Variable growth modes in Late Cretaceous ammonoids: implications for diverse early life histories

Abstract

Examination of ontogenetic changes in the septal angle of Late Cretaceous ammonoids (ten species representing seven superfamilies and four suborders) reveals four patterns: 1) a single abrupt change in septal angle; 2) two abrupt changes in septal angle; 3) cyclic fluctuations in septal angle throughout ontogeny; and 4) an almost constant septal angle throughout ontogeny. These various septal-angle patterns in Late Cretaceous ammonoids are in contrast with modern and fossil nautiloids, which have the common pattern displaying a single abrupt change in septal angles. Although the abrupt change of septal angles in nautiloids corresponds with the hatching event from the egg, change of septal angles in the examined ammonoids is hypothesized to correspond not to hatching but to the change from a planktic to a nektobenthic habit demarcated by the post-embryonic stage. Therefore, the variable patterns of septal angles within ammonoids suggest a diverse set of early life histories.

Ontogenetic trajectories of septal spacing in Early Jurassic belemnites from Germany and France, and their palaeobiological implications

“…These studies suggest that the ontogenetic trajectories of septal spacing show a variety of patterns, a wider variation than found in other cephalopods. Having such a variety of septal‐angle patterns in ammonoids contrasts with the situation seen in ancient and modern nautiloids, which retain almost constant septal angles with small fluctuations during the middle ontogenetic stages (Arai & Wani ). Such diverse ontogenetic patterns of septal spacing in ammonoids also contrasts with the situation found in belemnites.…”

“…; Okamoto & Shibata ; Korn & Titus ; Ebbighausen & Korn ; Kraft et al . ; Arai & Wani ; De Baets et al . ; Naglik et al .…”

Abrupt changes in distance between succeeding septa at the hatching time in modern coleoids Sepiella japonica and Spirula spirula

“…This is the same method commonly used to examine the septal angles of ammonoids and nautiloids (Wani and Mapes 2010; Arai and Wani 2012, and references therein). The centre of the rotational angles was approximated as the centre of the logarithmic shell spiral approximating the shell whorls (see Arai and Wani 2012 for detailed methods to determine the spiral's centre). The septal distances between succeeding septa were measured using a digital microscope (Keyence, VHX-900, with magnification of 25-1759).…”

“…In cephalopods with external shells (e.g., modern and fossil nautiloids and ammonoids), the external shells also have septa that delineate air chambers. Numerous studies have been dedicated to the ontogenetic analysis of the septate external shells of cephalopods to reconstruct ontogenetic growth in modern nautiloids (Landman et al 1983;Collins and Ward 1987;Tanabe and Tsukahara 1987;Landman 1988;Klug 2004;Chirat et al 2008;Klug et al 2008), fossil nautiloids (Ruzhencev and Shimansky 1954;Davis and Mohorter 1973;Landman et al 1983;Landman 1988;Chirat and Rioult 1998;Chirat 2001;Wani and Ayyasami 2009;Wani and Mapes 2010) and ammonoids (Kulicki 1974;Doguzhaeva 1982;Checa 1987;Landman 1987;Dommergues 1988;Bucher et al 1996;Okamoto and Shibata 1997;Korn and Titus 2006;Ebbighausen and Korn 2007;Kraft et al 2008;Arai and Wani 2012). However, little attention has been devoted to the study of internal cephalopod shells and their ontogenetic analysis (but see Hewitt and Stait 1988).…”

Evidence for semi-sessile early juvenile life history in Cretaceous ammonites

“…the highly variable septal distances observed during this stage of growth (cf. Arai and Wani, 2012;De Baets et al, 2015;Zell and Stinnesbeck, 2016).…”