As an extension to my earlier question/observation about the 8-arms-2-tentacles aspect of spirula being at odds with the spiral shell, I also note that Nixon & Young report that slow and awkward little spirula
has a giant fiber system related to that of other decapods, which would seem to suggest that fast squiddiness evolved before the loss of the coiled, chambered shell's buoyancy mechanism. The siphuncle in spirula
is also positioned ventrally along the inside of the spiral, rather than centrally as in Nautilus, so I'm pretty sure that this is the only living ceph with this arrangement. (see the green picture at http://tolweb.org/Spirula_spirula/19989
I'm sure Kevin and Phil can list off the various ammonids and nautiloids that share this arrangement, but modern nautilus has a central siphuncle. (as an aside, I was surprised that I couldn't find much in siphuncle location in the otherwise brilliant "fossils and history" articles... perhaps someone with a more encyclopedic knowledge of such things could remedy that? or perhaps my article searching skills have failed me and someone will point out where I missed it...)
As a starter, and a hat tip memorial to Sir Denton, I'll transcribe a bit from Ward The Natural History of Nautilus
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.
Figure 7.19 shows a "tortonic heteromorphic ammonite" which has a siphuncle placed strategically to always be above the fluid level. The "decoupling" mentioned above means that the gas/fluid ratio in a chamber can be changed much more easily when the siphuncle's opening is not under the cameral liquid. To reduce confusion, spirula's orientation of the shell is the opposite of Nautilus, in that the last opening faces straight down rather than horizontally, so there is a little confusion about dorsal vs ventral, but the unusual configuration Spirula
has is that the siphuncle is on the inside of the spiral, not the center as in Nautilus
or the outside as in many ammonoids.
edit: I neglected to mention that this post was inspired by a discussion with hallucigenia, who got me thinking about spirula again and helped me past some of my paleo-ignorance, as well as having a "find links, look in books, and discuss results" IM session (much like this