Is There Variation In the Microstructure of Ammonite Aptychi?

Like modern cephalopods, ammonites are equipped with a large buccal apparatus (Meek and Hayden, 1865; Lehmann, 1971). Many studies have speculated about the mode of feeding of ammonites based on the size and shape of the jaws, in comparison with modern species (Hewitt et al. 1993; Michalik, 1996). Other studies have documented the microstructure of the calcitic valves (aptychi) that cover the lower jaws (Farinacci et al. 1976). However, most of these analyses have been restricted to Jurassic Ammonitina, and the microstructure of these aptychi is usually assumed to apply to all forms. We describe for the first time the microstructure of the lower jaws of the Ancycloceratina, as represented by Baculites, Polyptychoceras, and Jeletzkytes, based on well preserved material from the Campanian and Maastrichtian of the U.S. Western Interior and Hokkaido, Japan. The lower jaws are in situ, that is, they occur inside the living chambers of the ammonites. The lower jaws of these species consist of an inner chitinous layer with a ridge along the midline. The outer layer consists of a pair of calcareous valves (the aptychus). All of these aptychi are thinner than those in Jurassic forms, such as Laevaptychus and Punctaptychus. In addition, the calcite in the Cretaceous aptychi occurs in regular layers whereas in Jurassic aptychi, most of the calcite occurs in a sponge like structure. These differences in microstructure may reflect an evolutionary trend. Moreover, these features may provide additional characters to use in reconstructing the phylogeny of the Ammonoidea.