OXYGEN ISOTOPE EVIDENCE OF HABITAT CHANGE DURING EARLY ONTOGENY IN CRETACEOUS HOPLOSCAPHITES

Ammonites are thought to have been part of the plankton during early ontogeny because individuals are small (~600 μm) and spherical at hatching. Morphological changes, such as the appearance of ornamentation, during early ontogeny are thought to reflect a transition from the planktonic to benthic habitats. Ammonite shell aragonite is precipitated in oxygen isotope equilibrium with seawater so changes in oxygen isotope ratio (δ¹⁸O) of ammonite shell indicate habitat change. Analysis of δ¹⁸O within early ontogenetic shell is difficult using mechanical sampling because the shell wall is thin (~10 μm). Secondary ion mass spectrometry (SIMS) measures the in situ δ¹⁸O of small domains (10 μm) with high precision (±0.3‰ 2SD) and provides the means to investigate ammonite habitat change during very early ontogeny. Analysis of δ¹⁸O from five Hoploscaphites sp. was conducted to test for variation during early ontogeny. Samples were from a concretion at the contact between the Trail City and Timber Lake members of the Fox Hills Formation (Maastrictian) of South Dakota. These members were deposited during the final regression of the Western Interior Seaway. These five individuals preserve nacreous aragonite microstructure that suggests δ¹⁸O was not altered by diagenesis and records equilibrium precipitation values. Diagenetic calcite and coeval bivalve fragments were also analyzed by SIMS.

All five ammonites show δ¹⁸O change with ontogeny. There is a ~3‰ range of δ¹⁸O within nacreous aragonite of each ammonite. There are there is a difference between pre-hatching and post-hatching δ¹⁸O. All individuals have an additional δ¹⁸O transition that may correspond to the appearance of ornamentation. These results show that ammonites did change habitat during early ontogeny. The range of δ¹⁸O within the ammonites suggests either the Western Interior Seaway was stratified during the Maastrictian or the ammonites used nearshore habitats with different δ¹⁸O during early ontogeny.