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. 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 … Read More

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