Chambered Cephalopod Shells, Buoyancy, Structure, and Decoupling: History and Red Herrings
The Paleontological Society Special Publications
Hooke articulated the first vision ()f the buoyancy function performed by the chambered shells of cephalopods. In the 17th century, he argued that the shell "vent up and down in the water c()]umn a,; pressurized gas filled and emptied the cham-bers. Although some prescient 19th and early 20th century workers took exception tt1 this view, it was not until the 19608 that Hooke's gas-pressure mechanism was sho\\/n to be . incorrect. Experimental "'Tork by Denton and Gilpin-Bro"vn delnonstraled
... chambered cephalopod shells are emptied osmotically, generating a partial vacuum in the chambers. Hooke's argument, that cephalopods changed their buoyancy on a daily basis, is not likely to have been true either. Ward (1982) argued cogently that the rate of fluid removal from the chambers, and hence the rate of buoyancy change, is proportional to siphuncular area. In ammonoids the siphuncular area is too small to support rapid change in buoyancy. Thus, ammonoids probably maintained neutral buo}'ancy throughout life. Despite the evidence for an osmotic pump and lo\v pressures in the chambers, text books continue to refer to gas pressure in cephalopod shells. Decoupling, the separation of fluid in the chambers from the siphuncle, ""as thought to shut off the osmotic pump and allow cephalopods to descend below 240m, the osmotic pressure difference between fresh and salt water, without flooding of the chambers. Denton and Gilpin-Bro\vn advanced this concept of decoupling because they did not known that the siphuncle could concentrate salts permitting pumping \veII beyond the osmotic difference between fresh and salt "vater. It is now clear that the decoupling explanation was not necessary, it would not have shut off the osmotic pump in the fashion envisioned, and there is no evidence of decoupled \vater in modem cephalopods. Despite the absence of an)t supporting evidence, workers continue to invoke decoupling to explain aspects of ammonoid morphology that they find inexplicable on other functional grounds. As a consequence of the absence of press~rized gas in the chambers, the chambered cephalopod shell must support large hydrostatic forces when the animal is submerged. Workers aware of this structural problem have attempted to explain cephalopod shell fonn in the context of this hydrostatic load. However, two distinct hydrostatic loading conditions occur sequentially in the chamber fannatian cycle. Forces operate through the lxxl)l chamber generating a h:ydrostatic load on the surface of the last fonned septum a phenomenon discussed by Pfaff in 1911. The other septa do not support a load on their surface, but are thought to support loads transmitted from the phragmoc()nc wall an issue associated ,vith Spath's 1919 \vork. Consequently, the fonn of septa and sutures in cephalopods cannot be an optinlal solution to single mechanical function, but must successfully satisfy these two distinct loading conditions, as \vell as other selective, anti hist()rical constraints on shell form. In 1836 Buckland argued that support of hydrostatic load required a functional relationship bet\\t'een sutural complexity and shell shape. SUbseqllently, it has been noted that sutural complexity evolves, t1ften iteratively, in response to changes in shell shape. Despite these previous observations it has recently been popular to interpret the function of sutural complexity in terms of a single number without considerdti()n of, the shape of the shell in question, the distribution of complexity along the sllture, or the placement of the taxa in an evolutionary context. Such approaches are not likely to resolve issues pertaining to the evolution of sutural function.