Monks, Neale Formerly, Department of Palaeontology, Natural History Museum, London, United Kingdom.
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An extinct subclass of Cephalopoda (phylum Mollusca). Besides the Ammonoidea (chambered ammonoids), the other subclasses of Cephalopoda are Nautiloidea (chambered nautiloids) and Coleoidea (squid, cuttlefish, octopus, and extinct belemnites), both of which include extant species. The Ammonoidea had buoyant external shells similar to those of the Nautiloidea, but their life cycle and anatomy resembled the Coleoidea. The Ammonoidea and Coleoidea most likely share a common ancestor with the straight-shelled and chambered Bactritida, which arose 390 million years ago in the Devonian Period. The Ammonoidea were very diverse and their fossil remains are abundant. Ammonoid fossils have been used widely for relative age dating and biostratigraphy (Fig. 1), as well as being objects of cultural or ritualistic significance. See also: Cephalopoda; Coleoidea; Fossil; Mollusca; Nautiloidea
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