Analysis of septal spacing and septal crowding in Devonian and Carboniferous ammonoids

Abstract

Septal crowding is widely known as a sign of maturity in conchs of ammonoids and nautiloids. However, reduced septal spacing may also occur as a consequence of adverse ecological conditions. Here, we address the question how septal spacing varied through ontogeny in representatives of some of the major clades of Devonian and Carboniferous ammonoids. We found that the degree of ontogenetic variation is similar between clades and that variation is only weakly linked with conch form. The results show that septal crowding alone is insufficient to identify adulthood in ammonoids; intermediate septal crowding is a common phenomenon and occurs in various growth stages. Changes in septal distances during ontogeny were, in addition to adulthood of the individuals, a passive reaction likely caused by fluctuating environmental conditions.

Stepwise growth changes in early post-embryonic stages among Cretaceous tetragonitid ammonoids

“…The ontogenetic trajectories of septal spacing enable us to understand details of chamber formation throughout the development of these cephalopods. These trajectories (Kulicki 1974;Lehmann 1976;Bucher and Guex 1990;Bucher 1997;Dommergues 1988;Landman and Waage 1993;Mignot 1993;Bucher et al 1996;Polizzotto et al 2007;Kraft et al 2008;Paul 2011;Arai and Wani 2012;Zell and Stinnesbeck 2016;Iwasaki et al 2020;Beck et al 2021;Kawakami et al 2022;Takai et al 2022) have mostly been analyzed using two-dimensional measurements (rotational angles or linear distance between two successive septa) (Naglik et al 2015a;Tajika et al 2020). The ontogenetic trajectories of septal spacing change at the transitions between ontogenetic stages (embryonic, neanic, juvenile, and mature stages) (e.g., Westermann 1958;Bucher et al 1996;Klug 2001;Kraft et al 2008).…”