219-15 Longevity of a Threatened Species: Allonautilus scrobiculatus from Papua New Guinea

Session: Paleontology, Diversity, Extinction, Origination (Posters)

Presenting Author:

Authors:

Abstract:

The chambered nautiloids Nautilus and Allonautilus, considered “living fossils”, are the last survivors of an extinct group of ancient, shelled cephalopods. The longevity of nautiloids has long been under debate, and some previous attempts have been made to estimate their age and lifespan. Determining longevity can provide insights into the lifespans of extinct nautiloids. We have quantified the age and lifespan of two mature specimens of Allonautilus scrobiculatus from Papua New Guinea by measuring the activity of the natural ²³⁸U-series radionuclide ²¹⁰Pb (half-life = 22.3 years) in the septa of these specimens. ²¹⁰Pb is incorporated from seawater into the shell material in excess of its grandparent ²²⁶Ra during growth. We used measurements of δ¹⁸O in the septa to calculate the temperatures of shell deposition and constrain the habitat depth of the animals. After hatching, the animals secreted their septa at depths of ~225 m. Tracking data collected using a radio transmitter device show that Allonautilus scrobiculatus migrates diurnally between depths of ~125m (night) and ~200m (day). The δ¹⁸O data suggest that septal deposition occurs mainly during the daytime when the animals are at depth. Excess ²¹⁰Pb activities show progressive decreases from recently deposited septa to those formed earlier in ontogeny. The constancy of depth during septal secretion suggests that the variation in ²¹⁰Pb cannot be explained by migration of the animals to different depths with potentially different ²¹⁰Pb activities, which in fact do not vary significantly with depth in the region. Instead, we interpret the change in excess ²¹⁰Pb through ontogeny as decay of ²¹⁰Pb during the lifetime of the animal. Our results suggest that the two animals lived for ~20 and ~30 years. Thus Allonautilus and, by extension, other species of Nautilus are the longest-lived cephalopods. This information is useful to provide guidance for management of these threatened species.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8733

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