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Re: What exactly IS a dinosaur?

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....

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

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): Triceratops and Stegosaurus are more
closely related to each other than they are to Tyrannosaurus or Diplodocus:
their "family" is called "Onithischia."  And Tyrannosaurus and Diplodocus
are more closely related to each other, apparently, than either one is to
the Ornithischians: THEIR family is caulled "Saurischia."  And-- this is a
little bit surprising, but there's lot's of detailed anatomy that suggests
it-- those two "families" are more closely related to each other than either
is to... Just about anything else.  They are more closely related to each
other than either is to, say, mammals or Pterosaurs or Plesiosaurs or

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

So, what's a dinosaur: an animal that's more closely related to Saurischians
and Ornithischians than it is to other animals.  In genealogical tree terms,
something descended from the most recent ancestor of S's and O's (and so
descended from something that WASN'T an ancestor of any of the non-dinosaurs

But-- you could skip this if time is limited-- scientists for good and
sufficient reasons prefer pickier definitions, so officially it's an animal
descended from the most recent common ancestor of Megalosaurus (a sample S)
and Iguanodon (a sample O).  With a tree to look at, it's obvious that this
gets the same effect as the previous definition.

Then you can point out that birds are all descended from something that was
clearly a Saurischian -- imagine a miniature Tyrannosaurus with longer arms
and you won't be far off -- so this definition makes birds out to be
dinosaurs!  At which point there are two options: (i) say birds are
dinosaurs (and stop saying dinosaurs are extinct) or (ii) use "dinosaur" (as
the authors of "Dinosaurs - the textbook" after explaining all this in an
early chapter say they will for the rest of the book) to mean "non-Avian
dinosaur."  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.


Allen Hazen
Philosophy Department
University of Melbourne