Ammonite wars

Neale Monks (UK)

Ammonites have been studied intensively for the last 200 years but, between experts, there is very little agreement on what ammonites looked like or how they worked as living organisms. Could they float? Did they swim? How did they catch their food? How long did they live? Why did they disappear at the end of the Cretaceous? All these questions remain essentially unresolved.

In fact, ammonites are a quite poorly understood group of fossils in many ways. By far the majority of scientific papers written about ammonites concentrate purely and simply on what is known as primary taxonomy — is this ammonite species distinct from all the others so far discovered and, if it is, how can it be recognised reliably and where else can it be found? The reason most scientists concentrate on these questions above all others comes down to the usefulness of ammonites for biostratigraphy.

Many ammonite species evolved and died out within fairly short periods of time, perhaps a few hundred thousand years, but their fossils are often abundant and, most crucially of all, often very widely distributed. So, if a particular ammonite species can be found in sediments at two different localities, it’s a good indication that those two sediments were laid down within the same, rather narrow period of time.

Just taking British palaeontologists as an example, virtually all the major scientists working on ammonites did so to further their studies of biostratigraphy: WJ Arkell, R Casey, MR House, MK Howarth, WJ Kennedy, HG Owen and LF Spath, to name just a few. While their publications have been hugely important in terms of ammonite systematics and evolutionary relationships, they wrote hardly anything at all about ammonite anatomy and ecology. You’d see much the same pattern looking at palaeontologists from other parts of the world and biostratigraphy, rather than biology, continues to be the focal point of most ammonite research, even among the younger generation of palaeontologists.

So what don’t we know about ammonites? What do palaeontologists argue about? That’s the focus of this article and you’d perhaps be surprised to know just how fierce some of these arguments can be.

Could they float?

It would seem self-evident that ammonites were neutrally buoyant. After all, modern nautiluses have very similar shells divided up into gas-filled chambers and they certainly do float in midwater. Their buoyant shells make it much easier for the nautiluses to swim about, so, unlike snails dragging heavy shells, nautiluses are comparatively speedy animals able to forage for food much more efficiently. In terms of design, ammonites have broadly similar shells with chambers and a siphuncle that connects them. Modern nautiluses use the siphuncle to replace the seawater in new chambers with gas and it isn’t unreasonable to assume that ammonites did the same thing (particularly since we know from fossils that they also possessed this tube). As they grew, they added new chambers to their shells, filled them with gas and, in that way, retained neutral buoyancy.

Sounds very neat and tidy, but not everyone agrees. German engineer, Klaus Ebel, has looked at ammonite shells and is convinced that they simply aren’t big enough to have provided neutral buoyancy. In a nutshell, to be neutrally buoyant, an ammonite must displace a greater weight of water than itself. Nautiluses clearly do this and, when examined closely, it can be seen that their hollow, gas-filled shells are rather large compared to their relatively small bodies. Ebel argues that ammonites didn’t have such a favourable ratio of body weight to shell size, so the weight of water they displaced wasn’t enough to counterbalance their body mass.

Ebel reconstructs ammonites as bottom-living animals, with shells that were buoyant enough to be lifted off the substrate, but not so buoyant that they pulled the animal up with them. Furthermore, he believes that the shape of the ammonite shell proves his point. As it grew, a typical ammonite shell continually ‘toppled over’ under its own weight, resulting in the classic spiral shape. Although Ebel’s interpretation of ammonite shell design is a minority viewpoint, it does at least fit in with what we can measure from ammonite shells, as opposed to vague assumptions that they worked in the same way as those of modern nautiluses.

On the other hand, the distribution of ammonite fossils is a problem, because very many ammonite fossils are found in sediments laid down under poorly-oxygenated conditions and notably lacking in fossils of unambiguously benthic (bottom dwelling) organisms such as crinoids, clams or brachiopods. That’s not a problem if the ammonite shell sunk down from above, but it seems very unlikely that ammonites could have crawled about in such habitats.

Could they swim?

