Interpreting ammonite fossils

Neale Monks (UK)

Ammonites are such popular and well-known fossils that suggesting they need interpreting may seem ridiculous. But for all their familiarity, there is still a good deal of debate over how they lived and what they did. If nothing else, ammonite experts all agree that they were ecologically diverse, with different species doing different things, and broadly speaking, they can be divided into ammonites that moved about close to the bottom, ammonites that actively swam about in mid-water and ammonites that drifted about on currents, rather like modern jellyfish.

The aim of this article is to help you extract the maximum amount of information from the ammonite fossils in front of you. The way an ammonite shell coils is important, but so too are things like the shape of the suture line and the ornamentation visible on the surface of the fossil, which means that even fragmentary specimens can be quite informative. But first, you need to find your fossils …

Where to collect ammonites

Ammonites only lived in marine environments, most often in moderately deep seas where water chemistry and salinity were more or less constant. So, the classic places to find ammonites are marine limestones (including chalks and oolites), marls, clays and shales. While ammonites seem to have inhabited a range of environments including reefs, their fossils are only occasionally common in places where coral reefs or crinoids dominate. On the other hand, sediments that contain lots of oysters, bivalves, belemnites and burrowing echinoids are often good places to find ammonites.

In terms of geological range, the earliest ammonites appeared during the Devonian, but in the UK at least, it is Jurassic and Cretaceous sediments that have proven to be the best places to find them. Partly, that reflects their higher diversity during the Mesozoic compared to the Palaeozoic, but it is probably also true that there happen to be more Jurassic and Cretaceous sediments of the sorts where ammonite fossils are most likely to be found.

The classic ammonite-rich localities include the Dorset ‘Jurassic Coast’ and its Somerset equivalent for Lower Jurassic ammonites; Burton Bradstock and Bearreraig Bay, Skye, for Middle Jurassic ammonites; Folkestone and Hunstanton for Lower Cretaceous ammonites; and the chalk cliffs of Kent and Sussex, such as those at Eastbourne and Beachy Head for Upper Cretaceous ammonites. Naturally, sites vary in their ease of access, fossil abundance and any restrictions on how fossils can be collected, so it is important to do your research before visiting, for example, on the UK Fossils website (http://www.ukfossils.co.uk/).

Coiling mode

The obvious starting point with any ammonite fossil is examination of its coiling mode. Broadly speaking, ammonites can be divided into the regularly coiled ones on the one hand and the irregularly coiled ‘heteromorphs’ on the other. Most of the heteromorph ammonites belonged to a single very diverse suborder called the Ancyloceratina, which appeared during the Late Jurassic, but was most diverse during the Cretaceous. Heteromorph ammonites are, as you might expect, a heterogeneous group; some were basically open spirals, but others were coiled (if that is the word) like paperclips, helical snail shells and even balls of string.

For the most part, it is assumed that heteromorph ammonites did not swim well and may even have floated about a bit like jellyfish. By contrast, the regularly coiled ammonites, which are by far the most common, are widely assumed to have been active swimmers. Not necessarily fast swimmers though; different shaped shells would create different amounts of drag and, the more drag, the more work it would have been to swim at a given speed. Experiments with model ammonite shells suggest that the ones that produced the most drag were those with the more open, horn-like coiling modes, where successive whorls are easy to see. Such ammonites are said to have evolute coiling. Many of the most familiar ammonites fall into the category, including Dactylioceras, Arietites and Asteroceras.

Other ammonites had what is called involute coiling, where each whorl partially overlapped the one before it. Living species of Nautilus have this kind of coiling, to such a degree that all that can be seen of the earlier whorls is the umbilicus (literally, ‘belly-button’) on each side of the shell. Among the ammonites, well-known examples include such beasts as Anahoplites, Hysteroceras and Macrocephalites.

