TRENDS IN AMMONITE SUTURE COMPLEXITY DURING THE TOARCIAN EXTINCTION

During the Toarcian time interval, a period of anoxia and high temperatures resulted in a mass extinction. These stressors should affect marine invertebrates by acting on their physiology. Because suture complexity may be related to metabolic rate in ammonites, they provide an opportunity to study the relationship between physiology and extinction selectivity. To assess this prediction, we measured suture complexity of 182 Pliensbachian-Toarcian ammonite species using fractal dimensions. Our results show that, on average, sutures were simpler during the early Toarcian, at the same time as anoxia and temperature stress. We also found more complex sutures are more sensitive to variations in the density of digitized points on the suture. Simpler sutures in the early Toarcian, possibly corresponding to lower metabolic rates, could suggest that anoxia was the most important factor in the extinction. However, ammonites with simpler sutures tend to occur after other extinctions, implying that physiology may not be the most important factor for survival and generalists, with simpler sutures, may instead be favored after extinctions. Finally, sensitivity in suture complexity to methodology suggests that caution should be used when interpreting taxonomic or temporal differences in suture complexity.