Coombs, Ellen Jane; (2021) Drivers of cetacean diversity: Evidence from the past and present. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In just 8-12 million years, cetaceans (whales, dolphins, and porpoises) underwent profound changes in adaptive zone. Their evolution from land-dwellers to aquatic inhabitants is an exemplar of macroevolutionary change. However, there has been little study of evolutionary dynamics that span their entire 50-million-year history. Using 3D geometric morphometrics and a rich dataset of 201 living and fossil species spanning Cetacea’s evolutionary history, I quantify cranial morphology and investigate shifts in evolutionary rates and disparity. I find three key waves of diversification throughout cetacean evolution. The first is in archaeocete (early whales) evolution as cetaceans evolved rapidly to fill a largely vacant aquatic niche. The second, in the mysticetes (baleen whales) and odontocetes (toothed whales) which diverged ~39-36 Mya and followed unique evolutionary pathways, facilitated by key innovations: echolocation in odontocetes and filter-feeding in mysticetes. The third wave, in the Miocene, is mostly an odontocete signal (~18-10 Mya). Further, I find asymmetry related to echolocation in odontocetes is driven by the pressures of acoustically complex environments, and that multiple ecological factors influence skull shape. I find climate fluctuations drive cranial evolution through deep-time. Importantly, ocean productivity drives evolutionary rates in mysticetes, whereas in odontocetes, these are driven by rates of temperature change. Finally, I switch from morphological to taxonomic diversity and investigate environmental and anthropogenic impacts on diversity in shallow-time, reinforcing the importance of long-term strandings data to monitor impacts. My results highlight the idiosyncrasies of species responses to environmental and anthropogenic impacts. Differences in diversity between suborders reflects their different early innovations and resultant ‘ecospace’ occupation. Importantly, this work highlights the differences in drivers behind mysticete and odontocete evolutionary rates, particularly with regards to climate change. The different historical responses of extant suborders highlight a requirement for separate, tailored conservation and mitigation of climate impacts for toothed and baleen whales.

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