Dinos and boundaries

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>Only one species has been found in
>more than one stratigraphic layer (species, not taxon).

Not quite true (Leptoceratops gracilis is in both the early-mid
Maastrichtian Meeteesee Formation and the late Maastrichtian Lance,
Anchiceratops ornatus in both the late Campanian Judith River Group and the
early-mid Maastrichtian Horseshoe Canyon, some of the species of Iguanodon
may cross over the Neocomian-Gallic boundary, and Coelophysis bauri might
be in both the Carnian and Norian), but for the most part a fair statement.

>The K/T " die out
>may just be representative of changing conditions world wide, and not be the
>result of some global catastrophe.
>  Second, by the systematics used in paleontology now, birds are dinosaurs.
>And Aves (or birds) had evolved by the late Cretaceous.  So why didn't they
>die out with the other dinosaurs?

Indeed, they'd been around since at least the Late Jurassic.

>  Third, we only know about 500 dinosaur species (correct it, but I think
>I'm in the ballpark) all thinly spread out through 100-150 million years.  I
>do not think that we have anywhere near a reasonable picture of the
>diversity of life in the past.  We have enough of the puzzle to learn a lot
>about vertebrate development, but not nearly enough to construct scenarios
>on mass extinction.  Even the mass extinctions took millions of years to
>happen.  Hardly a stunning and instantaneous loss.

However, the invertebrate, microfossil, and plant records do show pretty
well a rapid event of less than a few thousand years at the boundary,
although this was preceded by several million years of stepwise declines in
diversity.

And, welcome to the net.

                                
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.