[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Centrosaurines



In a message dated 95-10-29 23:31:30 EST, zooamy@zoo.latrobe.edu.au (Adam
Yates) writes:

> Most modern systematists agree (but by no means all - see 
>G.O. for dissent)  that classifications of 
>animals should strive to consist  monophyletic (that is it should include 
>ALL descendants of the common ancestor of the group in question). It is 
>this sort of classification that leads to the placement of Aves within 
>Dinosauria. Therefore a monophyletic genus that included Monoclonius, 
>Eucentrosaurus and Styracosaurus would have to include 
>Pachyrhinosaurus as well, wich most would judge is simply too different for 
>inclusion with the others.

Monoclonius is based on indeterminate material that cannot be distinguished
from immature _Styracosaurus_,  _Eucentrosaurus_, or _Pachyrhinosaurus_. A
monophyletic genus for those three genera would also include _Einiosaurus_
and _Achelousaurus_, Scott Sampson's new Two Medicine Formation genera. Most
paleontologists agree that this is too much variety for one genus--which is a
typological decision, not a cladistic one. That is, retaining one genus for
these forms is "ridiculous."

I have always agreed that taxa SHOULD be monophyletic--unless the
accumulation of evolutionary novelties in a subclade makes retention of that
subclade within the more inclusive clade "ridiculous"--like extant birds in
Dinosauria. Since my starting point before forming taxa is always the
cladogram, all phylogenetic information is preserved in my taxonomy. Derived
birds are treated as dinosaur descendants, not as dinosaurs; the phyletic
relationship is exactly homeomorphic to a set-inculsive relationship. Aside
from the trivial novelty of thinking of birds "as" dinosaurs, retaining a
monophyletic Dinosauria tells us nothing about birds that a paraphyletic
Dinosauria ancestral to Aves doesn't.

G.O.