DIMORPHISM IN CARBONIFEROUS AMMONOIDS FROM THE SOUTHERN MIDCONTINENT, UNITED STATES

Sexually mature individuals of the only living ectocochliate cephalopod, Nautilus Linné, can be easily distinguished by conch width: the conch is wider in males because of the spadix, a sex organ. Apparently sexually dimorphic pairs (antidimorphs) in many extinct ammonoid cephalopods differ in conch dimensions, shape, and ornament; the microconch-macroconch antidimorphs of most Mesozoic ammonoids are particularly striking. In Paleozoic taxa, antidimorphic pairs are characterized by subglobose conchs with large umbilicus and depressed whorls that occur with subdiscoidal forms with a narrow umbilicus and compressed whorls. There is no significant difference in suture configuration between antidimorphs, and early ontogenetic stages appear to exhibit identical patterns of change in conch proportions.

Before its destruction, the type section of the Rockford Limestone, (Early Mississippian - Osagean), southeastern Indiana, yielded abundant ammonoid specimens of several species, including Muensteroceras oweni (Hall), and M. parallelum (Hall). The two species of Muensteroceras at this locality differ only slightly in suture pattern, while the conch of M. parallelum is relatively compressed with a narrow umbilicus, compared to M. oweni with a wider, more openly umbilicate conch. These characters suggest that M. oweni and M. parallelum represent a single dimorphic species.

To test this hypothesis, conch dimensions (maximum diameter, whorl width, whorl height, and umbilical diameter) in Muensteroceras oweni and M. parallelum were measured and compared to the Carboniferous antidimorphic ammonoid Arkanites relictus (Quinn, McCaleb and Webb. Dimorphism in each measurement, estimated by the ‘mean method’ of Plavcan (2004), is expressed as ln(mean subglobose)-ln(mean subdiscoidal) (see table below). M. oweni/M. parallelum displays less pronounced dimorphism than A. relictus, supporting the hypothesis that the former two forms are antidimorphs of a single species. If analogy to the dimorphism in Nautilus applies, the subglobose forms (M. oweni) are male; the subdiscoidal forms (M. parallelum) are female.

W/D H/D U/D

M. oweni/M.parallelum 0.25 -0.24 0.58

Arkanites r. relictus 0.24 -0.39 0.63

A. r. redivivus 0.24 -0.42 0.60