Despite Ebel’s mathematical analyses, the majority of geologists continue to assume that ammonites were neutrally buoyant. At the very least, they were passive drifters, a bit like jellyfish. However, like Ebel, we were concerned that the shape of the ammonite shell wasn’t really comparable to that of the modern nautilus.

Ammonites generally have much longer and thinner body chambers than nautiluses, and this is important for two reasons. Firstly, jet propulsion in nautiluses – and, by extension, ammonites – works in the same way as a plunger. When the body is pulled into the shell, a jet of water is squeezed out and that jet can be used to propel the animal backwards. Nautiluses have broad but short body chambers, so the body doesn’t have to be pulled very far to get a good jet of water. However, the average ammonite has such a long and narrow body chamber that the body would have had to be pulled a very long way back to produce an equivalent force.

That leads to the second issue – stability. When a nautilus swims, there’s a slight back-and-forth rocking motion caused by shifts in the centre of gravity, as the body is pulled backwards and forwards. Ammonites have such long body chambers, and their bodies would have had to be pulled back so far to produce a useful jet of water, that any attempt at swimming in this way would have resulted in a far greater degree of rocking. Indeed, in extreme cases the ammonite would have rolled right over.

We argued that heteromorph ammonites couldn’t swim, at least not in the same way as nautiluses. For example, looking at the paper-clip shaped ones, we imagined that, when the body was at the front of the shell, the ammonite was tipped forwards, allowing the animal to forage for food. However, if the ammonite was disturbed, it pulled itself into its shell and that tipped the shell up and away from the substrate. Even without active swimming, water currents would hopefully carry it away from danger.

While that might not have been much of a defence against midwater fish, it would have been a useful counter to attacks by benthic predators like starfish and crabs, and comparable to what we see among modern scallops and file shells. This sort of defensive strategy also makes sense when used alongside spines and shell-strengthening ribs. Because these features increase drag, they don’t make much sense on actively-swimming species, and are seen today mostly on slow-moving and sessile animals, including sea urchins, thorny oysters and murex snails.

A third way of looking at ammonite shells has been put forward by Adolph Seilacher and Michael Labarber. They argue that, rather than swimming, ammonites worked more like submersibles, after the fashion of Cartesian divers. They argue that a living ammonite was neutrally buoyant and that the wall between the body chamber and the gas-filled chamber behind it wasn’t rigid but flexible. By using its muscles to compress the gas-filled chamber, the ammonite could increase its density, reducing its buoyancy and causing the ammonite to sink. If the ammonite wanted to float up again, it would expand the gas-filled chamber, reducing its density, thereby increasing its buoyancy. According to them, this unique functionality explains why the chamber walls on ammonite shells are so much more complex than those seen on nautiluses.

It’s often assumed that streamlined ammonites, such as Oxynoticeras oxynotum (left), were better swimmers than other types of ammonite, like the smaller one (top right).

How fast could they swim?

Nonetheless, the majority of palaeontologists continue to assume that ammonites were more or less active swimmers. In particular, John Chamberlain has tested this idea by placing models of ammonite shells in flume tanks and examined the way water flows past them. In this way, he has established what features of ammonite shells produce drag and thereby reduce swimming ability. It turns out that some shell shapes are actually quite smooth and stable, in particular, the discus-like ones typical of ammonites such as Oxynoticeras and Placenticeras. Their shells are tightly folded, so that the outer whorl almost completely covers the earlier ones and there’s little to no external ornamentation.

Similarly, Peter Allison has been applying computational flow dynamics to the problem of ammonite shell shape and swimming ability, and his results are interesting. Among other things, it may be that, far from being an encumbrance, ribs might actually have provided extra stability to the shell, reducing the shell’s tendency to rock. He has also been able to use computer modelling to take the soft body parts of the ammonite into consideration, something earlier workers using rigid models were unable to do.

What was the aptychus for?

