Essential collectibles #4: Orthoceras nautiloid

Orthocone nautiloids, loosely referred to as Orthoceras in the trade, are another type of fossil exported from Morocco in large numbers, like the Diacalymene trilobites discussed earlier in this series. Both their common and scientific names refer to the long, narrow, conical shape of their shell, the Greek prefix ‘ortho’ meaning ‘straight’, and contrasting them with most other nautiloids (not to mention ammonites) which have coiled shells instead.

Orthocones were a very common type of cephalopod during the Palaeozoic, some types reaching enormous sizes, well over six metres long, making them among the largest animals in the seas at the time. The ones sold to fossil collectors are generally smaller than those giants, anything up to 30 cm being typical, and because they’re usually sectioned and polished before sale, it’s easy to see some of their key anatomical features.

For a start, there’s the chambered external shell. Partly the shell was there to provide protection from predators, but the shell was also important for providing neutral buoyancy. As it grew the orthocone nautiloid would add additional chambers, allowing gases to diffuse out of its bloodstream and so replace the water in the chamber. The basic idea was not dissimilar to how a submarine works in terms of being able to hold its position in midwater without sinking or rising, but unlike a submarine nautiloids aren’t able to rapidly change the amount of gas and water in the chambers, so they couldn’t use their shells to quickly move up and down the water column. If an orthocone wanted to swim upwards, it had to use its built-in jet propulsion system for that, much as squid and octopus do today. But what neutral buoyancy does do is reduce the amount of energy needed for swimming in the same way that an airship only needs propellers to move it in a particular direction, not to keep it in the air.

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