The monster nautiluses of the Palaeozoic

The handful of nautilus species found in seas today are small, retiring animals that scavenge about at night, foraging for carrion and crustacean moults. However, nautiluses were not always so insignificant and, during the first half of the Palaeozoic Era especially, nautiluses were major predators, occupying the same niches in Ordovician and Silurian seas as sharks do today.

The first nautiluses

Compared to their cousins, the ammonites, the Palaeozoic nautiluses are relatively unfamiliar animals. That is a shame, because they are truly remarkable, in all likelihood being the first really big predators to evolve on Earth. But, to understand how they reached the top so quickly, we need to look back at their ancestors, the floating ‘snails’ of the Cambrian.

Nautiluses are the most primitive of all the cephalopods, the group of molluscs that also includes squids, octopuses, cuttlefishes, ammonites and belemnites. Nautiluses appeared during the Late Cambrian, about 500mya, but what their ancestors might have been remains uncertain.

The traditional explanation is that the first nautiluses, such as Plectronoceras exile, were derived from monoplacophorans. These are snail-like molluscs today, limited to a few species only found in relatively deep water, but in the past they were quite diverse. Although they look a lot like a limpet, their internal anatomy is distinctive, with unusual features such as serial repetition of the gills, kidneys and reproductive organs along the body.

At least some monoplacophorans had chambered shells. The Late Cambrian animal, Knightoconus antarcticus was one such species, but, unlike cephalopods, the chambers were all sealed off from one another. Only cephalopods have a siphuncle, a tube-like structure that runs through the chambers, allowing them to be connected to the living body of the animal. The siphuncle is important because it allows cephalopods to replace water inside the shell chambers with gas. By filling the chambers in this way, the cephalopod reduces its overall density and, once it acquires the same density as seawater, it neither floats to the top nor sinks to the bottom. Rather wonderfully, it simply hangs in midwater and can swim off in any direction it wants. Compare this to other molluscs, which have to drag their heavy shells around with them. Nautiluses and cuttlefishes continue to operate this way today, and ammonites and belemnites did so in the past.

Monoplacophorans, like Plectronoceras exile, had shells with chambers, but no siphuncle. Were the walls that formed the chambers for some other purpose? Did they work like bulkheads, so that the monoplacophoran could build shell that was thin and economical to construct, but just as strong as a thicker, unchambered shell? Perhaps, but recent work on the genetics of the molluscs suggests that cephalopods are more closely related to scaphopods (tusk shells) than they are to monoplacophorans. Modern scaphopods are small, specialised animals that live buried in the seafloor catching microscopic prey. While their genes might tell us they are closely related to the cephalopods, there are no obvious anatomical or ecological similarities.

Ortoceratiter (Endoceras sp.)
Dideroceras wahlenbergi (formerly Endoceras wahlenbergi) is an example of one of the larger orthocone nautiluses. From Billingen Hill, Skövde, in Västergötland, Sweden. Complete shells are rare, but fragments 30-45 cm long are common at certain horizons.

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