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




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