Tertiary cephalopods, or where did all the ammonites go?

Dr Neale Monks (UK)

Most geologists will be familiar with Palaeozoic and Mesozoic cephalopods, but their Tertiary counterparts are much less well known. It isn’t that Tertiary cephalopods are rare as such – at some localities they can be quite common – but their diversity is extremely low. For example, the Gault Clay is a Lower Cretaceous formation that has yielded hundreds of cephalopods species, including ammonites, belemnites and nautiloids. Fast-forward to the London Clay, an Eocene formation, and that diversity falls to about five species, at most.

At first glance, you would think this reflects the fatal decline of a group marching towards extinction. However, there are 700 cephalopod species alive today, so clearly that isn’t the case. In fact, what the lack of Tertiary cephalopod fossils shows is the switch within the group from forms with shells (such as ammonites and nautiluses) towards forms that don’t have shells (like squids and octopuses). Because they don’t have hard parts that fossilise easily, squids and octopuses have an extremely sparse fossil record.

Nonetheless, the Tertiary isn’t entirely devoid of cephalopods if you know where to look. The London Clay exposure at Sheppey is a particularly good place to find nautiloid fossils. Occasional specimens from other cephalopod groups occasionally turn up as well and these give us some fascinating glimpses into the evolution of the post-Cretaceous cephalopods.

Nautiloids

Perhaps surprisingly, the nautiloids seem not to have been much affected by the Cretaceous-Tertiary (K-T) extinction event. Geologist, Peter Ward, is fascinated by this and, as well as studying the fossil record of the nautiluses, he has also looked at their living descendants, now confined to the tropical, Indo-West Pacific region. Modern nautiluses produce only a few dozen eggs per year, each a little smaller than a ping-pong ball, and from these emerge offspring about an inch or so across. So far as we can tell, Tertiary and, indeed, Mesozoic nautiluses were not much different, their shells indicating that fossil nautilus species were already quite large and well developed at the moment of hatching.

This is all very different to what we know about ammonite reproduction. Ammonites seem to have produced large numbers of small offspring and these are likely to have lived in the plankton for the first few months of their lives. This, Ward argues, is the key to the puzzle – whereas juvenile nautiluses are large animals able to forage opportunistically on whatever prey or carrion they find, ammonite larvae were completely dependent on the plankton. Whatever else may be debatable about the K-T extinctions, the one thing that everyone agrees on is that the marine planktonic community was very hard hit. So, when the planktonic ecosystem collapsed, the larval ammonites died with it. By contrast, the nautiluses plodded on, more or less unscathed.

In any event, all three families of nautiluses present in the Late Cretaceous persist into the Tertiary, namely the Cymatoceratidae, Hercoglossidae and Nautilidae. On top of these, a fourth family appears in the Palaeocene, the Aturiidae. The Cymatoceratidae and Nautilidae were quite conservative groups, with smooth shells and simple suture lines, but the Aturiidae and Hercoglossidae were a bit more innovative, and developed shells with external ornament and complex sutures similar to those of goniatite ammonites.

The most common nautilus in the London Clay formation is Cimomia imperialis, one of the Hercoglossidae, and a species that reaches about 20cm in diameter and exhibits fairly simple suture lines, not much different to modern nautilus species.

The remaining species are all quite rare. Aturia ziczac is typical of the family Aturiidae and has a somewhat flattened, disc-like shell and remarkably sinuous suture lines. Even less often seen are Deltoidonautilus sowerbyi and Hercoglossa cassiniana (both members of the Hercoglossidae), and Euciphoceras regale, Eutrephoceras urbanum and Simplicioceras centrale (all members of the Nautilidae).

One thing that does appear to be as true about nautiluses in the past as it is today is that they preferred warm water, and the appearance of so many species in the London Clay can be seen as evidence that Southern England enjoyed tropical conditions at this time. Conversely, their absence from the younger sediments in this country would seem to reflect the change in global climate that took place between the Eocene and the Oligocene, during which time sea temperatures got considerably colder. While that does not explain why nautiluses today are only found in a relatively small area of the tropics covering the eastern Indian Ocean and the western Pacific Ocean, it does at least explain why we don’t have nautiluses swimming about in the English Channel.

