DRESSING UP FOR THE DANCE: PATTERNS OF SUTURE VARIATION IN SEXUAL DIMORPHS OF LOWER TOARCIAN HILDOCERATINE AMMONITES

Mesozoic ammonites frequently show evidence of sexual dimorphism. Adult microconchs (presumably male) are several times smaller than adult macroconchs, yet show evidence of mature modifications to the body chamber. Numerous studies have analyzed shell shape, ornamentation, body chamber, and apertural modifications in dimorphic pairs. Fewer analyses have considered the ontogenetic trajectories of dimorphs. Rarer still are studies focusing on differences in suture patterns between microconchs and macroconchs, in part due to the difficulty in quantifying disparities among these complex anatomical structures. However, studies of sutural differences between sexual dimorphs may provide evidence of the biological processes involved in septal formation and shell growth in ammonoids.

The Lower Jurassic subfamily Hildoceratinae is the earliest group of Ammonitina to show obvious sexual dimorphism. Five Lower Toarcian hildoceratine species belonging to two related lineages were included in the present study. Suture lines of macroconchs and microconchs were digitized from published figures and input into Geographic Information System (GIS) software to facilitate spatial analyses. Overall morphologies, areas of mismatch between pairs of specimens, and several measures of sutural complexity were calculated. All five species showed substantial differences between microconch and macroconch sutures. Microconch suture lengths are nearly identical across all five species, even though shell diameters of these specimens vary from 7.4 to14 mm and their shell shapes are not identical. Microconchs also show less sutural variation than macroconchs. The terminal suture length of one relatively small macroconch is 60% larger than that of similar-sized microconchs, suggesting that macroconchs have more complex sutures than microconchs at similar shell diameters. Hence, microconch septa are not identical to those of juvenile macroconchs of similar shell size, but show reduced complexity and variability. These results support the view that septal folding is related more closely to shell diameter than to shell shape, and suggest that microconchs may represent the physical limitations of septal building at a particular shell size while macroconchs were free to embellish on the basic septal Bauplan.