Captured at newsgroup alt.animals.dolphins
From: firstname.lastname@example.org (Clark Dorman)
Subject: Re: Dolphin Evolution
I am trying to find information about dolphin and other aquatic mammal evolution. The question is how they got their blowholes where they are. I believe that the blowhole is actually a fusion of the two nostrils that has moved back over time. But, isn't the hole behind the brain, and if so, how did it go around the brain?
and got several great responses. Thanks, Peter and George. I also got the following from Stacy Braslau-Schneck and thought that it would be good for the group as well:
I can't post to alt.animals.dolphins (maybe you can forward this if you think others are interested) but I can answer some of your questions about cetacean blowholes. In toothed whales (including dolphins) the blowhole is a single opening for both nasal passages. If you look just inside you'll see the septum, the dividing tissue between the two nasal passages (just like what we have between our two nasal passages). In baleen whales there are two external openings--two blowholes.
In both groups, the blowholes are still in front of the brain. I know, the dolphin looks real brainy with that big "forehead" (called the melon) but that's all fat, literally--lipids that make up the sound-transmitter system. Their brain, sizable as it is, is safely encased within the skull behind the blowhole area (what many people would probably label the dolphin's "back").
The skull got its current shape through a process called "telescoping," named after the old-fashioned brass telescopes that could be pulled out or collapsed. The upper part of the dolphin's rostrum or muzzle area is roughly the equivalent to the upper lip area on our skulls; this bone has been enormously lengthened to allow the nostril openings to be as high on the head as possible. (The rest of the skull got changed in interesting ways; it's kind of tilted and compressed in various ways. This is partially due to this telescoping and partially due to other factors like the need to hear sound through water with a system that originally evolved to hear sound through air.)
A lot of cetacean evolution is not nearly as well understood, since many of the intervening fossils are lost beneath the ocean. A recent National Geographic issue (within the last year) had a good short article on recent findings.
Some other references (sorry I don't have anything more recent, though I know there are some):
Barnes, LG. 1984. Search for the first whale: Retracing the ancestry of cetaceans. Oceans 17: 20-23.
Mead, JG, 1975. Anatomy of the external nasal passages and facial complex in the Delphinidae (Mammalia: Cetacea). Smithsonian Contributions to Zoology 207: 1-72.
Hope this helps!
Cetacean Research & Education
Honolulu, HI, USA