Mature Modifications and Sexual Dimorphism

Abstract

Allometric growth between different parts of the shell often hampers the identification of mollusk shells, particularly in such cases where preadult shell growth varies strongly. Especially in gastropods, the terminal aperture is often less variable and yields morphological information essential for species determination (e.g. Vermeij 1993; Urdy et al. 2010a, b). In fossil mollusk shells, the adult aperture (peristome)is often missing, partially due to an early death, and partially due to destructive processes, which occurred post mortem (taphonomy). Therefore, the entire shell ontogeny is known only from a small fraction of all ammonoid taxa (e.g., Landman et al. 2012). Nevertheless, knowledge of the adult shell of ammonoids is very important since it can yield morphological information essential for systematics and for the reconstruction of various aspects of their paleobiology.

Ammonoid Intraspecific Variability

“…The switch between forms is believed to be environmental in polyphenism (e.g., Fusco and Minelli 2010), while the switch is believed to be "almost always" genetic in (genetic) polymorphism (Ford 1966; this should not be confused with the use of the same terminology by molecular biologists for certain point mutations in the genotype, which do not necessarily correlate with recognizable phenotypic effects: Fusco and Minelli 2010). In ammonoids, polymorphism has been traditionally used to refer to two or more discrete coexisting forms within the same fossil population (Tintant 1980;Davis et al 1996 and references therein; Klug et al 2015), although others have used it more generally to include also continuous variation (e.g., Beznosov and Mitta 1995). We suggest using the term polymorphism only to refer to discontinuous variation in ammonoids to avoid confusion and to be in line with its most common use.…”

“…On the other hand, when the distribution is not normal or unimodal, it does not necessarily mean that the specimens belong to different species either (Tintant 1980). Such a distribution could originate from environmental influences, taphonomic biases, sampling biases or the fact that the distribution is not of a Gaussian kind (for example in the case of discrete variation within a species such as dimorphism or non-sexual polymorphism: Klug et al 2015).…”

The taxonomic status of the genus Hubertoceras Spath: A new light on sexual dimorphism from the Callovian ammonites of Kutch, India

“…One "has to be extremely careful when it comes to maintaining sexual relationships between hitherto blameless ammonites" (Lehmann, 1971) Sexual dimorphism in ammonites was established since the origination of the Order Ammonoidea during the Devonian (see Makowski, 1962Makowski, , 1963Davis et al, 1996;Page, 2008;Klug et al, 2015). Only a few Paleozoic and Triassic families (five) showed dimorphism in terms of size and shape variation (Klug et al, 2015 and references therein).…”

“…From the Jurassic onward sexual dimorphism became widespread. Out of 53 Jurassic families, sexual dimorphism occurred in at least 38 families (about 72%; see Callomon, 1963Callomon, , 1969Callomon, , 1981Donovan et al, 1981;Davis et al, 1996;Klug et al, 2015), and the same trend persisted in the Cretaceous when out of 55 families sexual dimorphism was recorded in 37 families (about 67%; see Wright et al, 1996, Davis et al, 1996Klug et al, 2015). Callomon (1963) provided detailed account of sexual dimorphism in many Jurassic ammonite families, for example, Graphoceratidae and Hildoceratidae of the Toarcian (Lower Jurassic) and Hammetoceratidae (Lower-Middle Jurassic).…”

“…One of the criteria of sexual dimorphism is based on the identity of the early ontogeny of putative sexual dimorphs, which have different adult stages and adult size. Much new information recently emerged in working out dimorphic pairs in ammonites (see Davis et al, 1996;Klug et al, 2015). We here explored all possibilities of finding out the true sexual counter part of Hubertoceras.…”