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.

Another thing you’ll notice about your Orthoceras specimen is that the chamber walls are curved, not straight. This meant that the joins between the outer walls of the shell and the walls in between the chambers are much stronger, and this seems to have been related to pressure. Over the course of evolution nautiluses (and later on ammonites) evolved steadily more complex ‘buttressing’ inside the shell that meant their shells could operate effectively in deeper water where pressure was greater, or resist the biting force of predators trying to break open their shells. At least, those are the standard explanations for what was going on; others have debated this, and believe the fluted junctions between the chambers and the outer walls were more to do with how the nautilus or ammonite emptied water from the chamber while filling it up with gas.

Sometimes Orthoceras fossils show what seems to be a tube running through the chambers, known as the siphuncle. This structure contained various tissues including blood vessels running from the back of the animal right the way along the entire chambered part of the shell. It played a crucial part in emptying the chambers as they grew, and because the precise position of the siphuncle within the chambers varied between major groups, it is an important taxonomic character as well. True Orthoceras will have their siphuncle at or slightly below the mid point of the chambers, whereas Endoceras, for example, have their siphuncle close to the ventral wall (the ‘bottom’ of the shell).

Why include Orthoceras among our essential collectibles? One reason is, of course, they’re popular fossils. You’ll see them on sale pretty much anywhere fossils and minerals are sold. But they’re also of note because they’re among the fossils most likely to be enhanced by the preparator, their goal being to increase the apparent size and quality of the fossil and therefore its selling price.

So just how do you spot a good specimen? The first thing to be aware of is that for all practical purposes, if it isn’t chambered, it isn’t an Orthoceras fossil. It is very common for the preparators to chip way the surrounding rock, or matrix, in such a way that they build up a conical or tapered oval shape around the fossil, making the fossil look a lot bigger than it actually is. This can be attractive, and there’s no reason to disregard a specimen simply because it’s been prepared this way. But the size of the actual fossil is the length of the chambered conical part of the shell, and you should bear that in mind if a specimen is being offered at a premium price because it’s purportedly a large size.

A more nefarious trick is to glue two or more fossils together to produce what seems to be a much larger specimen. This can be difficult to tell because the preparators really are very good at their craft! But the basic thing is this: a large, complete Orthoceras will be obviously conical, and towards the apex the chambers will get steadily smaller, their walls thinner, and the width of the successive chambers noticeably less. Fake specimens will have sudden changes in the thickness of the shell, even reversals of the normal trends, for example successive chambers that get bigger, then smaller, then bigger again. In short, anything that looks odd or irregular could well imply the fossil in front of you is a composite specimen. Again, there’s nothing necessarily wrong in this if all you’re after is an attractive specimen to place on the mantelpiece, but savvy shoppers should be aware of the possible tricks and act accordingly.

One last problem with these fossils is that the name Orthoceras is almost certainly not correct. This entire group of fossils is very difficult to identify, and though several attempts have been made over the years to properly identify the Moroccan orthocones, they remain problematic. Michelinoceras michelini and Orthocycloceras fluminense are two possible names, both known from the Devonian of Morocco, but which, if either, of these names belongs to the majority of the specimens traded is unknown.

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