Ammonites are iconic members of Jurassic and Cretaceous marine communities, but many questions remain about their ecology. Because it contains a diverse assemblage of well-preserved macro- and microfossils, the upper Maastrichtian Owl Creek Formation exposed in Tippah County, Mississippi, is an excellent site at which to compare carbonate isotopic patterns within and among taxa. A previous isotopic study of two ammonite groups common in the study section (baculitids in the genera Baculites and Eubaculites and scaphitids in the genus Discoscaphites) indicated that Owl Creek baculitids and scaphitids lived near the seafloor. However, their wide range of measured isotopic values, coupled with oxygen isotopic values suggesting that the ammonite shells sometimes were secreted at temperatures cooler than those experienced by other benthic mollusks suggested the possibility that these taxa may have migrated. Here, we test the migration hypothesis by comparing ontogenetic isotopic trends in the shells of co-occurring ammonites, bivalves, and gastropods. We found the range in δ18O values to be generally similar among baculitids, scaphitids, nuculids (Bivalvia), and various gastropods and that there are no consistent δ18O trends through ontogeny in baculitids or scaphitids (or other mollusks). Thus, there is no need to invoke migration for either ammonite group. Additionally, the variability in δ18O values within individual shells is low in scaphitids and baculitids, intermediate among gastropods, and high in nuculids, indicating that benthic temperature variability occurred on timescales that were long relative to the rate of shell formation for ammonites but short relative to that of the lifespan of the gastropods and nuculids. This interpretation implies rapid growth among baculitids and scaphitids, a conclusion consistent with lower δ13C values in scaphitids and baculitids compared to other mollusks.
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Vol. 34 • No. 6