Monster nautiluses of the Palaeozoic

Neale Monks (UK) 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 500Ma, 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, … Read More