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Re: What exactly IS a dinosaur?

On Tue, Apr 15, 2008 at 1:00 AM, Allen Hazen <allenph@unimelb.edu.au> wrote:
> The question was asked in order to prepare a talk (or poster?) for
>  non-specialists: high school students.  Somehow I don't think too much
>  knowledge of the details of the ICZN and phylogenetic nomenclature ought to
>  be presupposed....

I explained that I was answering the question in full; obviously it
would have to be boiled down.

>  So what ***I'd*** say is something like this:
>  You know some examples of dinosaurs: T. rex, Diplodocus, Triceratops,
>  Stegosaurus, etc.  They were all RELATED: in the same way that Tigers and
>  Lions and Lynxes and Tabby-cats are more closely related to each other than
>  any of them are to Aardvarks or Bears or Cows, your dinosaurs are more
>  closely related to each other than any of them are to Lizards or Snakes or
>  Frogs or People.  And "related" here means genealogy: they are all descended
>  from a common ancestor that the non-dinosaurs on the list aren't descended
>  from.

Not a bad boiling down.

>  Now, it turns out that the dinosaurs on the list can be grouped into two
>  main "families" (that used to be called "orders," and still are by some
>  people though not by Mike Keesey):

Certainly not.

Using the word "family" here could be misleading, given the Linnaean
rank of the same name. It would be best to use the word "clade", after
explaining what it means (an ancestor and all of its descendants). Of
course, there may not be time for this, given the general nature of
the talk.

>  (All this is most easily explained by pointing to branches and nodes (node
>  being technospeak for "branchpoint") of a genealogical tree.

Yes, visuals will be important, certainly.

>  As Mike Keesey says, phylogenetic taxonomy allows for the
>  definition of groups that aren't clades, and (ii) amounts to defining
>  dinosaurs-in-the-sense-we-will-for-practical-purposes-use-the-word as a
>  simple Boolean compound of clades.

I'm strongly against the idea of giving a name ("dinosaur") two
definitions, though. That's a recipe for confusion. Decide which way
to use the term, and then don't deviate from it. (And, preferably,
decide to use the clade meaning :)

Incidentally, the closest actually-scientifically-interesting and
semi-named paraphyletic group that I can think of to "traditional
Dinosauria" is the avian stem group (i.e., the avian total group,
including dinosaurs, _Marasuchus_, ?pterosaurs, etc., minus the avian
crown group).

T. Michael Keesey
Director of Technology
Exopolis, Inc.
2894 Rowena Avenue Ste. B
Los Angeles, California 90039