Abundance of chondrichthyan genera through time

As of September 2019, the Florida Museum of Natural History (FLMNH) had a
curated collection of 117,449 chondrichthyan specimens from Florida,
spanning the Eocene through the Pleistocene. Herein, I evaluated the
completeness of the chondrichthyan fossil record from Florida based on the
FLMNH collection, while analyzing trends in taxonomic and ecomorphological
diversity. At least 70 chondrichthyan taxa were recognized in the FLMNH
collection, representing 10 orders, 26 families, and 42 genera; of which,
20 taxa represent first occurrences from Florida. A sample of 107,698
specimens was organized into 12 time bins to analyze taxonomic and
ecomorphological diversity, with an expectation that diversity trends
would correspond with global climate events (e.g., the Eocene-Oligocene
Transition and the Middle Miocene Climatic Optimum). However, diversity
trends were obscured by pervasive sampling bias, attributable to variable
collection methods, research prioritizations, and regional lithologic
controls. Sampling is particularly poor for smaller specimens and older
geologic units (i.e., the Paleogene). Despite incomplete sampling of the
Florida chondrichthyan fossil record, there was an apparent turnover along
the Atlantic and Gulf coastal plains from a lamniform to carcharhiniform
dominated chondrichthyan fauna that occurred during the Eocene. This
turnover corresponded with the extinction of many lamniform taxa with
grasping-dominated dentition types (e.g., Brachycarcharias, Jaekelotodus,
and Macrorhizodus). Selachian taxa that survived the late Eocene
extinctions were predominantly represented by cutting-dominant dentition
types. Since cutting aids in the dismemberment of prey, this may reflect a
macroevolutionary trend among selachians towards active predation and
scavenging on larger prey, such as marine mammals.