| Summary: | Baleen whales (Cetacea: Mysticeti) are the largest animals to have ever lived on Earth; thus, they play critical ecological roles and have important implications for understanding evolution. In this thesis, I consider their phylogenetic patterns and evolutionary processes based on extant mysticete species; describe three Oligocene mysticetes (26–28 Ma) from New Zealand; and discuss some problematic issues (ancestor-descendant relationships, emergence of the living baleen whales, and character selection in phylogeny) in baleen whale evolution. Developmental trajectories reveal that disparate heterochronic processes occur in different baleen whale lineages: paedomorphic neoteny in the pygmy right whales, Caperea marginata and peramorphic acceleration in balaenopterids (Balaenoptera borealis and Megaptera novaeangliae). Juvenile specimens as separate operational taxonomic units show that juvenile morphology would result in tantalising phylogenetic patterns: juvenile Caperea marginata is closely placed with its adult morphology whereas juvenile Balaenoptera borealis is distantly related to its adult morphology. †Horopeta umarere, †Whakakai koroke, and †Rikoriko horapa (all newly described and named in this thesis) from New Zealand have ecological and evolutionary significance. †Horopeta umarere represents the earliest gulp feeder; †Whakakai koroke indicates the diverse niches in Oligocene mysticetes; and †Rikoriko horapa is the oldest described baleen-bearing Mysticeti. Their phylogenetic placements are problematic but in turn show potential roles revealing the emergence of the crown Mysticeti. Phylogenetic tests propose the first example of ancestor-descendant relationships with implications of punctuated equilibrium in Mysticeti: Miocaperea pulchra is the direct ancestor of the extant pygmy right whale, Caperea marginata. Philosophical consideration of character selection based on empirical experiments suggests that more morphological characters are not necessary giving better phylogenetic resolution in terms of reconstructing baleen whale phylogeny. Overall, this thesis flourishes our understanding and knowledge of baleen whale evolution both on the empirical and theoretical bases.
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