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po8crg comments on Where to Draw the Boundary? - Less Wrong

36 Post author: Eliezer_Yudkowsky 21 February 2008 07:14PM

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Comment author: po8crg 24 March 2012 06:37:30PM 2 points [-]

Fish, like reptiles are paraphyletic. The cladistic revolutionaries want to abolish the category altogether, or reduce it to just the ray-finned fishes - excluding coelacanths, lungfish, the cartilaginous fish (sharks, rays, skates and chimaeras), and the cyclostomes (hagfish and lampreys).

The result is that some sources will use fish as equivalent to the monophyletic group actinopterygii and others use the traditional polyphyletic pisces. Anytime you see a generalisation about fish that isn't true of sharks, there's a good chance that the original source was using fish to mean actinopterygii.

In many ways, it's a more useful classification - 96% of fish species are in actinopterygii, and there is an awful lot of anatomy that is shared by the actinopterygii but not by the rest of the fish. If you're going to exclude cetaceans because they have more in common with land animals than with actinopterygii then why not exclude lungfish and coelacanths for the same reason?