According to the experimental evidence, such shells generate less drag, but the ammonites that are assumed to have been the best swimmers combined evolute coiling with a strongly compressed, often sub-triangular, whorl section. The result was ammonites with shells that could cut through the water with minimal drag; notable examples familiar to most collectors are Oxynoticeras and Placenticeras. By contrast, ammonites like Macrocephalites had shells that may have been involute in coiling mode, but their near-circular whorl section would have still generated a fair amount of drag. Modern nautiluses have this sort of coiling too and, while they are relatively active animals, they rarely move quickly, unless alarmed.

Ornamentation

Another major factor that affected ammonite swimming ability was ornamentation, which, at its simplest, consisted of little more than fine radial ribs extended outwards from the umbilicus towards the underside (ventral surface) of the shell. More robust ribbing was very common though and it is assumed that these ribs served to strengthen the shell, thereby making the ammonite inside less vulnerable to predation. Some ammonite shells show signs of damage and repair, and often it is a deformity in the ribbing that makes this obvious. Therefore, we do know that at times ammonites survived predatory attacks, and went on to live long enough for them to patch up any damage and then carry on growing.

Most ammonite shells sport other types of ornamentation as well, most notably tubercles and keels. A tubercle is a short spine and, in fact, they are often assumed to be the bottom part of a much longer spine that does not normally preserve. It makes sense for spines to have been sealed off at the base: if a spine was damaged, any gas inside the chamber below it would have leaked out, reducing the buoyancy of the shell.

To be fair, sometimes the spines on ammonites may well have been short and blunt, and the tubercles we see on their fossils were pretty much what they looked like when the ammonite was alive. But occasional specimens are found that reveal certain species had very long spines, even if their fossil remains normally do not. For example, heteromorph ammonites seem to have been especially spiny and, while most fossil Ostlingoceras and Anisoceras specimens have only short tubercles, there are occasional specimens found where the spines are still intact.

In any event, long spines would likely to have increased drag, which is probably why the slow-moving heteromorphs were the ones with the spiniest shells. By contrast, it seems that keels served to improve swimming ability or, at least, improve stability. The basic idea is that they worked like the keel on a ship, acting against the tendency of a swimming ammonite to veer to the left or right. Keels are often sharp and well developed on those ammonites imagined to be the most active swimmers, like Oxynoticeras. However, they are often present on less obviously active forms too, such as Harpoceras, which had only moderately involute shells and fairly robust ribbing. On the other hand, those ammonites with the strongest ribs and spines often lacked keels, as was the case with, for example, Hoplites and Douvilleiceras.

Whether or not an ammonite swam quickly, slowly or drifted passively with the current would be an important aspect of its ecology. It is unlikely even the most active species were able to catch large fish or squid, but such ammonites were probably able to move about in mid-water, even if they mostly caught slow-moving prey, like crabs or shrimps. The less active swimmers probably concentrated on either sedentary prey (such as clams) or else sifted the substrate for things like worms or even carrion. As for the drifting heteromorphs, they may have been vertically-migrating plankton feeders, an important ecological niche still exploited by many oceanic squids today, which sink into the depths during the day to avoid predators, but rise up towards the surface at night to feed on planktonic crustaceans and other small prey.

Sex and sexual maturity

John Callomon has argued convincingly that many ammonites came in matched pairs, seemingly identical when small, different when mature, but always found in strata of the same geological age. Within these pairs are a large form called the macroconch and a small form called the microconch, and these are in fact males and females of the same ammonite species.

Which one is the male and which one is the female is a matter of conjecture, but it is widely assumed that the bigger one – the macroconch – is the female, because this is very often the case with other cephalopods. Evidence might come from the ornamentation on the microconchs, which is often more elaborate than on the macroconchs, in keeping with the observation that male animals commonly have elaborations used for display, such as brighter colours or longer horns, which the females lack.

