Geological Society of America

T307 Cephalopod Paleobiology and Paleoecology: A Tribute to Royal H. Mapes

Wednesday, 25 October 2017
8:00 AM - 12:00 PM
Room 607

USING IN SITU GEOCHEMISTRY TO INVESTIGATE THE DEPTH HABITAT OF AMMONITES

Paper number: T307-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Benjamin J. Linzmeier Presenting Author
Northwestern University, Evanston, IL
Benjamin J. Linzmeier Presenting Author
University of Wisconsin-Madison, Madison, WI
Neil Landman
American Museum of Natural History, New York, NY
Jocelyn A. Sessa
The Academy of Natural Sciences of Drexel University, Philadelphia, PA
Shanan Peters
University of Wisconsin–Madison, Madison, WI
Ian J. Orland
University of Wisconsin–Madison, Madison, WI
Kouki Kitajima
University of Wisconsin–Madison, Madison, WI
Reinhard Kozdon
Columbia University, Palisades, NY
John W. Valley
University of Wisconsin–Madison, Madison, WI

Abstract

Ammonites are the most iconic of extinct mollusks. Their wide spatial distribution and biostratigraphic utility make them ideally suited to investigate past climate. To date, stable isotope analyses of ammonites have focused on adult shells that can be sampled by bulk methods and have suggested adults had a nektobenthic mode of life. Eggs and hatchlings are thought to occupy a shallower position in the water column. Stratigraphic occurrence and analogy to modern cephalopods supports this hypothesis, but stable isotope analysis has not previously been applied to this question, primarily because the volume of material necessary for conventional techniques precludes sampling the diminutive embryonic shell (~700 µm diameter). Here we test for the depth that ammonite eggs and hatchlings inhabited by using secondary ion mass spectrometry (SIMS) for in situ sampling of the shell wall compared to environmental context.

Materials from the Maastrichtian Fox Hills and Owl Creek Formations of North America were sampled. Samples were assessed for diagenesis using backscattered electron and cathodoluminescence imaging. Original nacreous aragonite microstructure is visible. Electron backscatter diffraction confirmed aragonite mineralogy. Fossils from the Fox Hills Formation were from a single concretion. Samples from the Owl Creek Formation spanned several meters of section. Published bulk measurements from the location show no stratigraphic trend in δ¹⁸O, but ~1‰ range. Surface and bottom water conditions inferred from δ¹⁸O analysis of coocurring organisms.

Hatchling ammonite δ¹⁸O is variable within and between individuals at both sites. Shell wall precipitated before hatching in the Fox Hills Hoploscaphites suggest a benthic development (~-2‰ change at hatching) and carbon isotope data suggest influence of terrestrial runoff or the egg yolk carbon source at hatching (~-5‰ change at hatching). These pre- to post-hatching differences suggest the relationship between egg size and development depth amongst modern coleoids may not apply to ammonites. Owl Creek material shows no change in δ¹⁸O between varix and post-hatching shell wall, but a wide range of δ¹⁸O especially in later whorls (2‰ in whorls 4 or more). Our data demonstrate the utility of in situ geochemical methods to better understand life history of mollusks.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305790

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