Question

blue whale, dorudon, killer whale, ambulocetus, pakicetus, hippo, living in seawater, tail flukes, loss of external hind legs, carnivorous diet, underwater nursing

Explanation:

Since the problem (presumably related to evolutionary biology or phylogenetics of these species) isn't fully stated, but if we assume a question like "Which species share the 'tail flukes' trait?" or similar:

Looking at the phylogenetic tree, the node with "tail flukes" (blue node) connects Blue whale, Dorudon, and Killer whale. So these species share the tail flukes trait.

Answer:

Blue whale, Dorudon, Killer whale (species connected to the "tail flukes" node in the tree)

If the actual question is different (e.g., identifying a trait's distribution, evolutionary relationship), please clarify the specific question for a more accurate response.