EXPLORING AMMONITE NACRE TABLET THICKNESS AS A POTENTIAL PROXY FOR WATER DEPTH IN THE WESTERN INTERIOR SEAWAY

The Western Interior Seaway (WIS) is known to North American paleontologists for its abundance of well-preserved marine fauna, common among which are the ammonites. These extinct, externally shelled cephalopods grew their aragonitic shells through their entire lives, recording information about the environments in which they lived. Despite considerable research, there is much that remains unknown about their paleoecology. Recent work has postulated that the nacreous microstructure of mollusk shells, namely the thickness of the nacre tablets, may serve as a proxy for water temperature or depth. Previous studies suggest that increased nacre tablet thickness (TT) correlates with decreased water pressure and shallower depth.

Our study presents preliminary data from a suite of ~120 ammonites of varying morphotypes from a range of Late Cretaceous stages and locations across the WIS, examining the correlation between TT and oxygen isotope values (δ¹⁸O). δ¹⁸O is commonly used as a paleothermometer, in which lower δ¹⁸O values reflect warmer temperatures, though this can be complicated by changes in water δ¹⁸O. We paired stable isotope analysis with extensive SEM examination of shell microstructure and ImageJ measurements of TT. If water temperature was primarily controlled by depth, we would hypothesize that δ¹⁸O (inversely correlated with paleotemperature) would negatively correlate with TT, representing warmer water temperatures at shallower depths, and thus thicker tablets. In contrast, our initial results (n = 14 specimens) show a positive correlation between δ¹⁸O and TT (R² = 0.5), indicating a more complex explanation is needed. Previous work has proposed that the WIS exhibits water column salinity stratification, in which surface waters are influenced by freshwater with more negative δ¹⁸O values, which would be consistent with our results, though we cannot rule out other explanations.

TT does not appear to vary as a function of locality latitude, age of specimen, or shell preservation quality. Additionally, intra-sample variation in TT of a given specimen is very low, and TT values are comparable to prior studies. Our preliminary analyses suggest that ammonite nacre tablet thickness could be used as a proxy for water depth for these cosmopolitan and charismatic organisms, though further work is needed.