Saltations In Nautiloid Coiling

This paper is devoted to a critical re-examination of an old and almost a traditional concept which is usually encountered in elementary courses in paleontology. It has long been postulated that the evolution of nautiloids was characterized by a gradual and progressive change from straight shells, orthocones, to slightly curved ones, cyrtocones, loosely coiled gyrocones, and finally to nautilicones, in which the whorls are in contact or may be so closely coiled that the outer whorl embraces and may conceal the inn-er ones from external view. Dunbar (1924) elaborated this concept and correlated the changes in form with changes in the mode of life largely connected with varying buoyancy of the phragmocone. It was believed that the straight cephalopods held their shells horizontally in life, a matter attested by the known concentration of color bands on the dorsal side of the shell in quite a number of forms. When gas appeared in the camerae, it tended to lift the apex of the shell, and a natural response was curvature and eventually coiling. Uncoiling, in the Lituitidae, was suggested as an adaptation to an essentially planktonic existence. Asymmetrical curvature, in the shells called torticones or, later, trochoceroids, indicated a return of coiled shells to a benthonic mode of life. There were recognized examples of both progressive coiling and of uncoiling. The ammonites supply two excellent examples of uncoiling in the Triassic Choristoceratidae and again in the Cretaceous Baculitidae. In the last ten years there has developed some controversy as to whether certain stocks represent coiling or uncoiling. The Lituitidae, long regarded as a stock which is uncoiling (Hyatt, 1900), was regarded by Schindewolf (1937, 1942) as a stock which is coiling, the coiling appearing first only in early growth stages. Controversy still surrounds the Bactritidae, alternately regarded as primitive straight Ammonoidea, secondarily straight ammonoids developed from coiled ancestors, and homeomorphs of the ammonoids developed from straight nautiloids. These general but very interesting concepts were based upon concepts of evolution and classification of the nautiloids beyond which we have now progressed materially. Seventy-five years ago the nautiloids were classed in a few relatively simple form genera, Orthoceras, Cyrtoceras, Gyroceras, Nautilus, Trochoceras and Gorphoceras. Hyatt (1884, 1894, 1900) questioned whether this was not an oversimplification; whether there was not great homeomorphy in parallel stocks which could be differentiated by internal structures rather than external form. He developed a classification (Hyatt 1900) in which the major divisions of the nautiloids were based primarily on the form of the segments of the siphuncle, and in particular, the conformation of the septal necks. On this basis were distinguished three major and two minor orders. It was recognized that form changes were characterized by various stocks within these orders. This was an important step forward, and supplied a basis for paleontological thought which persisted for about forty years. Further investigations, however, showed that the structural types which characterized the ordinal groups of Hyatt were not monophyletic. The long septal necks characterizing the order Holochoanites appeared in only part of the