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Stem groups (was Re: Archosaur Origins...)



Dinogeorge Olshevsky writes:

> Now that we have a good enough handle
> on evolution, taxonomy, the fossil record, and so forth, we can with no
> difficulty expand all the high-level crown groups into stem groups. This
is
> the most natural way to extend the old Linnaean-style high-level
crown-group
> taxa to include fossil taxa.
>
> Specifically applying this method to the tetrapods, we get (for example):

[snipped; Olshevsky's new definitions for Aves, Crocodylia, Archosauria,
Lepidosauria, Diapsida, Chelonia, Reptilia, Mammalia, Amniota, Amphibia,
Tetrapoda, and Pisces]

> This method partitions the entire Tree of Life with no gaps. All fossils
> automatically have a group to which they most naturally belong.

Very true; that's the part I quite like about Olshevsky's system.  Makes
things a bit easier (from an "artistic" perspective at the very least).

> One good thing about this system is that it dispenses with all those
> contrived "Archosauriformes," "Dinosauriformes," "Eudinosauria,"
"Sauropida"
> and "Theropida" (not Theropsida) kinds of groups required if we fix
> nodes for the high-level groups somewhere within their inclusive stem
groups.
> We might find some use for these kinds of groups at lower levels in
taxonomy,
> but the high-level stem groups certainly should retain the
straightforward,
> more widely known, and least nit-picky names.

I'm not so sure about this part.  Yes, "Aves" is more straightforward,
widely known, and un-nit-picky than the current alternative
(Ornithosuchia?).  However, I think it is far less confusing (for both
scientists and the general public) to continue to use a term few
nonscientists have ever heard of, like Ornithosuchia, than it is to suddenly
start referring to all dinosaurs as birds, a term currently restricted (by
most people) to {_Archaeopteryx_ + Neornithes}  --  or, for the general
public, "things with feathers".

Same goes for Mammalia.  Yes, laymen have no idea what a synapsid is, but
they (and scientists) would be quite surprised to find that the term
"mammal", previously restricted to "furballs", now includes such animals as
_Dimetrodon_ (already mistakenly believed by many nonscientists to be a
dinosaur!).

My two drachmas.

-Grant Harding
amateur palaeontologist

Future home of the "Coelurosauria Cladogram" pages:
http://www.cyberus.ca/~sharding/grant