Ammonites as paleothermometers: Isotopically reconstructed temperatures of the Western Interior Seaway track global records

Ammonites are externally shelled cephalopods that were common in the North American Western Interior Seaway (WIS), and as they grew their aragonitic shells via accretion, they recorded aspects of their environment in the stable isotopic composition of their shells. While the mobility of ammonites may complicate efforts to reconstruct temperatures from their shells, they remain a potentially valuable target for paleothermometry. In this study, we reconstruct the spatial and temporal variability of WIS temperatures using a suite of ammonites (n = 113) spanning the last 25 million years of the Cretaceous along a North-South gradient ranging from the Canadian WIS to the Mississippi Embayment. We present a temporally high-resolution (∼0.6 Ma) oxygen isotope record from these ammonites that indicates cooling temperatures in the WIS of comparable magnitude (∼18 °C +/‐− 4°) to the temperature change seen in global studies, most notably cooling from the Cretaceous Thermal Maximum in the Turonian until the late Maastrichtian. Studies disagree regarding the development and strength of a latitudinal (pole-to-equator) temperature gradient during the Cretaceous; we do not see strong evidence for a latitudinal temperature gradient in the WIS. We do not observe any bias driven by ammonite morphology in our isotopic data, though we suggest that researchers consider the effects of taxonomy and ecological bias on their temperature records. As our ammonite δ¹⁸O record matches the direction and magnitude of global temperature reconstructions, our data imply that ammonites are viable targets for paleothermometry.