AMMONITE HABITAT REVEALED VIA ISOTOPIC COMPOSITION AND COMPARISONS WITH CO-OCCURRING BENTHIC AND PLANKTONIC ORGANISMS

Ammonites are amongst the most well known fossils of the Phanerozoic, yet where they lived within the water column is poorly known. At its type locality, the Upper Cretaceous (upper Maastrichtian) Owl Creek Formation offers an excellent opportunity to examine the depth habitat of juveniles and adults from three common ammonite families, the Baculitidae, Scaphitidae, and Sphenodiscidae, by comparing the isotopic composition of their shells to that of co-occurring planktonic and benthic organisms. Based on sediment composition and structures, and the micro- and macro-fauna of this outcrop, we infer a fully marine environment below wave base, ~50 to 70 m paleodepth. About 400 bivalve, gastropod, and ammonite specimens were collected from the 9 m thick exposure. Fossils were SEM’ed to screen for alteration; ~200 specimens were found to be well preserved. From these, we made over 300 isotopic measurements. In addition, planktonic and benthic foraminifera were picked from five stratigraphically dispersed horizons, resulting in 11 isotopic measurements of planktonic foraminifera and 14 of benthic foraminifera. No stratigraphic trends are observed in the isotopic data of any taxonomic group.

We estimate local bottom water temperatures via oxygen isotopes (δ¹⁸O) from benthic foraminifera, bivalves, and gastropods; serial samples from bivalve shells constrain seasonal seafloor temperature fluctuations. Surface water temperatures are estimated from the δ¹⁸O of planktonic foraminifera. The δ¹⁸O of baculitids and scaphitids suggest that they lived close to the seafloor. The range of variability in their isotopic data is similar to that seen in the seasonal profiles of bivalves, implying that these ammonites lived in one region and did not migrate with currents of a particular temperature. In contrast, the oxygen isotopic values of the sphenodiscids align with those of the planktonic foraminifera, implying that they inhabited the upper water column. This, combined with their rarity and poor preservation relative to the baculitids and scaphitids, suggests that perhaps the sphenodiscids lived closer to shore than the studied outcrop, and drifted seaward after death. These results provide evidence that the Baculitidae and Scaphitidae were nektobenthic/demersal, while the Sphenodiscidae lived within surface waters.