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Sterna, Whales, and Evolution



>>Birds merely represent a small part of the evolutionary diversity of the 
>>Dinosauria.
>
>One possible problem with this, is the breastbone seen in all bird
>species, but not seen in any dinosaur (including Archy).  It is
>possible that this feature succeeds in marking birds as "different
>enough" to place in their own category?

Big fused sterna are present in some other maniraptorans (Oviraptoridae, for
example), and have now been found in Archaeopteryx.  However, none
(including Mononykus) have the same sort of keel on the sternum as do
modern-style birds.  (Mononykus has a keel, but it is of somewhat different
morphology compared to advanced birds).

Do you suggest that the development of a simple ridge on a pre-existing
structure necessitates a Class-level division?

>I thought that whales evolved from wolf-like carnivores?  How do ungulates fit
>in this?

Whales evolved from wolf-like carnivorous ungulates (the "Mesonychidae", a
paraphyletic group).

> Assuming that whales did evolve from ungulates, I suggest that since
> there has been no evolution of new features, just modification and
> reduction of previously existing ones, then whales and ungulates to
> belong together.

But all avian features are modifications or reductions of previously
existing ones, as evolution IS descent with modification.

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