ANALYSIS OF THE EXPANSION RATE AND SEPTAL SPACING DURING THE ONTOGENY OF THE LOWER MISSISSIPPIAN AMMONOID MUENSTEROCERAS PARALLELUM FROM ROCKFORD, INDIANA

Theoretical studies of coiled invertebrates, particularly gastropods and cephalopods, by Raup (1966-67) demonstrated that idealized cephalopod shells can be described by changes in three parameters: whorl expansion rate, aperture shape, and umbilical diameter. In his model for an idealized planispiral shell, Raup assumed that these three parameters remained constant through ontogeny. Large populations of the genus Muensteroceras Hyatt are available from the famous Lower Mississippian ammonoid locality at Rockford, Indiana. Presumably mature individuals of M. parallelum, the most common taxon at Rockford, were sectioned in the dorsal-ventral plane to test Raup’s assumption and to examine concurrent changes in septal spacing. No systematic changes in whorl expansion rate were observed during the ontogeny of sectioned specimens. Interseptal angle, a measure of septal spacing, declines consistently throughout ontogeny, in contrast to a constant interseptal angle observed by Ward (1987) in Nautilus. Septal crowding has been observed in both nautiloids and ammonoids, and has been regarded generally as heralding the onset of sexual maturity. Indeed, approximated septa in Nautilus, which has determinate growth, are observed commonly in the conchs of mature individuals. Septal crowding occurs in the Rockford Muensteroceras. This study suggests that a reduction in septal spacing occurs throughout the ontogeny of all individuals. Approximated septa at maximum diameter were not observed.