Patterns of intraspecific variation through ontogeny: a case study of the Cretaceous nautilidEutrephoceras dekayiand modernNautilus pompilius

Abstract

The magnitude and ontogenetic patterns of intraspecific variation can provide important insights into the evolution and development of organisms. Understanding the intraspecific variation of organisms is also a key to correctly pursuing studies in major fields of palaeontology. However, intraspecific variation has been largely overlooked in ectocochleate cephalopods, particularly nautilids. Furthermore, little is known regarding the evolutionary pattern. Here, we present morphological data for the Cretaceous nautilid Eutrephoceras dekayi (Morton) and the modern nautilid Nautilus pompilius Linnaeus through ontogeny. The data are used to describe conch morphology and to elucidate the evolutionary patterns of intraspecific variation. We discovered a similar overall pattern of growth trajectories and the presence of morphological changes at hatching and maturity in both taxa. We also found that intraspecific variation is higher in earlier ontogeny than in later ontogeny in both taxa. The high variation in earlier ontogeny may imply increased flexibility in changing the timing of developmental events, which probably played an important role in nautilid evolution. We assume that the decrease in variation in later ontogeny reflects developmental constraints. Lastly, we compared the similarity/dissimilarity of ontogenetic patterns of variation between taxa. Results reveal that the similarity/dissimilarity of the ontogenetic pattern differs between E. dekayi and N. pompilius. We conclude that this shift in the ontogenetic pattern of variation may be rooted in changes in the developmental programme of nautilids through time. We propose that studying ontogenetic patterns of intraspecific variation can provide new insights into the evolution and development of organisms.

Significance of the suture line in cephalopod taxonomy revealed by 3D morphometrics in the modern nautilids Nautilus and Allonautilus

“…We suspect that our results also apply to fossil nautilids. Tajika et al 41 investigated the ontogeny of Late Cretaceous Eutrephoceras dekayi (Morton, 1834) and modern Nautilus pompilius . They discovered a similar ontogenetic pattern in whorl expansion rate, whorl width index, and septal spacing index between the two taxa despite the fact that the two taxa lived in different environments at different times (i.e., E. dekayi in shallow water epicontinental seas in North America during the Late Cretaceous vs. N. pompilius in much deeper water on steep forereef slopes in the Philippines today).…”

“…Thus, we presume that our results may be applicable to at least some groups of fossil nautilids. As mentioned above, however, detailed morphological studies are largely lacking in fossil nautilids with few exceptions 41 , 48 , 49 and, therefore, additional data on the conch morphology (e.g., siphuncular position, siphunclular thickness, soft tissue attachment, classical conch parameters) through ontogeny are needed to further discuss the potential application in fossil forms.…”

“…Then, we designated a total of 40 landmarks with equidistant intervals along the shell on the whorl cross-section (Fig. 1 B; compare Tajika et al 41 ). These landmarks were analyzed with the same methodology as the suture line.…”

“…We regard this parameter as the most commonly used character in the taxonomy of (mainly fossil) ectocochleate cephalopods. It is generally quantified using a linear morphometric approach [39][40][41] . We applied two-dimensional geometric morphometrics to analyze the conch geometry.…”

Isotopic evidence concerning the habitat of Nautilus macromphalus in New Caledonia

“…These parameters were used to calculate the whorl expansion rate [(dm 1 /dm 2 ) 2 ; WER], whorl width index (ww/dm; WWI), whorl height, and siphuncle position index (vd/wh; SPI). For details of these morphological parameters, see Tajika and Klug [ 25 ], and Tajika et al [ 33 ]. Direct comparisons between WER, WWI, and the isotope values are difficult because they require estimation of the position of the aperture at the time of septal formation.…”