Spirulirostra and Spirula

While nautiluses are bit-part players in modern cephalopod faunas, it’s the octopuses, squids and cuttlefishes that are the stars of the show. Unlike squids and octopuses, cuttlefishes have quite a good fossil record, because they have comparatively large and robust shells, which we normally call cuttlebones. Unlike nautilus shells, cuttlebones are internal shells that provide only buoyancy, not protection.

Three cuttlefish species are known from the London Clay: Belopterina levesquei, Belosepia blainvillei and Belosepia sepioidea. In fact, what we normally find are not the chambered shells themselves (what palaeontologists call ‘phragmocones’), but the calcitic tips (‘guards’) attached to the tail-ends of the shells, broadly analogous structures to belemnite guards. The guards of Belopterina are club-shaped structures, with slightly curved sockets that fitted onto the phragmocone. Belosepia had rather different guards that looked more like little claws or sharks’ teeth.

Whereas Belopterina and Belosepia are merely uncommon fossils, the fossil remains of the squid-like cephalopod, Spirulirostra anomala, are extremely rare, at least in the UK. As far as I am aware, the only two specimens in the collections at the Natural History Museum in London are a specimen found in Highgate in 1894 and a second specimen found in 1999 by a museum staffer, Simone Wells, on a field trip led by PA Jeffrey. As it happens, the second specimen was much better preserved than the first, allowing Ms Wells and I to describe this species in considerably more depth than ever before (Monks and Wells, 2000).

Spirulirostra anomala is interesting because its shell begins with an open spiral similar to that of modern Spirula spirula. However, as the animal grows, the shell unwinds into a straight cone and even develops a sort of guard-like structure like those possessed by belemnites. Therefore, to understand how Spirulirostra anomala worked, we are forced to make analogies with both belemnites and modern Spirula.

When the animal was young, it presumably lived a head-downwards lifestyle similar to that of modern Spirula, with the buoyant shell at the ‘top’ end of the animal and its head and tentacles hanging below. Modern Spirula mostly move up and down the water column, rising into shallow water at night to feed on plankton, and then sinking into deep water during the day to avoid predators. However, once Spirulirostra anomala reached a certain size, it would switch to a more traditional horizontal orientation, with the head and tentacles at the one end, the buoyant shell in the middle, and the guard acting as a counterweight at the other end.

Our interpretation of Spirulirostra anomala was that the lineage to which it belonged was ancestral to the one that ultimately gave rise to Spirula spirula. In short, we hypothesised that Spirula spirula is an animal that retains in its adult morphology characteristics (and likely ecological traits) that were possessed only by juvenile Spirulirostra anomala. This sort of evolutionary process is known as neoteny, the most famous example of which can be seen in the axolotl, a neotenic version of salamander that essentially spends its adult life as a giant, sexually mature tadpole.

One interesting question is whether Spirulirostra anomala lived in the warm, shallow seas that gave rise to the London Clay formation. Certainly, the nautiluses seem to have lived in such habitats, if their abundance is anything to go by. Modern Spirula spirula favour deep water habitats though, typically spending the days at about 500m, while rising up to about 100m to feed during the night. Nonetheless, Spirula shells do wash up on beaches from time to time and perhaps the ones we find in the London Clay got there in a similar way, drifting in from far offshore. That could explain why they are so rare.

So why no Tertiary ammonites?

In one way at least, this question is easy to answer – there are no ammonites in the Tertiary because they had all died out by the end of the Cretaceous. Thus far at least, no convincing post-Cretaceous ammonite fossils have yet been found (though Cretaceous ammonites reworked into Palaeocene sediments are not unknown). However, what is more interesting is why the ammonite way of living didn’t re-evolve during the Tertiary. When we look at Tertiary nautiluses, we see at least two families that seem, at face value at least, to be re-evolving ammonite-like traits, namely complex suture lines and external ornament. By contrast, the standard issue nautiluses generally lack these traits – they have simple sutures and shells that either lack any kind of external ornament or have, at most, very fine growth lines.

