Re: Apomorphy-based definitions (of mammals and birds)

["Ken Kinman" <kinman@hotmail.com>]

     I think cladistic analysis is great, so I am delighted to see 
cladograms appearing in high school texts.  It is just the mandatory 
translation of cladograms into strictly cladistic classifications that 
bothers me.  I also reviewed a chapter in another recent textbook (not sure 
if it published yet), that dealt with mammals, and it had a monotreme, 
marsupial, and two "ungulates".  Unfortunately one of the ungulates was a 
sirenian (i.e., a pseudoungulate) and the synapomorphy for Ungulata was 
"hooves".  But the nail-hooves of pseudoungulates probably evolved 
convergently with the true hooves of the true ungulates.  So that cladogram 
also had to be modified, so that it would not be soon obsolete.
      As for the refined scientific definition of a mammal, this is the way 
I understand it.  Tritheledontidae (ictidosaurs), an advanced cynodont 
therapsid family, has both reptile and mammal jaw articulations 
(transitioning to the mammalian condition).  The mammal boundary has been 
barely crossed in the Upper Triassic mammal _Adelobasileus_, which 
possesses:  an incipient promontorium housing the cochlea (and absence of a 
reptilian jaw articulation).
    So that is probably going to be the precise definition of a mammal for 
quite some time.  Hadrocodium (which is not much more advanced) has a 
fully-developed promontorium.  Therefore, Adelobasileus is as close to the 
first mammal as we are going to see for a while.  A fossil lacking the 
promontorium and the reptile jaw articulation would definitely be on the 
reptile side of the line.  A mammal with an even more incipient promontorium 
might eventually be found, but I wouldn't expect it to happen any time soon. 
 And it would almost certainly be called a mammal unless it happened to 
still have both the mammal and reptile jaw articulations (possible I 
suppose, but doubtful in my opinion, and would require a bit more refinement 
of the definition at that time).  This is a extremely well-refined 
synapomorphy, and if there are future refinements, they will be very minor.
     Anyway, it would certainly be great if we could do with Aves what has 
been so successfully done with Mammalia.  This would not only entail the 
precise definition of the "true" semilunate carpal block, but also precise 
transitions which occur in bones immediately adjacent to it.  I do not have 
the literature necessary to determine if such "adjacent" synapomorphic 
transitions could be determined, but it would be very helpful if there were.
       ----Enough for tonight,
             TGIF,    Ken


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