Nautilida
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Nautilida is an order of mostly prehistoric cephalopods that includes the modern nautiluses and their immediate ancestors and relatives. All recent nautiloids are included in this group. This was a large and diverse group from the Late Paleozoic through to the Middle Cenozoic. Between 24 and 34 families and 165 to 184 genera are recognised, making it the largest order of the Nautiloidea. Today the group is represented only by the five species of the genera Nautilus and Allonautilus.
Diversity and Evolutionary History
In 1942 Otto Schindewolf proposed that the Nautilida evolved from straight-shelled ("Orthoceras") nautiloids, which gradually developed a curved shell through transitional forms like the Ordovician Lituites. It is now widely agreed that they evolved from Oncocerids. Curt Teichert (Teichert 1988 p.41) suggests that the best candidates for an ancestor are the families Acleistoceratidae and Brevicoceratidae, both of whom have the same sort of shells and internal structure as that found in the very earliest true nautilida of the Devonian period.
Beginning in the Early Devonian, the number of genera progressively increased to about 22 in the Middle Devonian. During this period the shells of nautilids were much more varied in shape then those of the species of the living nautilus, ranging from curved (cyrtoconic) through loosely-coiled (gyroconic) to tightly-coiled like the modern nautilus.
In the Late Devonian the group declined, but once again diversified in the Carboniferous, the acme of the group, when some 75 genera and subgenera in some 16 families are known to have lived. Although there was considerable diversity, curved and loosely-coiled shells were rare or absent, except in the superfamily Aipocerataceae. For the rest, nautilids adapted the standard planispiral shell form, although not all were as tightly-coiled as the modern nautilids. (Teichert 1988, pp.42-43). There was, however, a great diversity in surface ornamentation, cross section, and so on, with some genera, such as the Permian Cooperoceras and Acanthonautilus, developing large lateral spikes (Fenton and Fenton 1958, p.187).
Despite again decreasing in diversity in the Permian, nautilids were far less affected by the Permian-Triassic extinction event than their relatives the ammonoids. During the Late Triassic there was a tendency - represented by several families (in the Clydonautilidae, Gonionautilidae, and Siberionautilidae - to develop sutures similar to those of some Late Devonian Goniatites. Only a single genus, Cenoceras, with a shell very like that of the modern nautilus, survived the much more minor (relatively speaking) end Triassic extinction.
For the remainder of the Mesozoic, nautilids once again flourished, although never at the level of their Paleozoic glory, and 24 genera are known from the Cretaceous. Again, they were not affected by the end Cretaceous mass extinction, which completely annihilated the ammonoids. Three families and at least five genera of nautilids are known to have survived this crisis in the history of life. There was a further resurgence during the Paleocene and Eocene, with several new genera, the majority of which had a worldwide distribution. During the Late Cretaceous and Early Tertiary, the Hercoglossidae and Aturiidae again developed sutures like those of Devonian goniatites. (Teichert 1988, pp.43-44)
Miocene nautilids were still fairly widespread, but today the order includes only two genera, Nautilus and Allonautilus, limited to the south-west Pacific.
References
- Fenton and Fenton (1958), The Fossil Book (Doubleday & Co., Garden City, New York)
- Kummel, B. (1964) "Nautilida" in Treatise on Invertebrate Paleontology, Part K. Mollusca 3. (Geological Society of America, and University of Kansas Press)
- Moore, Lalicker and Fischer, (1952) Invertebrate Fossils, McGraw-Hill Book Company, Inc., New York, Toronto, London
- Teichert, T. (1988) "Main Features of Cephalopod Evolution", in The Mollusca vol.12, Paleontology and Neontology of Cephalopods, ed. by M.R. Clarke & E.R. Trueman, Academic Press, Harcourt Brace Jovanovich,