An Ancient Fleet Driven from the Seas – The Nautilus

Modern distribution ranges of nautiluses (red) and pinnipeds (yellow)（By Shui-Ye You）

During the Cenozoic era, nautiluses once possessed a broad geographic distribution and considerable diversity. Today, however, their range has shrunk dramatically, confined mainly to the central Indo-Pacific region stretching from the Indian Ocean to the western Pacific. For a long time, the reason behind this large-scale contraction remained unresolved. Some researchers proposed that declining ocean temperatures might have been responsible, yet this explanation does not align with the historical changes observed in nautilus distribution throughout the Cenozoic. A study published last September, however, offered a new perspective on the mystery.

The researchers proposed that marine mammals—particularly pinnipeds—played a decisive role in shaping the modern distribution of nautiluses. To examine this hypothesis, they compared the spatial and temporal distributions of pinnipeds, cetaceans, nautiluses, and the extinct nautiloid Aturia. Although Aturia belongs to the nautiloid lineage, it differs substantially from modern nautiluses. It likely possessed faster reaction speeds, higher reproductive rates, and a thinner shell. Because of these biological differences, the researchers treated Aturia separately when analyzing evolutionary patterns.

Their results revealed a striking pattern. Beginning in the late Oligocene, whenever pinnipeds appeared in a region, nautiluses disappeared from the same area. Aturia, meanwhile, expanded its distribution widely during the middle Miocene. By the late Miocene, however, its range had contracted dramatically, becoming largely restricted to mid-latitude regions before ultimately vanishing near the end of the Miocene. In the northeastern Pacific and the northwestern Atlantic, the disappearance of nautiluses coincided with the emergence of short-snouted toothed whales.

Cetaceans were among the earliest marine mammals to evolve during the Cenozoic. Within this group, the archaeocete family Basilosauridae shows dental wear patterns similar to those of pinnipeds, suggesting comparable feeding habits. Yet most early whales lacked echolocation, a sensory ability that allows predators to detect prey in dark or murky waters. Because of this limitation, early cetaceans were probably not highly effective predators of nautiluses. Large marine predators also tend to target larger prey, making it unlikely that small organisms such as nautiluses were their primary food source. As a result, the early appearance of whales did not immediately place significant evolutionary pressure on nautilus populations.

The situation began to change during the Oligocene with the emergence of certain short-snouted odontocetes, including members of the families Agorophiidae and Simocetidae. Their shortened rostrums are thought to represent adaptations for suction feeding. Moreover, their dentition appears well suited for handling hard-shelled prey such as nautiluses. Fossil evidence indicates that the appearance of these whales coincided with the disappearance of nautiluses in several regions. Even so, compared with pinnipeds, cetaceans appear to have exerted only a relatively modest influence on the evolutionary trajectory of nautiluses.

The arrival of pinnipeds in the late Oligocene produced a far more direct effect. Wherever pinnipeds appeared, local nautilus populations vanished during the same period. Pinnipeds possess complex feeding behaviors and are capable of manipulating prey with considerable dexterity. For a slow-moving animal such as the nautilus, this made them ideal prey. Consequently, pinnipeds likely exerted a much stronger influence on the survival and evolution of nautiluses than whales did. Once nautiluses were eliminated from a region, the continued presence of pinnipeds prevented their return. The ecological territory had effectively been occupied.

Another environmental factor may have intensified this pressure. From the Eocene into the Oligocene, oceanic upwelling became increasingly prominent. Upwelling systems often generate oxygen minimum zones (OMZs)—regions of the ocean where dissolved oxygen levels are extremely low. These zones form largely because aerobic bacteria consume oxygen while decomposing organic matter. When nautiluses were pursued by predators, they could no longer retreat safely into deeper waters if an OMZ blocked their escape route. In such circumstances, the deep ocean refuge that once protected them became inaccessible.

The Caribbean region presents a puzzling exception. Pinnipeds are largely absent there, aside from the recently extinct Caribbean monk seal. Nevertheless, after the middle Eocene, the fossil record shows no nautiluses in the Caribbean except for Aturia. Some studies have suggested that the disappearance might be related to upwelling and the development of oxygen minimum zones. Yet the extinction of Caribbean nautiluses occurred earlier than the onset of strong upwelling in that region. The true cause of their disappearance therefore remains unresolved.

If pinnipeds also contributed to the decline of Aturia, their impact appears to have been less immediate than in other nautiloids. The species’ faster movement and higher reproductive rate may have allowed it to temporarily withstand predation pressure. Beginning in the Oligocene, Aturia also evolved smaller body sizes. This size reduction likely made the animals harder to capture and reduced their energetic value as prey. At the same time, however, the smaller size may have increased their vulnerability to attacks from other, smaller predators.

By the end of the Miocene, Aturia had disappeared entirely. Earlier interpretations suggested that global cooling caused this extinction. Yet this hypothesis conflicts with the fossil record, which shows that Aturia vanished from tropical regions while continuing to occur in temperate environments during the late Miocene. The study therefore proposed an alternative explanation. When upwelling was absent, nautiluses under predation pressure could retreat to deeper waters. The thin shell that gave Aturia greater speed, however, may have become a disadvantage in this situation. Because of its lighter shell structure, Aturia may have been unable to descend to the same depths as other nautiloids, leaving it more exposed to predators and eventually driving it to extinction.

Some might argue that the apparent disappearance of nautiluses in the fossil record could simply reflect a taphonomic bias. Yet during the interval when nautiluses vanished across large regions, Aturia fossils remained abundant. Since the shells of both groups share similar composition, structure, and preservation potential, this explanation is unlikely.

Three key conclusions emerge from the study.

First, marine vertebrates have played an active role in shaping the evolutionary history of cephalopods.Second, newly evolved predators can rapidly eliminate prey species that lack effective defenses. The fate of the dodo provides a familiar illustration of this principle.Third, if current trends continue and pinnipeds expand into the remaining habitats where nautiluses still survive, these ancient animals could disappear within the next few million years. Yet the study also emphasizes a more immediate threat: modern human fisheries may pose an even greater and more direct danger to nautilus populations.

Author: Bai Leng

Reference:

Kiel, S., Goedert, J. L., Tsai, C. H. (2022). Seals, whales and the Cenozoic decline of nautiloid cephalopods. Journal of Biogeography.