Although most ammonite fossils are shells, another anatomical structure known as the aptychus is also widely preserved in the fossil record. Actually, aptychi normally come in pairs, each one approximately semi-circular in shape and made from calcite, as opposed to the aragonite typical of ammonite shells. Sometimes, they are found in fossil ammonite shells and this makes it clear that the aptychi were associated with the soft body parts of the animal and located within the living chamber. It should also be noted that primitive ammonites had a single, more or less oval-shaped anaptychus that resembled a pair of aptychi fused together.

Initially at least, it was assumed that pairs of aptychi worked as trapdoors that sealed off the front of the ammonite shell when the animal withdrew its head and tentacles. Certainly, nautiluses have a somewhat similar (though uncalcified) structure known as the hood and many snails also have trapdoors, known as opercula, calcified or not as the case may be. However, there are some problems with this interpretation, the main one being that, while some aptychi fit the opening of the shell quite neatly, many of them do not.

An alternative explanation advocated by Ulrich Lehmann and Nicol Morton, among others, is that ammonite aptychi were part of their jaws. They hypothesise that ammonites had beak-like jaws like squids and the aptychi formed part of the lower jaw. This calcified lower jaw was used like a sort of rigid dredge and, as the ammonite pushed through the sediment, the aptychus helped to scoop up small prey animals, such as the benthic foraminifera and crustaceans found in the very few ammonite fossils with recognisable gut contents.

On the other hand, the sheer size of these hypothesised jaws is remarkable and many palaeontologists have found this interpretation difficult to accept. Certainly, jaws of this type would be far larger than those seen in modern nautiluses or, for that matter, any of the other living cephalopods. Recall also that most researchers have rejected the idea ammonites were strictly benthic animals, because their fossils are commonly found in environments where benthic animals could not possibly have survived, let alone fed. Quite why ammonites, living away from the substrate, needed massive, heavy, scoop-like jaws remains unclear.

Lehmann went on to develop a modified version of the aptychus-as-jaws idea with Cyprian Kulicki, arguing that these structures might have worked as both trapdoor and feeding apparatus, depending on whether the head and tentacles were withdrawn or extended.

Why are ammonites so contentious?

Given the abundance of ammonite fossils, as well as the expertise of the scientists studying them, it can seem odd that such fundamental questions as whether they could swim remain unanswered, or at least, not completely settled. However, this isn’t actually all that surprising once you start looking at how palaeontologists answer these sorts of questions.

The easiest way to explain how fossil organisms work is to compare them to their living relatives. Ammonites are most closely related to cuttlefish, squids and octopuses, but these animals are very different in terms of anatomy. Most obviously, they don’t have external shells. Nautiluses do have external shells, but they’re more distantly related to ammonites and, in terms of anatomic detail, the shells of nautiluses are only somewhat similar to those of ammonites. Unlike nautiluses, ammonites have shells with complex chamber walls and their shells are also far more ornamented on the outside, with ribs, keels and spines. So, while palaeontologists can draw some conclusions on ammonite anatomy by comparing them to nautiluses, such inferences are limited.

Palaeontologists looking at ammonites have another problem – the lack of information that fossil shells provide. For example, vertebrate bones have an intimate relationship with the muscles connected to them. Looking at the skeleton of a dinosaur or mammoth tells you a great deal about how the animal was put together and what it looked like in life. Much the same thing can be said about trilobites and echinoderms – their complex skeletons reveal lots of detail about the structure of the animal within them. By contrast, the shell of an ammonite is mute. Apart from a few vague muscle attachment scars, there’s little to be gleaned about the size and shape of the ammonite animal’s soft body parts.

The bottom line is that ammonites are both familiar and mysterious. Palaeontologists have had to use their imaginations rather more with ammonites than with most other groups of invertebrates and the result has been a more varied selection of interpretations. One day, perhaps someone will find an ammonite fossil with soft body parts preserved. And, should that happen, that specimen will almost certainly be found by an amateur — perhaps even a reader of this august journal.

Further reading

“Ammonites“, by Neale Monks and Philip Palmer, The Natural History Museum, London (2002), 159 pages (softback), ISBN-13 : 978-15883404-7-4.