phyletic bracketing

["Ken Carpenter" <Kcarpenter@dmns.org>]

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