Tertiary cephalopods or where did all the ammonites go?

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.

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Fig. 1. Warden Point, on the Isle of Sheppey, is one of the best places to collect Tertiary cephalopods (UKGE photo).

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.

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Fig. 2. After death, Spirula shells float around for months and may, as here, end up encrusted by organisms, such as bryozoans (courtesy Natural History Museum; scale bar in mm).

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.

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Fig. 3. In life, Spirula is oriented head-downwards, as shown here (photo by Neale Monks).

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.

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Fig. 4.These images of two Spirulirostra anomala specimens show the basic arrangement of this unusual fossil (courtesy Natural History Museum; scale bar 1cm).

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.


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