RE: On the subject of mysterious absences...The Answer

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Taylor wrote:


>> So you would lump together _Euparkeria_, proterosuchians,
>> erythrosuchians, phytosaurs, aetosaurs, rauisuchians,
>> non-crocodylian crocodylomorphs, basal ornithodirans
>> ("lagosuchians", "silesaurs"), and even basal dinosaurs (if
>> _Eoraptor_, _Herrerasaurus &c lie outside both Saurischia and
>> Ornithischia)...?
>
> Yes ... when and only when that was the group of animals that I wanted
> to talk about, i.e. the "ancestral stock". It's pointless to pretend
> that we don't need to talk about these animals 


I don't think we need to talk about these "ancestral stocks".  I'm not sure 
that these "ancestral stocks" really exist.  Whether it's Thecodontia, 
Condylarthra, Proteutheria, Labyrinthodontia, Apterygota, or even Protozoa - 
all these 'wastebasket' groups were just created for the sole purpose of 
lumping together taxa that were considered "primitive" by comparison with 
certain "advanced" groups (= the presumed descendents).  Such groups have 
proven very unhelpful for elucidating relationships.  They're all unnatural 
groupings, and so don't deserve to be maintained.


For the "thecodonts", I think it's best to talk about these animals as 
individual clades (proterosuchians, phytosaurs, sphenosuchians, etc), rather 
than as a "whole".


> -- the very existence of the term "thecodont" shows that it's needed.


I would put it another way.  The fact that the term "thecodont" has pretty much 
been abandoned for the past 20 years or so shows that it is not needed at all.  
The term has surfaced a few times - mostly by the BANDits, who use it to gloss 
over the awkward question of which particular archosaur(iform?) they 
hypothesize to be the sister taxon to birds.  The "Thecodontia" is sufficiently 
vague and so disparate in content that it's a convenient term to hide behind.


I don't want to pick on "Thecodontia".  Other time-honored paraphyletic groups 
are equally problematic.  Many vertebrate paleontologists who specialize in 
mammals hold a deep and abiding dislike toward the term "condylarth".  I've met 
entomologists who wish that "Apterygota" would crawl away and die.  The word 
"Protozoa" will not pass the lips of any self-respecting microbiologist.   And 
so on.  It's not just the historical baggage that these groups carry that 
evokes such misgivings; it's the fact that these paraphyletic terms can be very 
misleading.


Such paraphyletic taxa might be erected (or resurrected) with the best of 
intentions; but I suspect they will take on a life of their own and cause more 
trouble than what they're worth.  Your idea sounds reasonable in theory, but in 
practice it would lead to all sorts of problems if the rather esoteric (to 
non-experts) distinction is lost between "paraphyletic" and "monophyletic".  
Under the current system, only monophyletic groups (clades) are approved, so 
there's no scope for this kind of confusion.


> I surely don't need to tell you that I'm not recommending that this
> name be used _instead_ of Archosauromorpha or any of the other names
> that we also find useful.


Several years ago the clade Thecodontia appeared, and was defined as 
_Protorosaurus_ + _Thecodontosaurus_.  This would effectively make it a junior 
synonym of Archosauromorpha.


Kischlat, E. E. (2000 or 2002 - I've seen both).  Tecodoncios: a aurora dos 
arcossaurios Tríassico. Pp. 273–316. In: Holz, M. & De Ros, L. F. (eds) 
Paleontologia do Rio. Grande do Sul. 273-316.


Thecodontia was one of several unhelpful clades to be defined in this paper.
 



Cheers

Tim
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