Ammonoid Intraspecific Variability

Abstract

Because ammonoids have never been observed swimming, there is no alternative to seeking indirect indications of the locomotory abilities of ammonoids. This approach is based on actualistic comparisons with the closest relatives of ammonoids, the Coleoidea and the Nautilida, and on the geometrical and physical properties of the shell. Anatomical comparison yields information on the locomotor muscular systems and organs as well as possible modes of propulsion while the shape and physics of ammonoid shells provide information on buoyancy, shell orientation, drag, added mass, cost of transportation and thus on limits of acceleration and swimming speed. On these grounds, we conclude that ammonoid swimming is comparable to that of Recent nautilids and sepiids in terms of speed and energy consumption, although some ammonoids might have been slower swimmers than nautilids.

Describing Ammonoid Conchs

“…Variability is often the lowest in the early and the latest whorls, i.e. these are the most characteristic, but still variable (De Baets et al 2015).…”

“…1.7. In any case, intraspecific variability of ammonoids is so poorly studied that it yields a wide array of possibilities for future studies (De Baets et al 2015).…”

“…By contrast, ornamentation and sutures can be essential for the systematics of Late Paleozoic and Mesozoic ammonoid groups, while conch shape might play a lesser role. Additionally, intraspecific variability differs strongly between ammonoid clades and thus, small differences between some forms might justify the introduction of a new species whereas in other clades, such a small difference could fall within the broad range of intraspecific variability (De Baets et al 2015).…”

Mature Modifications and Sexual Dimorphism

“…In the case of polymorphism, there are more than two (usually three or more) different adult morphologies. Polymorphism in modern biology refers to natural genetic variation (with phenotypic expression or not), undetectable in fossils; here, we use the term to refer to morphological differences between supposed conspecific phenotypes, which could have either a genetic or an environmental cause (compare De Baets et al 2015a).…”

“…It was also demonstrated for Recent coleoids in captivity that environmental factors such as light intensity, temperature or food availability can have an effect on maturation and therefore adult size (e.g., Gabr et al 1998;Moltschaniwskyj and Martínez 1998;Tafur et al 2001;Jackson and Moltschaniwskyj 2002). Callomon (1988) criticized Matyja's ideas about polymorphism, suggesting that a larger database would be needed to test some of his hypotheses (see also De Baets et al 2015a). Later, Dzik (1990a), analysing a rich collection of Callovian Quenstedtoceras ammonites from the classic locality at Łuków in eastern Poland, did not find any evidence for polymorphism.…”