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Memoir 12—The nautiloid order Ellesmeroceratida (Cephalopoda)

By R. H. Flower, 1964, 234 pp., 53 figs., 32 plates, 1 appendix, 1 index.

The order Ellesmeroceratida is reviewed in terms of three new suborders, their families, and genera. Species are listed under the genera in which they occur, with appropriate indications where transfer from one genus to another has been required. Detailed specific descriptions are confined to new species or to forms discussed and refigured for morphological purposes. Particular attention has been given, wherever material permitted, to the structure of the connecting rings, which are thick and commonly complex and show generally layered structure. Diaphragms, developed in the Plectronoceratina, Ellesmerocera-tidae, and some Proctocycloceratidae, appear to be extensions of the rings. They are suppressed in the Baltoceratidae, and in higher members of that group a ventral rod, presumably originally aragonitic, is developed and is found also in some forms otherwise assignable to the Protocycloceratidae, in which homeomorphic stocks may yet be included. Relationships and evolution of the stock are particularly investigated, and ranges are indicated, with some attention necessarily given to the derived orders Endoceratida, Tarphyceratida, and Michelinoceratida. The table of contents supplies a summary of the taxa to the generic level. A systematic appendix includes inadequately known genera, some forms removed from the Ellesmeroceratida, and some genera of doubtful position. A stratigraphic appendix includes detailed discussions of some sections and problems of ranges of some of the genera and species.

This work deals with a study of the Ellesmeroceratida, the archaic order of the Nautiloidea and thus also of the Caphalopoda as a whole, for the Ammoniodea and Coleoidea are developed from later nautiloids, both possibly appearing in the Devonian. It seems generally true that the archaic members of every group have proved elusive, difficult to interpret, and the subject of conflicting opinions. This has certainly been true of the Cephalopoda. In another section, some of the vicissitudes of conflicting opinions and interpretations have been reviewed. It is evident now that the old concept of Orthoceras as primitive is false, and also we must abandon the view, so widely held in the 1920s, of the generally prevalent conditions of holochoanitic structure among the older cephalopods. Indeed, prior to the development of long necks, it is evident that there was established a pattern of shells with siphuncles of short necks but with thick, commonly layered rings, the sort of structure generally developed in what is here called the suborder Ellesmeroceratina, though similar rings are found in the derived Endoceratida with edocones, and the derived exogastric Tarphyceratida, of simpler internal construction. But this is not all. Beginning earlier and disappearing earlier-the first form in the Franconian of the Upper Cambrian and the later ones appear in the Wanwanian of Manchuria-is a stock in which the siphuncle segments are strongly inflated between the necks, the Plectronoceratina of the present work. These forms, so oddly at variance with what one expected as the oldest cephalopods that they were considered aberrant or were discredited completely, are, surprisingly, the oldest cephalopods, and scrutiny of our objections to accepting them as primitive shows that such objections are perhaps not really valid.     The Ellesmeroceratida are a diverse lot. The older Plectronoceratidae with their siphuncular bulbs are in this respect so diverse from the Ellesmeroceratidae and their derivatives that they could be placed in a separate order. However, the shells are so closely similar in other respects that, without exceptionally well-preserved material, the two could be easily confused, and as a compromise, the two groups are recognized as suborders. To these are added a third suborder, the Cyrtocerinina, an odd, little-known group characterized by siphuncles in which the rings are swollen and extend as lobes into the cavity of the siphuncle. Such shells are a disparate lot, and were it possible to find some other possible origin form Cyrtocerina, which, though the best-known genus morphologically, seems the most isolated in form and in range, the group would be considered as polyphyletic.

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