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Re: Stratigraphy, biogeography & cladograms



At 06:36 PM 1/18/99 -0800, Betty Cunningham wrote:
>It seems to me that (modern birds) may simply be all that's left of a
>major group of animals (major prebird creature group) that extended from
>the early Jurassic right on into the late Cretaceous.  And rather than
>trying to fit such fossils as Archie, Seino, Caud, etc as direct
>ancestors of (modern birds), the whole mess is more like fitting a newly
>found fossil hominid into the (modern humans) family tree. Very very few
>hominid fossils are direct ancestors of (modern humans).

In cladistics one doesn't search for ancestors, but for sister taxa.  Thus,
no one states that any of the above are explicitly ancestral to later birds,
only that they share some form of nested hiearchical relationship with later
birds (say, for example, that Archie shares a more recent common ancestor
with modern birds than with _Caudipteryx_).

Note that some sister taxa in an analysis might potentially be true
ancestors (i.e., if they occur earlier in time than all of the sister taxa;
that they share some derived features with the sister taxon; that they don't
have autapomorphies (shared derived features of their own).  However, one
cannot demonstrate that a fossil IS ancestral (that is, it is part of the
population that gave rise to the later grou) rather than a primitive sister
taxon.  On the other hand, one can demonstrate that the fossil is NOT
ancestral if you find a member of the more advanced clade at the same
stratigraphic level or below.  Nevertheless, the primitive sister taxon
might well be important in understanding the morphological conditions of the
true ancestral population.

BTW, who is "Seino"?  The compsognathid _Sinosauropteryx_?  The
enantiornithine _Sinornis_? 

>If that is the case is it possible that (modern birds) might actually be
>derived/descended from more than one line of (prebird creatures) from
>this (major prebird creature group)?   
>
>How would you say that in linnean or cladistic terms?

Possible, yes, and even has been suggested.  However, it requries a
helluvalotta convergence throughout the anatomy, as modern birds are pretty
derived compared to (and share the same derived features with each other
relative to) members of the Mesozoic bird groups (i.e., enantiornithines,
ichthyornithines, etc.).

You would say this in linnean or cladistic terms that Aves, as traditionally
envisioned, is polyphyletic (same term is used in both systems).

HOWEVER, under phylogenetic taxonomy, "modern birds" (Gauthier's Aves, most
people's Neornithes) has been defined as all descendants of the most recent
common ancestors of paleognaths and neognaths (or _Struthio_ and _Passer_,
or equivalent).  Under PT the name would apply to the clade meeting that
description.  If that clade also contained dromaeosaurids or oviraptorosaurs
or coelophysoids or brachiosaurids (!?!), these other forms would then be
incorporated into Neornithes.

However, as noted above, monophyly of ratites, tinamous, and neognath birds
outside of ichthyornithines, enantiornithines, _Patagopteryx_, etc. (much
less _Archaeopteryx_, _Caudipteryx_, or dromaeosaurids) is pretty damn secure.

One good source for some of this info is in Dingus & Rowe's "The Mistaken
Extinction".

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