EVOLUTIONARY STASIS IN AMMONOID SUTURES ISOLATING THE LATERAL LOBE

Ammonoid suture complexity is commonly measured using fractal dimension. Generally, an entire hemisuture is used, and it is measured using finite units of length. Lutz and Boyajian introduced size-independent metrics. “Conversion values” can be added to the fractal equation to extrapolate an appropriate fractal dimension for the whole hemisuture from just the lateral lobe and one adjacent saddle (LLS). LLS is useful in plotting the ontogenetic sequence of a given ammonoid’s septal complexity, as it is unaffected by whorl overlap. Gould indicated that all biological systems increase complexity over generations, before which point, a “left wall of minimal complexity” acts as the threshold for a basal level of complexity required for life to exist. Conversion values may have taxonomic value and communicate morphometric qualities of the suture’s general geometry, as they are consistent at the level of individual genera of Jurassic and Cretaceous ammonites. However, septal complexity varies directly with ammonoid biodiversity: the most primitive ammonoids (in order Agoniatitida (Average Df 1.092)) have no statistically significant difference between converted and unconverted LLS data, but other Paleozoic ammonoids (in orders Goniatitida (1.210), Prolecanitida (1.243) , and Clymeniida (1.203)) exhibit conversion values that are consistent at the level of order. Paleozoic ammonoids exhibit static conversion values for over 100 million years, until the appearance of Ceratitida. Ceratitids were the first order to exhibit fluting, or infolding of individual saddles and lobes into smaller sections. This served as a key adaptive shift which coincided with dramatically thinner shells and greater septal complexity thought to afford them greater capillary control over cameral fluids. As complexity increases, periods of stasis shorten. Ceratitida (1.282) and Ammonitida (1.673) have conversion values that are only consistent at the level of genus.