Generalized ammonoid hydrostatics modelling, with application to Intornites and intraspecific variation in Amaltheus

Abstract

A numerical procedure for calculating the buoyancy, apertural orientation and rotational stability of ammonoids can accommodate noncircular apertures and allometries. The hydrostatic properties of the Triassic oxyconic ammonite Intornites nevadanus were estimated from measured coiling parameters, body chamber length and digitized whorl section. The calculations indicate close to neutral buoyancy. The orientation of the aperture is 33 degrees from the horizontal, and the stability index relatively high at 0.054. Unlike for many other ammonites, these values are comparable to those of Nautilus, suggesting a similar nektobenthic lifestyle. An apertural interior shell callus does not contribute to stability in this genus, and its function remains unknown. In Amaltheus margaritatus, hydrostatic properties and body chamber length vary significantly as a result of intraspecific variation in shell shape. Covariation between septal spacing and whorl shape can be given a functional explanation in terms of hydrostatic properties. Buckman's second law of covariation between ribbing and sutural complexity can possibly be thought of in terms of heterochrony.