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Phyl Tax roundup (was Re: Quick cladistics question)

At 06:23 AM 3/31/97 -0800, jwoolf wrote:
>> Cladists are presently trying to move definitions of taxonomic groups away
>> from characters ("character-based" definitions) and toward definitions based
>> on relationship ("node-based" and "stem-based" definitions). Key characters
>> may change with the acquisition of new information and the discovery of new
>> fossils, whereas relationships are more stable against such discoveries. 
>I don't know very much about cladistics, but based on what I do know the
>above seems to be an inherently contradictory position.  OK, you define
>clades based on relationships.  How do you define relationships?  How
>else can you define relationships except by physical characteristics?   

Important distinction:
Definition: a formula describing the content or basis for a taxonomic decision.
Diagnosis: the features which distinguish the members of a taxon.

Under phylogenetic taxonomy, taxa are DEFINED by evolutionary relationship,
but DIAGNOSED by characters.  Finally, relationships among organisms are NOT
defined, but discovered (or at least hypothesized) by their physical

Jonathan Wagner wrote:

>>   Dinosauria is defined as the most recent common ancestor of _Triceratops_
>>and modern birds.
>        There is, as you point out, a slight inconsistancy in this
>definition, as it does not reflect the intention of the origional namer of
>the taxon. If anyone ever gets the oppurtunity to act in the capacity of
>first reviewer with regards to whatever paper this was published in (if it
>has indeed been published), I would urge them to change it to:
>        = { + Megalosaurus, + Iguanodon }

As would I, but it is too late, if we use date of phylogenetic definition
for priority.  Saurischia was defined phylogenetically in 1986 (by
Gauthier), Ornithischia in 1993 (by Padian and May), and Dinosauria in 1996
(by Novas).

>        Indeed, it may be appropriate to add to this the other dinosaur, the
>name of which escapes me, named by Owen in his definition.


Jeff Poling wrote:
>   I did a search of the DML archives, usenet, and the web, and didn't find
>a definition for a crown clade.  What is a crown clade?

A taxon defined as "all descendants of the most recent common ancestors of
the living members of this group".  Example: Mammalia is the crown group of

Note that crown groups are node-based taxa, but not all node-based taxa are
crown groups (e.g., Allosauroidea, Euhadrosauria, etc.).

>   One other question:  I carefully reread the definitions for stem and node
>based clades (and then did the requisite search of the archives).  It looks
>like the only difference between a node and stem based clade is how each is
>defined.  There has to be more to it than that ... what IS the difference?

Nope, there is NO OTHER difference.  These are simply two different ways of
defining taxa.  One could argue the merits of one sort vs. the other, but
they are simply two different ways of definig taxon names.

>   And one other one other question:  how is it decided when one definition
>has precedence over the other?  As part of my example, you mentioned that
>under those conditions Dinosauria would become synonomous with Archosauria,
>and Archosauria would be sunk.  I thought Archosauria had been named first?

The current favored practice is priority based on the first explicit
phylogenetic definition of a taxon.  Thus, Archosauria was defined
phylogenetically before Dinosauria (1986 vs. 1996), although the traditional
terms were defined in the opposite order (i.e., Dinosauria first).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661