Abstract
Nautilus remains 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 to Nautilus ecology, 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 Recent Nautilus pompilius and two Middle Jurassic ammonites (Normannites mitis) were reconstructed using computed tomography and grinding tomography, respectively. Both of the ontogenetic growth trajectories from the two Normannites demonstrate logistic increase. However, a considerable difference in Normannites has been observed between their entire phragmocone volumes (cumulative chamber volumes), in spite of their similar morphology and size. Ontogenetic growth trajectories from Nautilus also 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 between Nautilus and Normannites. 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.
Highlights
Ammonoids and nautiloids are well-known, long-lived molluscan groups, both of which faced devastation at the end of the Cretaceous, but with different responses: extinction versus survival
The aims of this study are to answer the following questions based on empirical 3D models reconstructed from real specimens: (1) How did chamber volumes change through ontogenetic development of ammonites and nautilids? (2) How much did the volumetric growth trajectories differ between two conspecific ammonites? (3) What was the intraspecific variation of volumetric growth trajectories of modern Nautilus? (4) Are the differences in chamber volumes between male and female nautilids significant? (5) Is there a difference in construction of chambers between the ammonites and modern Nautilus?
We virtually reconstructed the conchs of two Middle Jurassic ammonites (Normannites mitis) and 30 specimens of Recent nautilids (Nautilus pompilius) using grinding tomography and computed tomography (CT), respectively, to analyse the intraspecific variability in volumetric change of their chambers throughout ontogeny
Summary
Ammonoids and nautiloids are well-known, long-lived molluscan groups, both of which faced devastation at the end of the Cretaceous, but with different responses: extinction versus survival. A number of palaeontologists investigated the ecology and anatomy of living Nautilus as an analogy for those of extinct ammonites over the last decades (e.g., Collins, Ward & Westermann, 1980; Saunders & Landman, 1987; Ward, 1987; Ward, 1988) It was Jacobs & Landman (1993) who argued that, despite its superficial morphologic similarity, Nautilus was an insufficient model to reconstruct ammonoid palaeoecology, given their phylogenetic positions, which are distant within the Cephalopoda. This argument is widely accepted (e.g., Warnke & Keupp, 2005). Whereas palaeoecology and evolution of ammonoids need to be discussed based on their own fossil record, those of modern Nautilus can be satisfactorily analogized to fossil nautilids, which have borne persistent conch morphologies throughout their evolution (Ward, 1980)
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