At a small meeting at the Open University about ten years ago, I mentioned this and made the point that what we actually see in the Tertiary is that cephalopod evolution occurs in two distinct phases. During the first half of the Tertiary, there is a high diversity of cephalopods with chambered shells, both internal (as in the case of Spirulirostra anomala) and external (as with the various nautilus families). However, from the Miocene onwards, many of the shelled cephalopods disappear, never to return. Instead, it is the unshelled cephalopods that seem to become predominant – of the 700 modern cephalopod species, fewer than 130 have chambered shells of any sort, and only six (the extant nautilus species) have external chambered shells. Apart from Spirula spirula, all living cephalopods with chambered shells are cuttlefish.

So what happened in the Miocene that knocked the shelled cephalopods back so hard? There was certainly a lot going on. For a start, global climate cooled considerably and, as we’ve noted, nautiluses prefer tropical climates. However, one of the things that happened during the Miocene, which might have been important, was the evolution of cetaceans, that is, whales and dolphins. Early whales, like Basilosaurus, had been around since the Eocene, but these cetaceans hunted by sight. The ability to hunt and find their way around using a sort of sonar – what we call echolocation – appeared with a family of whales known as the Squalodontidae, which evolved in the late Oligocene.

Of course, correlation doesn’t imply causation. Yes, echolocating whales were diversifying at the same time cephalopods with chambered shells were declining, but that doesn’t mean that the one is causing the other. However, it’s something worth thinking about. Why? Because a gas-filled chambered shell would produce a very clear ping that an echolocating whale could hear. By contrast, a squid without a gas-filled shell has a density close to that of water, and its echo would be much weaker and more difficult to detect.

It’s not hard to imagine that primitive whales would have found shelled cephalopods, like nautiluses, much easier to find and eat than unshelled cephalopods, such as squids. Over time, an arms race of sorts would develop, with steadily more advanced whales becoming better and better at detecting squids and octopuses, while squids and octopuses would be coming up with new strategies for avoiding predators, perhaps the most important of which is improved jet propulsion, so they can escape from predators more effectively.

In such an arms race scenario, the shelled cephalopods would have lost out at a very early stage and, indeed, that seems to be borne out by what we see today. There’s just the one open ocean cephalopod with a chambered shell, Spirula spirula, and it probably survives by living in deep water where dolphins can’t get at it, while being too small to be of much interest to the large, deep-diving whales. The nautiluses, of which there are only six species, have carved out a unique but limited niche in the Indo-West Pacific, which keeps them close to rocky reefs in fairly deep water and, again, this habitat probably minimises their interaction with echolocating cetaceans. On the other hand, cuttlefish are quite diverse and dolphins and whales do eat them in considerable quantities. How do they manage to survive? It’s hard to say, but perhaps a combination of rapid growth (most live for only a year or two) and stealth (in particular, hiding under sand or among seaweeds) help cuttlefish to minimise their loses.

Finding Tertiary cephalopods in the UK

The best place to find Tertiary cephalopods is undoubtedly the London Clay at Sheppey, Kent, particularly the highly fossiliferous localities such as Warden Point. Nautiluses are reasonably common in the large nodules found in the clay. These nodules sometimes yield other interesting fossils as well, such as lumps of wood bored through with shipworms (Teredo spp.). Original shell material will often be found on the nautilus fossils, visible as a pinkish-white or pearly layer. It’s also possible to find small nautiluses in the pyritised debris where the seeds, seashells and other small fossils are usually found.

In theory at least, other cephalopods, such as Spirulirostra anomala, could be found at any London Clay exposure where shells are abundant, but in practise these tend to be rather rare. Nonetheless, they do get found from time to time and it’s likely they’ve actually been found more often than people suppose, because they are so difficult to identify, and simply get put to one side with all the other unidentified bits and bobs. If you’re lucky enough to find cephalopods other than nautiluses, you’ve found a scientific treasure. So, if in doubt, take it along to the Natural History Museum to get it identified – you may well have a fossil there that hasn’t been collected for decades.

Reference

Monks N. & Wells S. 2000. A new record of the Eocene coleoid Spirulirostra anomala (Mollusca: Cephalopoda) and its relationships to modern Spirula. Tertiary Research, 19, 47-52.

Further reading

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