The chronology of mysticete diversification (Mammalia, Cetacea, Mysticeti): Body size, morphological evolution and global change

A comprehensive revision of the localities of discovery of fossil mysticetes is presented together with a highly inclusive phylogenetic analysis in order to provide the basis for a chronology of the main mysticete diversification events. The results suggest that the origin of Mysticeti (that include earlier toothed taxa together with today baleen-bearing cetaceans) occurred c. 38 Ma; the origin of Chaeomysticeti (that include only baleenbearing cetaceans) occurred c. 28 Ma; the origin of Balaenomorpha (crown mysticetes) occurred c. 23.3 Ma. Additional chronological inferences are provided. Within this chronological framework, we analyzed diversity trends, origination and extinction patterns and body size evolution, and looked for eventual causal relationships between evolutionary processes, marine and terrestrial ecological turnovers and geodynamic events. We found four main diversification events corresponding to peaks in originations and, in a few cases, with the origin of different feeding strategies adopted by the differen mysticete families. We found that different mechanisms are correlated to specific diversification events and these include changes in temperature and ocean circulation patterns, nutrient availability in the water column and diatom abundance and diversity. Resuming, no single mechanism explains all the diversification events occurred during mysticete evolution; rather, we found that each diversification event was correlated to different combinations of biotic and abiotic factors, suggesting that this group experienced major adaptation processes to the changing paleoenvironments in the last 38 Ma.