Ontogeny and sexual dimorphism ofEurycephalites gottschei (Tornquist) (Ammonoidea) of the Andean Lower Callovian (Argentine-Chile)

Mature Modifications and Sexual Dimorphism

“…The growth rate measured by the relative whorl ventral height is dimorphic (Fig. 7.4a upper) with an increase in both dimorphs up to about 10 mm diameter, after which both dimorphs inverted the trend; from a diameter of about 18 mm onward, the microconch diverged by increasing the rate of growth towards the peristome (Parent 1997). The shape of the whorl cross section changed strongly during growth, but the ontogenetic trajectory is the same in micro-and macroconchs, i.e., monomorphic (Fig.…”

“…Many Paleozoic and Mesozoic ammonoids display a more or less strong change in coiling near the termination of growth (e.g., Trueman 1941;Parent 1997;Klug 2001;Klug and Korn 2003). In the earliest ammonoids such as Metabactrites, Anetoceras and Erbenoceras, the last whorl is usually more openly coiled than the preceding ones (e.g., De Baets et al 2013a, b).…”

“…In any case, many antidimorphs display a congruent pattern of development of various shell parameters, which diverge at some point with the microconchs maturing and stopping growth at a size-wise earlier point (e.g., Makowski 1962;Guex 1973;Parent 1997;Parent et al 2008bParent et al , 2009.…”

“…The microconch trend diverged from a diameter of 18 mm compared to that of the macroconch, which is taken as the standard. Parent (1997). c from data in Parent (1998) …”

“…Makowski 1962Makowski , 1971Makowski , 1991Callomon 1963;Brochwicz-Lewiński & Różak 1976;Bucher and Guex 1990;Brooks 1991;Bucher et al 1996;Davis et al 1996;Schweigert and Dietze 1998;Parent 1997;Parent et al 2008a;Zatoń 2008;Landman et al 2012). The reliable identification of mature shells is the logical prerequisite to determine sexual dimorphism.…”