[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Re: definition of dinosaur
>I appreciate Tom and GO responding to my question. Of course, I don't
>want to teach outmoded taxonomic thinking. But a point Tom brought up
>has me back in my persistent cladistic haze:
>>We (i.e., phylogenetic taxonomists) WANT to
>>leave behind grade-based thinking of taxa. Taxa in this system are NOT
>>defined by characteristics; they are soley defined by ancestry.
>Have we really gotten away from grade-based taxa when we say that the
>Maniraptora, for example, have: "ulna bowed posteriorly; metacarpal III
>long and slender;", etc.? Then, the Arctometatarsalia have: "elongate
>tibia and metatarsus; metatarsals deeper anteroposteriorly than
>mediolaterally;", etc. (from Tom Holtz's 1994 Jour. Paleo. paper). Do
>those synapomorphies not represent grades, so that the taxa are
>recognizeable (within their clade) on the basis of the features
>mentioned? It seems to me that "going up" the cladogram, more and more
>precise distinctions are added to those already specified, with each step
>analogous to a grade in the other taxonomic system.
What we have here is the difference between DEFINITION of a taxon (in the
case of phylogenetic taxonomy, based on ancestry) and the DIAGNOSIS of that
taxon (the anatomical, genetic, behavioral, etc. characteristics by which we
recognize the taxon). (Sorry for the caps, but I don't know how to embolden
the letters over email).
The clade Maniraptora is defined as birds and all theropods closer to birds
than to Ornithomimidae. This definition says nothing about bowed ulnae,
thin mcIIIs, or whatever. The definition is based soley on ancestry. If
additional data (i.e., my paper) suggest that the synapomorphies or
component taxa of a clade are different than previously
suggested, then the synapomorphies or component taxa of that
clade are different than previously suggested. The clade
retains its orignal defintion.
(Incidentally, I used the name "Maniraptora" incorrectly in my paper, and have
taken steps (see J Paleo issue 3 of the current volume) to correct this).
Taxa are recognizable on the basis of possessing the derived characters of a
clade, but are only members of said clade is they descend from the ancestor
refered to by the definition. Snakes and whales are tetrapods, although
each have lost their hindlimbs, because they descend from the common
ancestor of all tetrapods. _Eoraptor_ and Herrerasauridae can only be
dinosaurs if they descend from (or are) the common ancestor of birds and
_Triceratops_ (or _Megalosaurus_ and _Iguanodon_).
For further discussions of this, you may wish to check the papers by De
Queiroz and Gauthier in the 1990 volume of Systematic Zoology, the 1992
volume of Annual Review of Ecology & Systematics, and the 1994 volume of
Trends in Ecology and Evolution. I don't have the page numbers on me, but
they shouldn't be too hard to find.
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742