironmental reconstruction and reveal persistent ecological patterns in Cretaceous marine ecosystems. The consistent isotopic offsets between morphotypes likely reflect differences in metabolic rates and habitat depths, providing new constraints on ammonite paleoecology and Western Interior Seaway water column structure.

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

doi: 10.1130/abs/2025AM-412352

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