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RE: Bakker's Brontosaurus and Late Cretaceous populations

Tom Holtz wrote:

In one of the episodes Bakker explains why he
> doesn't
> > feel that a comet killed the dinosaurs, One thing
he said
> was
> > that before their sudden disappearance from the
> fossil
> > record the dinosaurs were already dying out. This
> > blatantly contradicts what another paleontologist
> in a
> > different episode of the same series says,
> claiming
> > they were thriving. I could be mistaken, but in
> "When
> > Dinosaurs Roamed America" I thought something was
> > mentioned that in the time leading right up to
> their
> > extinction that there was a relatively low number
> of
> > species yet in an issue of National Geographic
> from
> > 1999 on Sue, it explains that the dinosaurs were
> > rapidly diversifying. How can their be such
> blatant
> > contradictions on the matter?
> To a large degree, it depends on what time scale and
> taxonomic level you are looking at. Most of the
> traditional work supporting "no
> evidence for decline" has been looking at the
> species or family level within the Hell Creek or the
> Lance, and there is no sign of
> decline that way.

Right, but by that point 'lambeosaurines',
'centrosaurines', 'nodosaurs' and possibly sauropods
had all gone extinct in N. Am. Arguably, all but
sauropods went extinct before even Alamosaurus-times
(~69Ma pre-Lance/Hell Creek).. So there's a very
strong argument for declining diversity before the K-T
boundary... at some point in the Edmontonian, very
probably associated with onset of the final regression
of the WIS.

However, yes, within the final million years or so
covered by the Lance and Hell Creek, you don't see any
taxa go extinct (until the boundary of course).
Historically, you could argue that we see a small
diversification amongst Chasmosaurines; instead of
just one species (as was the status quo throughout the
N. Am Late K), there are two. But this is false since
both Torosaurus and Triceratops are present in the
Alamosaurus fauna, so this diversification presumably
happened about the same time as centrosaurines and
lambeosaurines disappear. Hmmm....... now that's

> Most of the traditional work supporting a decline
> looked at the subfamily level over the last 15-20
> million years or so.
> The recent work by Fastovsky (supporting a possible
> increase, or at least stable) looked at the species
> level over the whole of the
> Cretaceous.

I don't really agree with the oversplitting of alot of
the North American taxa (especially ornithischians);
it gives a very false idea of dinosaurian diversity.
For example, it is extremely difficult (if not
impossible) to tell one lambeosaurine from another
postcranially, even size- wise there isn't huge
variation, yet the likes of Lambeosaurus,
Corythosaurus, Parasaurolophus etc are afforded
genus-level distinction. Substitute any of the
ornithischian subfamilies and the result is similar.

Contrast this with my old buddy Iguanodon, which shows
as much variation between the contemporaneous
I.bernissartensis and I.atherfieldensis as is seen
between the Hadrosaurinae and Lambeosaurinae (noone
ever suggested hadrosaurs and lambeosaurs were
male-female, and they shouldn't for Iggy either). Do
we really see endless diversity of species/genera in
North America in the late K, or is it just a single
representative of any given 'group', at any one time? 

> Too bad no one's looked at Campano-Maastrichtian
> dinosaurian diversity with an eye towards
> phylogeny... Oh, wait. Right.  See you at
> Mesa, people! :-)

I'll be very interested to see what you have to say


>               Thomas R. Holtz, Jr.
>               Vertebrate Paleontologist
> Department of Geology         Director, Earth, Life & Time
> Program
> University of Maryland                College Park Scholars
>       Mailing Address:
>               Building 237, Room 1117
>               College Park, MD  20742
> http://www.geol.umd.edu/~tholtz/
> http://www.geol.umd.edu/~jmerck/eltsite
> Phone:        301-405-4084    Email:  tholtz@geol.umd.edu
> Fax (Geol):  301-314-9661     Fax (CPS-ELT):
> 301-405-0796

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