randman

Whatever. Clearly some animals have more than one raw of teeth, just as the guy said. Piranhas are fish, are they not?

As far as 7 rows, I am a little unclear as to how this was a step towards the evolution of today's species. Seems like an anamoly, which says very little evolutionary-wise. The fact few animals have mutiple rows of teeth would seem to substantiate that.

Valentine Pontifex

Traditionally a shark has been classified as a fish: in the class Pisces. Pisces is no longer considered a valid taxon. If anything, "fish" today is an informal taxon.

And the classifying evolutionary closeness ("a dog is more closely related to a fish than is a shark") is cladistic classification pure and simple. A snake is more related to a bird than a a turtle. Traditional classification cares more that certain changes have happend then about evolutionary "closeness" and thus allows a classification that would no cladist would accept. Certain crustaceans are more closely related to insects than to certain other crustraceans. And yet traditional classification recognizes crustaceans. Cladistics which only cares about how closely related things are by evolution does not recognize the crustaceans. Molecular evidence indicates that a hippo is more closely related to a whale then to either a deer or a pig. Traditional classifications will still keep whales in one order and puts pigs and deer in another.

The reason why tradional classification has such problems is that, unlike cladistic classification, it allows paraphyletic taxa. Cladistic taxa are always true clades: a common ancestor plus all of its descendants. Paraphyletic groups groups is a common ancestor and some of the clades descended from it. Reptiles are paraphyletic group since it makes up of all members of clade Amniota minus the avian and mammalian clades.

Question: do you consider lungfish or lobefish to be fish?

Please consider that they are more closely related to humans than to trout.

lpetrich

In more recent classifications, Pisces becomes a superclass, alongside of Tetrapoda (all the vertebrates with walking limbs and their aquatic descendants). In the classifications I've seen, Pisces has these classes as subtaxa:

Agnatha (jawless fish)
Placodermi (extinct early fish)
Chondrichthyes (cartilage fish -- sharks)
Osteichthyes (bony fish)

And I think that the traditional approach to taxonomy may be described as part-cladistic and part-phenetic (classification by degree of similarity); the phenetic part justifies its various paraphyletic taxa.