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Re: Quick cladistics question



Jeff Poling wrote:
>   Here's a cladistics question I do not recall ever being addressed:
        Your question really deals with Phylogenetic Taxonomy, the branch of
Phylogenetic Systematics dealing with the application of the principles of
PS to taxonomy. You can be a "cladist" in some senses of the word without
conforming to the principles of PT.

[note: read all equals signs as 3-bar "is defined as".]
>   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 }
        Indeed, it may be appropriate to add to this the other dinosaur, the
name of which escapes me, named by Owen in his definition.

>   What happens if birds were to, for example, be found to have evolved from
>a crocodile?
        Since so many clades are defined using birds as an anchor taxon (as
well they might, it has been noted by many authors that any taxonomy should
be rooted to crown clades of one form or another), what happens is a lot of
animals suddenly end up without taxa.
        As Archosauria is defined as = { + Aves, + Crocodilia }, this
definition is moved up to that node, and "dinosaurs" become Archosauromorpha
= { + Aves, - Lepidosauria }. Sauricha = { + Aves, - Ornithiscia } defines
the clade leading to Archosauria, and the bird half of Archosauria takes the
name Ornithosuchia = { + Aves, - Crocodilia }. Dinosauria = ( + Triceratops,
+ Aves } is still a valid taxon, but it now includes crocdilians, probably
pterosaurs, and a whole lot of other critters. "Dinosaurs" are sunk into
Ornithiscia = { + Iguanodon(?), - Aves }, and the definition of Pterosauria
= { + Pterodactylus, - Aves } (I am assuming it is something like this)
disappears.

Archosauromorpha = { + Aves, - Lepidosauria }
        Dinosauria = ( + Triceratops, + Aves }
                Ornithiscia = { + Iguanodon(?), - Aves }
                        "pterosaurs"
                        "dinosaurs"
                Saurischia = { + Aves, - Ornithiscia }
                        Archosauria = { + Aves, + Crocodilia }
                                Crocodylotarsi = { + Crocodilia, - Aves }
                                        (syn. pseudosuchia)
                                        "aetosaurs"
                                        "pseudosuchians (parphyl.)"
                                        "crocodylians"
                                Ornithosuchia = { + Aves, - Crocodilia }
                                        "birds"

>Would clade Dinosauria be moved down the line to be
>synonomized with what appears as the clade uniting ornithodira and
>crurotarsi in Weishampel's book?
        Nope. Ornithodira = { + Pterosauria, + Dinosauria } or somesuch. The
clade you want is Archosauria  = { + Aves, + Crocodilia }, which is composed
of the sister taxa Crocodylotarsi(syn. Pseudosuchia) = { + Crocodilia, -
Aves } and  Ornithosuchia = { + Aves, - Crocodilia }. See above for where
Dinosauria sits.

>Would the Dinosauria be redifined?
        You can't just redefine taxa at will, you have to do it right the
first time. The situation above does demonstrate a possible weakness in
phylogenetic taxonomy, in that by concentration too much on defining taxa
based on a certain phylogenetic hypothesis (cladogram), we run the risk of
later losing the meaning of well known and useful taxa to synonomy or
drastic changes in composition. In the case of dinosauria, this results
solely from the inclusion of birds in the definition. This is fine, in the
sense that we ought to try to relate taxa to living taxa in our definitions
whenever possible, but it escapes the origional meaning of the taxon. A
similar problem could come about with Pterosauria.
        But to answer your question, yeah, someone would probably try.


>Would it be dumped as a polyphyletic group?
        Impossible, it is *defined* so as to always be monophyletic. It
could be lost to synonomy, but it can never be poly- or paraphyletic.
----------------------------------------------------------------------------
      Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
            Web Page:  http://faraday.clas.virginia.edu/~jrw6f