NAUTILUS: NAUTILIDAE

Nautiluses and Their Fossil Relatives

Nautiluses are much closer to the first cephalopods that appeared about 500 million years ago than the early modern cephalopods that appeared 100 million years later — the ammonoids and coleoids. 

The fossil record of Nautilidae begins with Cenoceras in the Late Triassic, a highly varied genus that makes up the Jurassic Cenoceras complex.

Cenoceras is evolute to involute, and globular to lenticular; with a suture that generally has a shallow ventral and lateral lobe and a siphuncle that is variable in position but never extremely ventral or dorsal. 

Cenoceras is not found above the Middle Jurassic and is followed by the Upper Jurassic-Miocene Eutrephoceras.

Eutrephoceras is generally subglobular, broadly rounded laterally and ventrally, with a small to occluded umbilicus, broadly rounded hyponomic sinus, only slightly sinuous sutures, and a small siphuncle that is variable in position.

Next to appear is the Lower Cretaceous Strionautilus from India and the European ex-USSR, named by Shimankiy in 1951. Strionautilus is compressed, involute, with fine longitudinal striations. Whorl sections are subrectangular, sutures sinuous, the siphuncle subcentral.

Also from the Cretaceous is Pseudocenoceras, named by Spath in 1927. Pseudocenoceras is compressed, smooth, with subrectangular whorl sections, a flattened venter, and a deep umbilicus. The suture crosses the venter essentially straight and has a broad, shallow, lateral lobe. The siphuncle is small and subcentral. Pseudocenoceras is found in Crimea and in Libya.

Carinonautilus is a genus from the Upper Cretaceous of India, named by Spengler in 1919. Carinonautilus is a very involute form with a high whorl section and flanks that converge on a narrow venter that bears a prominent rounded keel. The umbilicus is small and shallow, the suture only slightly sinuous. The siphuncle is unknown.

Obinautilus has also been placed in Nautilidae, though it may instead be an argonautid octopus.

Learning and Behaviour

They have a seemingly simple brain, not the large complex brains of octopus, cuttlefish and squid, and had long been assumed to lack intelligence. 

They are able to learn simple tasks and have the ability to remember food triggers. There was an experiment done that shone a blue light each time they were fed and then flickered on later without food. The nautilus responded to the light without delay when food was given. They quickly learned that the light was not an indicator of food when the light was turned on without issuing a tasty snack. 

The cephalopod nervous system is quite different from that of other animals, and recent experiments have shown not only memory but a changing response to the same event over time.

Life on Deep Reef Slopes

Nautiluses usually inhabit depths of several hundred metres. It has long been believed that nautiluses rise at night to feed, mate, and lay eggs, but it appears that, in at least some populations, the vertical movement patterns of these animals are far more complex. Nautiluses are found in only the Indo-Pacific, from 30° N to 30° S latitude and 90° E to 175° E longitude. They inhabit the deep slopes of coral reefs. 

Nautilus pompilius may be the deepest ocean dwelling nautilus. They have been spotted at depths of 703 m (2,306 ft) — very close to the estimated implosion depth of 800 m (2,600 ft). 

Only in New Caledonia, the Loyalty Islands, and Vanuatu can nautiluses be observed in very shallow water, at depths of as little as 5 m (15 ft). This is due to the cooler surface waters found in these southern hemisphere habitats as compared to the many equatorial habitats of other nautilus populations – these usually being restricted to depths greater than 100 m (300 ft). Nautiluses generally avoid water temperatures above 25 °C (75 °F).

Explore more ammonites in the Fossil Huntress Guide to Ancient Oceans.

Compare these living shelled cephalopods with extinct ammonites in Ammonites: Chambered Beauty.