STABLE ISOTOPE INVESTIGATION OF EARLY ONTOGENY IN UPPER CRETACEOUS OWL CREEK FORMATION AMMONITES

The strong contrast between egg size and hatchling ecology of ammonoids and nautiloids is thought to explain why ammonoids were more vulnerable to extinction and more prone to diversification than nautiloids throughout the fossil record. Nautiloids had large slowly developing eggs (20-30 mm) and nektobenthic juveniles and ammonoids had small eggs (0.7 mm) and potentially planktonic juveniles. Stable isotope analysis of δ¹⁸O(CaCO₃) from early shell wall has the potential to elucidate living depth from early ontogeny that morphology and stratigraphic occurrence cannot. Conventional mechanical separation for stable isotope analysis cannot readily isolate very small domains such as the embryonic shell wall (~10 μm). Secondary ion mass spectrometry (SIMS) overcomes the limitations of mechanical separation, because a focused Cesium ion beam performs in situ sampling of the δ¹⁸O of small domains (10 μm) with high precision (±0.3 ‰ 2SD).

To observe the isotope pattern from early ontogeny, we sampled two ammonites from the late Maastrichtian Owl Creek Formation in northern Mississippi. This formation was chosen because of exquisite preservation, faunal diversity that includes foraminifera and bivalves, and a preexisting dataset of bulk stable isotope analyses. Scanning electron microscopy shows the two ammonites sampled by SIMS preserve prismatic and nacreous microstructure with little diagenetic alteration.

The two Discoscaphites iris that have been analyzed show no trend in δ¹⁸O values through the first several whorls of growth (n = 14, 1.0‰ 2SD and n = 5, 1.4‰ 2SD). This suggests that these individuals may have hatched and lived in the same water mass for these early few months of life. Additional sampling from other individuals and later whorls may reveal, via δ¹⁸O, habitat change through ontogeny. Analysis of foraminifera will allow for intermethod comparison with published datasets from this location.