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Fossil Record

The fossil record of cephalopods is rich, with over 7500 species recorded. The illustration below shows the time ranges and diversities of the three cephalopod subclasses (from E. Clarkson).

The earliest known cephalopod is Plectronoceras from the Cambrian of China, approximately 520 million years (My) old. At the end of the Cambrian there was a phase of explosive radiation among the cephalopods and this continued into the Early Ordovician. During this period, the order Endoceratida gave rise to many other orders, together forming the subclass Nautiloidea, of which the order Nautilida was the last to arise, approximately 410 My ago. However by the end of the Triassic, all the members of this subclass had become extinct apart from Nautilus, which has survived to the present day.

Many nautiloids became extinct during the Devonian. Then, the subclass Ammonoidea arose, a group whose members were exceptionally well adapted to life in the deep ocean as their shells could resist implosion. They may also have been ecological specialists that could adopt a wider range of life modes than the scavenging nautiloids. Ammonoids were hit hard by the end-Permian and end-Triassic mass extinctions, but they recovered in the Jurassic to become dominant marine predators, the famous ammonites. The Ammonoitida finally became extinct at the end of the Cretaceous, at the same time as the belemnites (and the dinosaurs of course).

How did Nautilus survive, when all its relatives died out long ago? Some have suggested that it might have been because Nautilus hatchlings are large hatchlings, and they are immediately able to take up the deep water foraging habits of their parents, while the Ammonitida had small hatchlings that were dependent on the planktonic ecosystem. When this ecosystem collapsed, they lost their main source of food.

The Subclass Coleoidea is the largest extant subclass of cephalopods, but their fossil record is relatively poor compared to other cephalopod groups. They are now more diverse than they have been in the past and because only the belemnites are normally preserved as fossils. Squid, octopi and cuttlefish are mostly composed of soft parts and so are not as readily fossilizable.

Author: James Tarver
Last updated: 14/11/2004
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