Reading the age of an ammonite from its jaws

Abstract

International audienceAssessing ammonites lifespan is a challenging issue for macroevolutionary studies. Age estimations based on growth features and chemical compositions of the low Sr aragonitic shell have been attempted on few exceptionally preserved specimens, but remain debated (Bucher et al. 1996). Compared to the aragonitic shell, the calcitic covering of the lower jaw (called aptychus) is poorly investigated although in Recent cephalopods, chitinous beaks are considered a relevant proxy for the age of the animal (Perales-Raya et al. 2014). Sclerochonological studies carried out on recent and fossil mollusks have provided a methodological framework that could be applied to ammonite lower jaws (Lartaud 2010, Bougeois 2013). Following these studies, well-preserved aptychi from the Campanian of Alabama (Mooreville Chalk Formation) were investigated in order to assess its potential for schlerochonological studies. Microstructural observations indicate that fine growth features (up to 2µm thick) are well preserved and that a part of paleobiological information is conserved. We analyzed growth lines spacing-out, microstructure, and variation of the chemical composition along the growth direction (∂18O and trace elements) in order to provide the first age estimations for ammonites obtained from non-shell structures