Growth trajectories of some major ammonoid sub-clades revealed by serial grinding tomography data

Abstract

Molluscs such as ammonoids record their growth in their accretionary shells, making them ideal for the study of evolutionary changes in ontogeny through time. Standard methods usually focus on two‐dimensional data and do not quantify empirical changes in shell and chamber volumes through ontogeny, which can possibly be important to disentangle phylogeny, interspecific variation and palaeobiology of these extinct cephalopods. Tomographic and computational methods offer the opportunity to empirically study volumetric changes in shell and chamber volumes through ontogeny of major ammonoid sub‐clades in three dimensions (3‐D). Here, we document (1) the growth of chamber and septal volumes through ontogeny and (2) differences in ontogenetic changes between species from each of three major sub‐clades of Palaeozoic ammonoids throughout their early phylogeny. The data used are three‐dimensional reconstructions of specimens that have been subjected to grinding tomography. The following species were studied: the agoniatitid Fidelites clariondi and anarcestid Diallagites lenticulifer (Middle Devonian) and the Early Carboniferous goniatitid Goniatites multiliratus. Chamber and septum volumes were plotted against the septum number and the shell diameter (proxies for growth) in the three species; although differences are small, the trajectories are more similar among the most derived Diallagites and Goniatites compared with the more widely umbilicate Fidelites. Our comparisons show a good correlation between the 3‐D and the 2‐D measurements. In all three species, both volumes follow exponential trends with deviations in very early ontogeny (resolution artefacts) and near maturity (mature modifications in shell growth). Additionally, we analyse the intraspecific differences in the volume data between two specimens of Normannites (Middle Jurassic).

Intraspecific variability through ontogeny in early ammonoids

“…One might suggest that these differences are rooted in the widely documented juvenile (post-hatching) morphological intraspecific variability of mollusk shells (e.g., Jacobs 1992;Jacobs et al 1994;Jacobs and Chamberlain 1996;Urdy et al 2010a, b;De Baets et al 2013, 2015Naglik et al 2015a). This is in contrast with the fact that the conch morphology of some of the juveniles of early late Viséan ammonoids lies at the edge or even outside of the morphospace of adult Palaeozoic ammonoids (Figs.…”

“…As documented by many authors, the ammonite conch underwent several morphological changes during ontogeny, possibly reflecting changes in habitat and habit (e.g., Drushchits et al 1977;House 1996;Westermann 1996;Klug 2001;Lukeneder et al 2010;Ritterbush et al 2014;Robin et al 2014;Lukeneder 2015;Mironenko and Rogov 2015;Naglik et al 2015a). These ontogenetic changes can be profound to subtle.…”

Non-invasive imaging methods applied to neo- and paleo-ontological cephalopod research

“…The most important thing is to recognize that the PVE is also present in grinding tomography images (Fig. 12), but this has not yet been taken into account (see Naglik et al, 2014 andPascual-Cebrian et al, 2013). A second issue is the non-isotropic voxel size produced during grinding tomography.…”

“…A few years earlier Sollas (1904) invented a method using serial sections that allowed the visualization of internal structures of fossils. This approach has formed the basis of modern grinding tomography (e.g., Pascual-Cebrian et al, 2013). Much has been improved since the earliest attempts, invasive or not, for 3-D reconstructions of fossils and their internal structures (Sollas and Sollas, 1914;Tipper, 1976;Luo and Ketten, 1991;Herbert et al, 1995;Hurum, 1996;Stock, 2006;Tafforeau et al, 2006;Sutton et al, 2001, Hoffmann and Zachow, 2011;Falkingham, 2012).…”

Wrinkle layer and supracephalic attachment area: implications for ammonoid paleobiology

“…In many taxa, this could have been a rather heavy part of the body including the head, jaw apparatus (with large calcified aptychi in the case of ammonites with aptychus-type jaws), hyponome and arms (although it is unknown what they looked like and whether they were muscular). At the same time, in most of all computations published to date, it is explicitly or implicitly assumed that the soft body of ammonoids with planispiral shells was located entirely within the body chamber (Trueman 1941, Raup & Chamberlain 1967, Saunders & Shapiro 1986, Westermann 1996, Naglik et al 2014, Tajika et al 2015. Since these authors did not take the protrusion of the cephalic region out of the body chamber into account, the apertures of many ammonites are located in a nearly horizontal position (Trueman 1941, figs 14, 15;Saunders & Shapiro 1986, fig.…”