Theoretical Modelling of the Molluscan Shell: What has been Learned From the Comparison Among Molluscan Taxa?

Describing Ammonoid Conchs

“…Constrictions are less frequent than the previously mentioned radial elements; they usually occur in a lower number than ribs (often between one and five per whorl) and commonly are produced at growth halts (megastriae; see Bucher et al 1996;Urdy 2015). At least on the internal mould (steinkern), constrictions are visible as furrows.…”

“…The conch diameter has often been used as proxy for size (and relative age). However, other properties like body chamber volume might be more suitable as a proxy for size because it better reflects the volume of the soft body than the conch diameter, especially when comparing forms with very different conch geometries (e.g., Bucher et al 1996;Dommergues et al 2002;De Baets et al 2012, 2013a, 2015. In extant coleoids (Nixon and Young 2003;Boyle and Rodhouse 2005), mostly the (dorsal) mantle length (which would correspond with the body chamber length in ammonoids) is used as a measure of size.…”

“…Caused by allometric growth, the change in certain parameters through ontogeny is not necessarily perfectly linear in a loglog-space (Kant 1973;Kant and Kullmann 1980;Klug 2001;Urdy et al 2010aUrdy et al , 2010bKorn 2012;Urdy 2015). In order to quantify these changes, parameters taken from transverse cross sections or values measured in the plane of symmetry can be used.…”

“…In the case of characters, where transitions in character states between the supposedly new species and closely related species are known, intraspecific variability can be examined based on some tens of specimens of one size class in order to use such a character to explain the separation of a new species (De Baets et al 2013a, 2015. The obvious disadvantage of the quantitative evaluation of intraspecific variability and its description as well as illustration is that they cost a lot of time and that they require a lot of printed space.…”

Ammonoid Intraspecific Variability

“…For instance, a large part of the intraspecific variability in shelled mollusks could be caused by differences in growth rates (Urdy et al 2010a) and development 9 Ammonoid Intraspecific Variability Crônier 2003, 2005). This could also explain certain recurrent patterns in intraspecific variation in the shells of ammonoids and other mollusks with coiled shells (e.g., Dommergues et al 1989;Urdy et al 2010bUrdy et al , 2013Urdy 2015). Extant cephalopods can comprise a high intraspecific variability, particularly in their variable size-at-age, which can be related to intrinsic as well as extrinsic (environmental) factors (Boyle and Boletzky 1996;compare De Baets et al 2015a; Keupp and Hoffmann 2015 for pathologies affecting growth).…”