Intraspecific variation of phragmocone chamber volumes throughout ontogeny in the modern nautilidNautilusand the Jurassic ammoniteNormannites

Abstract

Nautilusremains of great interest to palaeontologists after a long history of actualistic comparisons and speculations on aspects of the palaeoecology of fossil cephalopods, which are otherwise impossible to assess. Although a large amount of work has been dedicated toNautilusecology, conch geometry and volumes of shell parts and chambers have been studied less frequently. In addition, although the focus on volumetric analyses for ammonites has been increasing recently with the development of computed tomographic technology, the intraspecific variation of volumetric parameters has never been examined. To investigate the intraspecific variation of the phragmocone chamber volumes throughout ontogeny, 30 specimens of RecentNautilus pompiliusand two Middle Jurassic ammonites (Normannites mitis) were reconstructed using computed tomography and grinding tomography, respectively. Both of the ontogenetic growth trajectories from the twoNormannitesdemonstrate logistic increase. However, a considerable difference inNormanniteshas been observed between their entire phragmocone volumes (cumulative chamber volumes), in spite of their similar morphology and size. Ontogenetic growth trajectories fromNautilusalso show a high variation. Sexual dimorphism appears to contribute significantly to this variation. Finally, covariation between chamber widths and volumes was examined. The results illustrate the strategic difference in chamber construction betweenNautilusandNormannites. The former genus persists to construct a certain conch shape, whereas the conch of the latter genus can change its shape flexibly under some constraints.