Re: Stormbergia dangershoeki, new Early Jurassic ornithischian from South Africa

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Taylor wrote:

> Is it just me, or does anyone else feel uncomfortable about the idea that, 
> once the PhyloCode is implemented, there will be a much stronger tendency 
> to respect strict priority in the definitions of clades, so that we don't 
> have the kinds of options that Mike and Tim are arguing about here?

No, it's not just you.  Having definitions set in stone, with the first 
definition having priority, could (and will) lead to all sorts of trouble.  
There needs to be some wiggle-room.

Let's look at Herrerasauridae.  Not everybody agrees that _Staurikosaurus_ 
and _Herrerasaurus_ form a clade to the exclusion of (other) theropods and 
sauropodomorphs.  (Okay, Langer and others present a strong case that these 
two guys are closely related, but who knows what future phylogenies may 
reveal, 10 or 20 years from now?)  If the Herrerasauridae is defined as a 
node-based clade containing _Staurikosaurus_ and _Herrerasaurus_ and all 
descendents of their most recent common ancestor - what happens if 
_Staurikosaurus_ and _Herrerasaurus_ are found to be paraphyletic (or even 
polyphyletic)?  As Mickey says, if _Staurikosaurus_ is found to be a basal 
sauropodomorph (as Kischlat found), then Herrerasauridae expands to include 
the entire Saurischia or Eusaurischia (depending on the position of 
_Herrerasaurus_).  Nobody wants that.

Thus, a small shift in the position of _Staurikosaurus_ relative to 
_Herrerasaurus_ can dramatically change the content of Herrerasauridae.  As 
I see it, there are two ways to remedy this:

(a) Allow the Herrerasauridae to be re-defined.  In the above example 
(_Staurikosaurus_ is a basal sauropodomorph), _Staurikosaurus_ would be 
removed as as one of the specifiers (i.e., replace it with another genus).

(b) Include a qualifier in the definition of the Herrerasauridae.  In the 
above example, the definition could be emended to "All descendents of the 
most recent common ancestor of_Staurikosaurus_ and _Herrerasaurus_, but not 
_Passer_ or _Apatosaurus_."  This way, the Herrerasauridae self-destructs 
rather than swallow up most or all of the Saurischia.

(a) is ad hoc, whereas (b) is more pro-active.  However, PhyloCode may not 
allow clades to be re-defined, so (b) may be the only solution.

Cheers

Tim