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Re: Running speeds



>        I was wondering if fossil records show faster dinosaurs
>in more recent periods.  I'm not that well up on which period every
>dinosaur lived in but thought that the length of time they were around
>was plenty for selection pressure to act and therefore faster dinosaurs
>to evolve, (could it have been this selection pressure that made them
>smaller => faster?),

Among theropod dinosaurs, at least, those with the most elongate legs and
other cursorial adaptations (ornithomimids, troodontids, tyrannosaurids,
etc.) appear in the Cretaceous, and are most abundant in the Late
Cretaceous.  Smaller is not necassarily faster - ostrich-sized
ornithomimids were probably faster than dog-sized troodontids, since a
larger body size gives a larger stride.

[Gratuitous plug - some of your questions are addressed in:
Holtz, T.R., Jr.  1995.  The arctometatarsalian pes, an unusual structure
of the metatarsus of Cretaceous Theropoda (Dinosauria: Saurischia).
Journal of Vertebrate Paleontology 14: 480-519.]

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
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
U.S.A.