This was mentioned on another thread, but there was a recent National Geographic special on lake monsters/sea serpents and it showed a photograph of one of these basilosaurus type, maned sea serpents that had been recovered from the stomach of a sperm whale, (but regrettably not preserved), so I would have to agree with Paulwhale that the Zeuglodon would be the loser in a life and death struggle with a Sperm whale.
Sperm whales could easily kill another large whale or shark, but normally wouldn't, for they can only swallow whole prey because of the lack of upper teeth to tear it apart.
However, a Sea Serpent/zeuglodon creature, giant squid, and sharks at least 4 meters in length could be swallowed whole and are, as sperm whale stomachs reveal. Perhaps this is why "Sea Serpents" are so rare!
It is not so much the case that Sperm Whales would kill Meg Sharks in battle, but that they would outcompete them in the hunt for gaint squid, which was probably the principle prey of both creatures. If it came to a fight though, a sperm whale could probably fit the entire Meg Shark's head in its jaws and crush it. Normally large predators simply avoid each other, not risking injury in pointless combat. Megs would gradually disappear if Sperm whales ate most of their food. This seems to be what happened.
I don't know where anyone got the idea that the zeuglodon was the largest whale, unless it was the false assumption that anything "prehistoric" must be "bigger" than today. This, of course, is not true, and the biggest living creature ever recorded, the blue whale, is still with us today, though it is now possibly being rivaled by a huge, newly discovered Ichthyosaur called the Shonisaurus, a creature that probably was the "Sperm Whale" of its day, in the ecological niche of chief giant squid predator. With its extinction, along with the other great sea reptiles, the Meg attained supremacy until the arrival of the Sperm Whale.
Edited by draconic chronicler, 05 October 2005 - 01:14 PM.