PATTERNS OF INTRASPECIFIC VARIATION IN CEPHALOPODS - NEW DATA FROM 3D MORPHOMETRY OF NAUTILUS SHELLS

Knowledge of intraspecific variation of organisms is of great importance because it is a prerequisite for correct taxonomic assignments, and thus biostratigraphy, biogeography, biodiversity as well as evolutionary studies. Externally shelled fossil cephalopods such as ammonoids and nautiloids have often been used for research in these fields. However, their intraspecific variation is rarely examined in detail when a new species is introduced, which often results in oversplitting. Naturally, this can produce biases affecting the above-mentioned fields of paleobiological research.

Although ammonoid intraspecific variation is sometimes discussed by paleontologists, the number of specimens is often too low to yield statistically sufficient data. Also, many studies on intraspecific variation use only one or a few measurements per specimen without considering ontogenetic change. Furthermore, time-averaging may bias the interpretation of intraspecific variation of fossil organisms. Therefore, a no time-averaged example should be examined as a first step to understand this phenomenon in externally shelled cephalopods. In this study, we examined the intraspecific variation of Recent Nautilus through ontogeny. We applied computed tomography to 93 conchs of Nautilus pompilius from three geographically separated populations (Philippines, Malaysia and Indonesia). Measurements of whorl expansion rate (WER) and whorl width index (WWI) were taken at about 14 different ontogenetic stages from each specimen. Additionally, phragmocone chamber volume (PCV) was measured (a measure that can only be quantified using tomographic methods). Standard deviation of WER and WWI and coefficient of variation of PCV were calculated through ontogeny as measures of intraspecific variation. Our data revealed that intraspecific variation is high early in ontogeny and decreases subsequently until short before maturity in all three populations. This data was compared to those obtained from ammonoids and belemnites, it turned out that this pattern of variation is also present in the examined ammonoids and belemnites.