QUOTE(Greengrass @ Jan 7 2008, 10:31 PM)

Not as currently defined by ???
I'm thinking gills, fins, lives in water, swims - a fish. Even identified as a member of the fish classification "Chrondrichthyes".
Modern taxonomy, which no longer is based on the Linnaean system (except in the case of "species," but not even universally with that), is based on common ancestry, not on physical characteristics. We distinguish between
definition and
diagnosis. The
definition of a taxon (which is any natural evolutionary group consisting of an ancestor and all of that ancestor's descendents) simply is a statement of who belongs in that group. So, for example, the most commonly accepted definition of "mammal" is all of the descendants of the last common ancestor of the three living groups of mammals (monotremes, marsupials and placentals) and all of that ancestor's descendants. Of course, when we look at an animal we cannot see it's ancestry, but only can infer it from it's physical (including its genetic) features. This is where
diagnosis comes in. The diagnosis of a mammal is fur, mammary glands, warm-bloodedness, a particular kind of jaw joint, and having three middle ear bones. These are not what makes a mammal a mammal, but merely are the features that allow us to identify a mammal as a mammal. If a mammal were to lose all of the diagnostic features of a mammal, it still would be defined as a mammal. A mammal
is a descendant of the last common ancestor of monotremes, marsupials and placentals; we
can tell whether something is a mammal by looking for fur, milk glands, middle ear bones, etc.
It's very similar to the way diagnosis is used in medicine. Influenza
is a disease caused by a particular virus. it is diagnosed by a set of symptoms that, in our experience, reliably are associated with the viral infection. But there is no necessary relationship between the definition and diagnosis. The symptoms themselves are not the disease, but only signs of the disease.
In the case of sharks and fishes, features like gills, fins, etc, all are diagnostic, not definitional. Moreover they are not good diagnostic features because they do not reliably separate one taxon from another. Sharks and trout both have fins, but trout are far more closely related to salamanders and birds and humans than they are to sharks.
So far, very little of this has worked its way into pre-college or even undergraduate science courses, and the vast majority of taxonomic information available on the web from general sites is archaic if not downright incorrect (although the amount of good information is growing. Berkeley's website-- www.ucmp.Berkeley.edu-- is excellent). It's annoying to those of us in the business that so many high schools and colleges still teach the Linnaean system (i.e., Kingdom, Phylum, Class, Order, Family, Genus), and confuse definition and diagnosis (which is a powerful logical distinction that can clarify thinking far beyond biology.