How seals made Nautilus a ‘Living Fossil’

Increasing predation pressure by pinnipeds through the late Cenozoic drove Nautilus into its present-day refuge in the deep tropical Indo-West Pacific Ocean

Above: Reconstruction of the fossil Nautilus taiwanus inhabiting deeper waters of the tropical Indo-West Pacific Ocean about 20 million years ago. Illustration by Cheng-Han Sun.

Predator-prey interactions are important drivers of evolution. For example, the iconic ‘living fossil’ Nautilus is thought to inhabit deep water mainly as a way to avoid predation. Due to the general difficulties of exploring the deep-sea, very little is known about which animals actually prey on Nautilus, and how. This makes it particularly difficult to infer which animals might have preyed on Nautilus in the geologic past. As a consequence, the question of why Nautilus is today restricted to the tropical Indo-West Pacific Ocean despite having had a worldwide distribution only 50 million years ago, had always been assumed to be related to changes in oceanic water circulation patterns and temperature; whether or not this distribution was instead predator-driven had never been asked. Indeed, we had nothing like this in mind when we went on a fossil collection trip to Taiwan in early 2019. One locality unexpectedly produced a fossil Nautilus specimen and while my co-workers tried to identify the species they noted, based on their own field and research experience, that Nautilus disappeared from the fossil record of western North America and Japan as soon as pinnipeds – the seals – appeared. Realizing that we might be on to something, we embarked on an extensive literature review involving the global distribution of nautiloids and their potential mammalian predators through the entire Cenozoic era, writing to colleagues about specimen images, advice on local geology and geologic ages, and hard-to-get literature.

Editors’ choice article: (Free to read online for two years.)
Kiel, S., Goedert, J. L., & Tsai, C.-H. (2022). Seals, whales and the Cenozoic decline of nautiloid cephalopods. Journal of Biogeography, 00, 1– 8. https://doi.org/10.1111/jbi.14488 

These compilations nicely demonstrated that the appearance of pinnipeds in the geologic record of any given region coincided with the disappearance of Nautilus from that region. A few now extinct groups of early whales may also have played a role in the local extinction of Nautilus. Some of these early whales had different cranial morphologies – and hence feeding strategies – than the living whales, opening the intriguing possibility that a wider diversity of prey items had triggered this wider range of feeding adaptations. This is particularly interesting as there is one unusual fossil nautiloid – Aturia – that did not immediately become extinct with the appearance of pinnipeds. Aturia is special as it shows adaptations to fast and agile swimming, such as a rather flat and thin shell, that might have enabled it to escape hungry mammalian predators.

This raised the question  as to whether there was direct evidence for predation on fossil nautiloids, such as bite marks on shells. There are numerous reports on predation scars on ammonites and nautiloids from the Mesozoic era, including bite marks by mosasaurs. But to our surprise, we found nothing about bite marks on Cenozoic nautiloids. This may or may not suggest that the Cenozoic marine mammals had a different – and perhaps more efficient – hunting style compared to Mesozoic marine reptiles. It certainly calls for a closer look for predation scars in Cenozoic nautiloid fossils, to better understand the interactions between marine vertebrate predators and their invertebrate prey, and how this has shaped their evolution and biogeography.

Written by:
Steffen Kiel, Swedish Museum of Natural History

Additional information:
https://www.researchgate.net/profile/Steffen-Kiel