Size and function of ammonite aptychi in comparison with buccal masses of modern cephalopods

Morton, Nicol & Nixon, Marion 1987 07 15: Size and function of ammonite aptychi in comparison with buccal masses of modem cephalopods.
Previous impressions that the size of ammonite aptychi is unusually large, thereby posing a problem to their interpretation as part of the jaw apparatus, are shown to be incorrect. The relative length of the aptychus for at least the Jurassic ammonite groups studied, at approximately 15% of the length of the body chamber, is remarkably constant in different taxonomic groups and sizes. This is well within the range for buccal mass length as percentage of mantle length of living cephalopods, being most similar to Octopus and Sepia and much smaller than Nautilus. The height and width of aptychi relative to whorl height and width are larger but again ammonites are probably not significantly different from modern cephalopods. The size and design, with no obvious structures for biting or crushing, suggest that ammonites were adapted to a particular type of relatively unspecialised feeding in which mostly small animals were ingested, possibly with some external digestion. The large shovel-like lower jaw may have functioned like a scoop for collecting large quantities of water and small prey, and movement of the buccal complex with the upper jaw almost closed against the lower jaw could have expelled water while retaining captured prey. Calcification of aptychi may have been protective, but more likely acted to weight the buccal mass for nektobenthic feeding and to make it more rigid.