RE: phyletic bracketing

["Tim Williams" <twilliams_alpha@hotmail.com>]

I will venture a thought on this.  Might it not be a good policy to invoke 
phyletic (or phylogenetic) bracketing (PB) for every character, up until the 
point that there is direct evidence to the contrary?  In other words, until 
we have hard evidence that a novel adaptation has appeared we do not assume 
that more basal taxa also had this adaptation?

This would apply to cheeks.  "Papp and Witmer (1998) attempted to debunk 
cheeks in ornithischians by noting the absence of such structures in crocs 
and in birds."  In this instance, I don't think PB can be used to justify 
the notion that ornithischians did *not* have cheeks because we have changes 
in jaw morphology (inset teeth, bone texture) that clearly deviate from the 
primitive condition (retained in crocs and most birds).  In fact, I would 
say that PB supports the assertion that *all* ornithischians above 
_Lesothosaurus_ had cheeks, simply because ankylosaurs and ornithopods show 
evidence of cheeks, and the most parsimonious interpretation is that their 
most immediate common ancestor did as well.  (AFAIK, the teeth of 
_Lesothosaurus_ are not inset, therefore there are no indicators of cheeks 
in this critter.)

Overall, I think PB is a safe 'default' setting for phylogenetic 
discussions.  With ornithischian cranial anatomy, I think we can say that 
the setting has changed.

Another example: feathers.  We know of many non-avian theropod taxa that 
show feather-like integument structures: alvarezsaurids, compsognathids, 
deinonychosaurs, oviraptorosaurs, therizinosauroids.  We can therefore 
confidently assert that feathers were most likely present in the most recent 
common ancestor of these taxa.  Thus feathers are primitive for the clade 
that includes these taxa (Maniraptoriformes).  This is PB.  However, as yet, 
there is no *direct* evidence of feathers in taxa outside of this clade - 
and to suggest that (for example) coelophysids or allosaurs also had 
feathers is to move outside the bracket.  This does not necessarily mean 
that these more basal taxa were *not* feathered - it means that we do not 
assume that a given lineage has a particular character until there is direct 
evidence saying that it does.

(I am aware that skin impressions are known for _Carnotaurus_ and 
_Tyrannosaurus_, and both show a rather leathery and tubercular texture.  
However, I don't believe this automatically *disproves* the existence of 
pre-maniraptoriform feathers.  Juveniles may have been feathered, and/or 
feathers may have been limited to the extremities in adults.  But, invoking 
PB, this is just speculation until we have direct fossil proof.)

My 2c.

Tim

> Thought I would stir the pot here, then sit back to see what boils to the 
> surface.
>
> Phyletic bracketing analysis (PBA) has become the latest tool in the 
> analysis of dinosaur structures and behavior. For example, because guarding 
> of nests occurs in crocs and in most birds, then nest guarding must have 
> occurred in dinosaurs (this is a general statement to illustrate how PBA 
> works to those who are not familiar with it).
>
> The same principle is used to interpret the function of dinosaur 
> structures. But this is also where the limitations of PBA become apparent. 
> Papp and Witmer (1998) attempted to debunk cheeks in ornithischians by 
> noting the absence of such structures in crocs and in birds. Fair enough at 
> face value. But, as Greg Paul has noted, the California condor has a 
> fleshy, non-mammalian cheek. Furthermore, the presence of osteoderms in the 
> cheek region of some ankylosaur skulls indicates that there must have been 
> tissue (i.e., a cheek) present in which the osteoderms grew. Finally, the 
> PBA model does not explain the inset of the teeth in ornithischians and 
> change in bone texture at the point the inset begins.
>
> The problem of PBA is that it cannot explain novel structures, and indeed 
> in the strict adherence to the techniques does not allow for such 
> structures (fireworks at this point, I am sure). I offer as examples: the 
> predentary bone of all ornithischians, the rostral bone of all 
> ceratopsians, etc. Neither of which occur in either birds or crocs.
>
> Now don't get me wrong, PBA  IS a valuable tool - but it is just that: a 
> tool.
>
> OK let the fireworks begin. ;-)
>
> Kenneth Carpenter

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