The nautiloid cephalopods: a strange success

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Abstract
The origin of the cephalopods from the Monoplacophora is briefly considered. The first rare Upper Cambrian Plectronoceras are now known to be succeeded by a later Cambrian radiation involving the Plectronoceratida, Ellesmeroceratida, and two other orders, all well documented from Chinese occurrences. The greatest success of the nautiloid cephalopods came in the Ordovician Period with three evolutionary pulses in the Tremadoc, Arenig, and later Ordovician. Three particular Palaeozoic problems are treated: gigantism in cephalopods, the orthocone operculum Aptychopsis, and the Chinese Pagoda Limestone as an example of an 'Orthoceras' limestone. Devonian developments included the origin of the ammonoids through their first suborder, the Bactritina; the origin of the coleoids; and the beginning of the long history of the coiled Nautilida. After a brief aside on the classification of the cephalopods, the Nautilida are treated in terms of their survival after the extinction of the ammonoids. Finally, there are comments on the living Nautilus.