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Re: Upper size limits in dino...

>If the smaller dinosaurs such as Compsygnathus or such were
>extinct at the time of the Late Cretaceous, then the dinosaurs were all of a
>gigantic type at the time.

Although Compsognathus itself was long dead by the Late Cretaceous, there
were still some small dinosaurs (Velociraptor, Troodon, Stegoceras,
Parksosaurus, and several others were man-sized or smaller)

>Did the presence of a niche of small dinosaurs
>lead to the long term survival of dinosaurs as a genus throughout all those
>millenia?  On the timeline that was wandering around this mailing list a
>while ago, it seems that each small extinction was followed by a smaller
>species becoming gigantic to fill an open niche, rather than a gigantic
>species proliferating other niches left open by an extinction.  Is this
>possible?  Am I misreading something?

That does seem to be a common thread in dinosaurian history.  For example,
large carnivores appear to have evolved independantly several times
(neoceratosaurs, megalosaurids, spinosaurids, allosauroids, tyrannosaurids,
and dryptosaurids).

>Did we get gigantic species spreading
>out to other niches, or were these splitting into species and subspecies done
>in a period of genetic and geologic calm (such as the ceratopian splitting
>into subgroups, or the saurapod splitting into subgroups)

Actually, evidence suggests the opposite: speciation seems to have occured
during periods of climatic and geologic change.  For example, in the North
American Western Interior, the advance and fall of the seaway seems to have
caused changes in the dinosaur populations, while periods of geologic
stability show periods of evolutionary stasis.  This pattern has been much
better documented in the late Cenozoic, and has been termed the "turnover
pulse hypothesis".

Thomas R. Holtz, Jr.                                   
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092