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FIGURE 1. A. Prey size of baleen whales modified from Gaskin (1982) with prey information in Jefferson et al. (2008). B. Modern baleen whale phylogeny and information of prey types + prey capture tactics. Phylogeny was combined the tree of the Balaenopteridae in Rosel et al. (2021) and the tree in Steeman et al. (2009) for the relationships of others.






FIGURE 2. Example semi-landmark in the ventral view of the basihyal and thyrohyal with anatomical terms. The one of Balaenoptera musculus number 66 in Table 1 is used. The origins for the muscles were modified from Schulte (1916) with minor modification following Reidenberg and Laitman (1994) on the omohyoid muscle insertion.






FIGURE 3. Outlines of analyzed true baleen whale specimens. Numbers are given in Table 1 and Appendix 1. Abbreviations mean prey capture tactics (Sk: Skim, Mu: Multiple, Lu: Lunge) and prey types (Sm: Small, Bo: both large and small prey, La: Large, Un: Unknown).





FIGURE 4. The results of principal component analysis. Ovals represent 90% confidence intervals for prey types of the extant taxa. Diagrams of the shape changes in the positive directions are given along each axis. Numbers and letters are IDs and abbreviations of scientific names (see Table 2).






FIGURE 5. Same results as in Figure 4 with 90% confidence intervals for combinations of prey capture tactics and prey types. Numbers and letters are IDs and abbreviations of scientific names (see Table 2).





FIGURE 6. Morphological traits of the basihyal and thyrohyal among the Chaeomysticeti separated by prey types. Boxes gray in colour are extinct baleen whales, which ID number 4, 8, 11, 33, 40, 53, 61, 70 in Table 2 are used here. Piscobalaena nana shows two different types of phylogenetic hypotheses (see in cladogram section).