Denton (1974) recognized the possible importance of decoupling in fossil nautiloids. Endocerids show obvious evidence of decoupling adaptations through the use of long septal necks, in a manner similar to that of
Spirula. The very common Tertiary nautiloid
Aturia showed a similar adaptation, but in a planispiral shell similar to that of
Nautilus, rather than in a straight shell as in the Paleozoic endocerids. The long septal necks of
Aturia would effectively produce decoupling at even lower percent emptying volumes than in
Nautilus (Ward 1980) (Fig 7.17).
Aturia evolved at the end of the Cretaceous Period, and became the most diverse nautiloid genus of the Cenozoic. Fossil
Aturia can be found from Cenozoic marine deposits on most continents. In contrast to the centrally localted siphuncle of most nautiloids, and the outer marginal position of the siphuncle in most ammonoids, the siphuncle in
Aturia is dorsal in position on the whorl, or placed against the inside shell wall. In this characteristic it resembles the clymeniid ammonoids of the Paleozoic, the endoceratid nautiloids of teh Paleozoic, and modern
Spirula. The living position of
Aturia was such that the last formed chamber would be in a position similar to that of
Nautilus, with connecting rings of the siphuncle oriented vertically. Well-preserved specimens of
Aturia from the Pacific Coast of North America sometimes still preserve the original aragonitic composition of the shell and shell structures. From these specimens detailed ultrastructural studies can be made (Fig 7.18). The interior of the connecting rings show large fields of aragonitic needles, that would serve as fluid reserves. The long neck in neck structures of the siphuncles would cause decoupling to occur immediately after chamber formation.
Aturia may have been among the most active and deeply living of the nautiloids, judging from the shell's large size, excellent streamlining, and siphuncle configurations.
Other examples of decoupling strategies include siphuncle migration in heteromorphic ammonites (Ward 1979, Klinger 1980) (Fig. 7.19). Most ammonites show no obvious mechanical adaptations to produce cameral liquid decoupling, although the position of the siphuncle in most ammonoids, against the outer margin of the whorl, would produce decoupling automaticall in those chambers situated high up on the whorl. Conversely, chambers located at the bottom of the whorl would always have liquid in contact with the siphuncle as long as liquid was present within the chamber.
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Originally Posted by BigSquid8Me
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