Final answer:
Dorudon is more closely related to the modern blue whale than to Ambulocetus. Dorudon and the blue whale are both cetaceans with a common ancestry alongside other even-toed ungulates. Ambulocetus represents an earlier, less derived stage of cetacean evolution.
Step-by-step explanation:
Dorudon, an extinct genus of ancient whales, is more closely related to the modern blue whale than to Ambulocetus. Both Dorudon and the blue whale fall under the classification of cetaceans. The phylogenetic tree shows that cetaceans, which includes both toothed whales and baleen whales, are descended from even-toed ungulates and share a common ancestry with animals such as hippopotamuses.
Ambulocetus, known as a 'walking whale,' represents an earlier stage of whale evolution, showing adaptations for a semi-aquatic lifestyle, but it is not directly ancestral to Dorudon or modern baleen whales such as the blue whale.
Additional evidence suggesting the relationship between Dorudon and modern cetaceans comes from shared characteristics, such as the anatomical structure of the inner ear and similarities in the skull and teeth. The discovery of a fossil species such as Rodhocetus, which exhibits a type of ankle bone unique to a group of land mammals, also supports the terrestrial ancestry of whales. However, Rodhocetus is more closely related to Dorudon and modern whales than Ambulocetus.