But the most convincing evidence comes from the lappets – paired flap extensions, rather like sideburns that extend forward from the aperture of the shell and only fully developed on microconchs. In the past, palaeontologists have suggested these had a function during mating, perhaps helping the small male ‘hold onto’ the female, but this does not seem likely, given that the ammonite animal was already equipped with more or less squid-like tentacles used for gathering food. Probably, the lappets did not do much of anything and, like the peacock’s train or the guppy’s tail, they were mere advertisements meant to signify the sexual maturity of the male carrying them and his genetic fitness in being able to grow them, particularly since such structures doubtless made it less easy for him to move, feed or escape from predators.

Size

Ammonites ranged widely in size, from species that were little more than 1cm across when fully grown, through to ones more than a metre in diameter. Obviously, size is an important aspect of any animal’s ecology and, doubtless, the miniature species occupied very different ecological niches to the giants.

However, determining the size of the ammonite in front of you is rather tricky. For a start, it is important to remember that the fossil in front of you is not necessary complete. Because of the way it is divided up into reinforced chambers, the flotation part of the coiled shell (known as the phragmocone) is by far the strongest part of an ammonite’s shell. It is also likely that those predators, such as ichthyosaurs and sharks that fed on ammonites, were able to extract the meaty part of the animal from its shell, crushing the living chamber that contained the ammonite animal and discarding the nutritionally worthless phragmocone.

Even if the ammonite died a natural death, the fact that the living chamber was not reinforced in any way would make it much more likely to be damaged by scavengers, impacts caused by local water currents or the weight of the surrounding sediment. In those places where both phragmocone and living chamber get preserved, it is very common for the phragmocone to be preserved as a three-dimensional cast of some sort, while the living chamber is little more than a flattened impression; you often see this with ammonites in the Gault Clay, for example.

The distinction between phragmocone and living chamber is important, because ammonites had long living chambers that may account for as much as a whole whorl. So, if the living chamber is lost, the resulting ammonite will be much smaller and, of course, you will not be able to see those modifications to the aperture that are only present when the ammonite becomes fully grown, such as lappets, constrictions or collars. Basically, if you can see the jigsaw-like suture lines between each chamber, then you’re looking at the phragmocone, which means the living chamber will be partially or completely lost from your specimen, and the size of the ammonite in life can be estimated to be substantially bigger.

Suture lines

Having a chambered shell was clearly useful to ammonites since they seemed to have evolved the most complicated shells in the entire evolution of the Cephalopoda. But exactly why they had complicated junctions between each chamber — what we see as the suture line in fossils — remains open to debate.

The commonest interpretation is that the complex walls between the chambers added strength, helping the shell to resist crushing, either from water pressure or a predator’s bite. So, the more complex the suture, the stronger the shell and, broadly speaking, ammonites do indeed show a consistent trend towards greater complexity from the earliest species in the Devonian onwards through the Palaeozoic into the Mesozoic. To a degree, this may reflect the way ammonites were becoming better adapted to life in deeper water, where the crushing pressure on gas-filled shells is greater than in shallow water. But it is also likely this trend was driven by the evolution of predators, including sharks and the various marine reptiles like ichthyosaurs and plesiosaurs.

Pulling it all together

So, by examining the ammonite fossil in front of you, it is possible to make inferences about its adult size and possibly its gender, its swimming ability and consequently what sort of things it may have eaten, and any adaptions it may have had to resist predators and water pressure.

While the fossils of their shells provide lot of information in some ways, the frustrating thing about them is that they provide very little evidence about what the actual ammonite animal looked like. Without fossil remains of the soft body parts like tentacles and eyes, we do not really know what they looked like compared to living cephalopods. Did they have the crude, pinhole-camera eyes of nautiluses or the sophisticated, lensed eyes of octopuses? Did they have the many whisker-like tentacles of nautiluses or the eight or ten muscular arms that squids have with hooks and suckers? Were ammonites capable of communicating using colour patterns on their skins like cuttlefish do? Nobody really knows, so you may as well use your imagination when it comes to thinking about what your ammonite fossil might have looked like when it was a living, swimming and feeding animal, all those millions of years ago.

Further reading

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