The ammonite septum is not an adaptation to deep water: re-evaluating a centuries-old idea

Abstract

The shells of ammonoid cephalopods are among the most recognizable fossils, whose fractally folded, internal walls (septa) have inspired many hypotheses on their adaptive value. The enduring explanation for their iterative evolution is that they strengthen the shell against pressure at increasing water depths. The fossil record does not definitively support this idea and much of the theoretical mechanical work behind it has suffered from inaccurate testing geometries and conflicting results. By using a different set of mathematical methods compared with previous studies, I generate a system of finite-element models that explore how different parameters affect the shell's response to water pressure. Increasing the number of initial folds of the septa ultimately has little to no effect on the resulting stress in the shell wall or the septum itself. The introduction of higher-order folds does reduce the tensile stress in the shell wall; however, this is coupled with a higher rate of increase of tensile stress in the septum itself. These results reveal that the increase in complexity should not be expected to have a significant effect on the shell's strength and suggests that the evolution of ammonitic septa does not reflect a persistent trend towards deeper-water habitats.

A comment on Lemanis (2020): The ammonite septum is not an adaptation to deep water

“…R. Soc. B [1] refutes the idea that septal complexity in ammonoids is an adaptation to strengthen the shell against hydrostatic pressure. Although there is some evidence suggesting that septal complexity is involved in a metabolic rather than a biomechanical function [2], the author adequately indicates that there are still reminiscences of the strengthening hypothesis in some well-known textbooks.…”

Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture

“…Recent approaches have used sophisticated modeling techniques to explore shell stress. Simulations by Lemanis 44 demonstrate that higher-order folds, characteristic of ammonitic septa, do not reduce the tensile stress of the shell, however, increased point load resistance is suggested 45 . Johnson et al 46 investigated shell strength by performing physical compression experiments on 3D-printed models and found that sutural complexity does not relate to compression resistance.…”

“…Johnson et al 46 investigated shell strength by performing physical compression experiments on 3D-printed models and found that sutural complexity does not relate to compression resistance. Furthermore, others have found that fluted septa may actually decrease certain types of stress resistance , 44 , 47 . Concerning hydrostatic pressure, fossil occurrences and lithological data do not support a relationship between increased septal complexity and deeper habitats 48 – 50 .…”

The Intertwined Evolution and Development of Sutures and Cranial Morphology

“…It is well known that vertebrate suture morphology responds to stresses imposed by behaviour and ecology ( Jaslow and Biewener, 1995 ; Clack, 2002 ; Goswami et al, 2013 ; Buezas et al, 2017 ). It has similarly long been postulated that ammonoid shell suture morphology may also respond to ecological stresses such as hydrostatic pressure ( Hewitt and Westermann, 2007 ), although there is controversy within the literature and this idea is still debated ( Lemanis, 2020 ). Remarkably, these non-homologous sutures may have convergently evolved to respond to similar external pressures.…”