Ontogenetic change and intra-specific variation of shell morphology in the Cretaceous nautiloid (Cephalopoda, Mollusca)Eutrephoceras clementinum(d'Orbigny, 1840) from the Ariyalur area, southern India

Abstract

Morphometric analyses of shell morphology in the Cretaceous nautiloid Eutrephoceras clementinum (d'Orbigny, 1840) (Cephalopoda, Mollusca) from the Ariyalur area, southern India, reveal ontogenetic change from hatching to maturity as well as intra-specific variation in shell morphology. the shell breadth has a negative allometric relationship with shell diameter and with whorl height, and the umbilicus diameter has a positive allometric relationship with shell diameter. This shows that shell shape became relatively thinner with less variation, and the umbilicus diameter became relatively broader with growth. the siphuncle position moves from a dorso-central to ventro-central position with growth. A constriction was recognized on the early whorl at 20 mm in shell diameter, and the interval angles of succeeding septa were changed at the 8th septum, indicating that they hatched at this stage. the bending of umbilical walls of apertures toward the center of coiling suggests that E. clementinum attained maturity at about 115 mm in shell diameter. the comparison of the shell morphology of E. clementinum with that of E. bouchardianum (d'Orbigny, 1840) reported in the literature clarifies their difference in whorl shape and umbilical size, especially in the adult stage. This kind of morphometric study of nautiloids is essential for elucidating their adaptive designs for environment and mode of life, functional shell morphology, taxonomy, phylogeny, and evolution.

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

“…Furthermore, based on the results of this study, we estimate that Sepiella japonica attains maturity when septal distances dramatically decrease after about 135th septum. These decreasing trends of septal distances at hatching and maturity have been similarly observed in modern and fossil nautiloids (Wani and Ayyasami 2009;Wani and Mapes 2010;and references therein).…”

“…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).…”

Taxonomy and palaeoecology of Cretaceous nautilids Angulithes galea (Fritsch in Fritsch and Schlönbach, 1872) and Angulithes westphalicus (Schlüter, 1872)

“…Wani & Ayyasami 2009) during ontogeny greatly assist in other areas of study such as taxonomy (most important in Cretaceous nautilids, e.g. Hyatt 1884, Kummel 1956, Wiedmann 1960, Shimansky 1975, phylogeny (e.g.…”

“…Tintant & Kabamba 1985) and taphonomy (e.g. Wani 2004Wani , 2007. In comparison with ammonoids (Ward 1980) and with respect to the relative conservativeness of the morphology of nautilid shells, studies of any morphological changes are very valuable.…”

“…One of the first studies utilising morphometric analysis of intra-specific variation and ontogenetic changes in shell morphology (Wani & Ayyasami 2009) focused on the species Eutrephoceras clementinum (d'Orbigny, 1840) from the Cretaceous of the Ariyalur district, southern India. This study required a large number of well-preserved specimens, preferably from the same locality and horizon (see also Kummel 1956).…”