Generic diversity and septal complexity in Cretaceous ammonoids. Effects of oceanic anoxic events on the ammonoid evolutionary dynamic

Abstract This paper analyzes the relationship between the number of genera of Cretaceous planiespiral ammonoids and their sutural complexity estimated by their fractal dimension. It is confirmed that the dynamics of generic diversity is associated with the appearance/disappearance of simple suture genera, which are not ascribed to a particular family. The maximum oscillations of generic diversity occur in the Aptian and Albian which are associated with large variations in the number of genera with simple sutures. From the Middle Campanian there is a continued loss of diversity until the end of the Maastrichtian linked to a net reduction of single suture genera. The direct relationship between the number of genera and the range of septal complexity in each substage indicates that different degrees of septal complexity are different ecomorphological strategies of adaptation to different niches. This result confirms that the specialist forms are those with simple sutures while the genera with complex septa are either generalist forms or are adapted to environments that do not undergo major fluctuations over geological time. The ammonoid generic diversity recorded dynamics is different from that of other marine invertebrates, reflecting paleobiological and/or taphonomic differences. The symmetrical distribution of septal complexity values points to the relative fidelity of the fossil record of Cretaceous ammonoids. The effect of seven oceanic anoxic events of the Cretaceous on the number of genera and septal complexity is analyzed, obtaining no statistically distinguishable effects of the oscillations of these variables with respect to those occurring in other intervals where such events are not recorded.