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RE: Bakker's Brontosaurus and Late Cretaceous populations

--- Denver Fowler <df9465@yahoo.co.uk> wrote:
> 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).. 

 Centrosaurines were apparently gone by Lancian time,
but nodosaurs and lambeosaurs appear present, albeit
rare, in the lower Lance and Hell Creek, and therefore
may have become extinct within the Lancian. Boyd made
a case for Hypacrosaurus in the lower Hell Creek, and
nodosaur material has been found in the Ferris, Lance
and Hell Creek. A 1986 JVP article noted that all of
the Lance/Hell Creek nodosaur material was
stratigraphically low. But I don't know the level of a
more recently discovered partial skeleton from
Greasewood creek. Several years ago, Sroka mentioned
another nodosaur from about the middle of the Hell
Creek but the identity and level are still uncertain.

>So there's a very
> strong argument for declining diversity before the
> K-T
> boundary... at some point in the Edmontonian,

 Maybe at the boundary between the Edmontonian and
Lancian. AFAIK, Hypacrosaurus is still abundant in
unit 4.

> very
> probably associated with onset of the final
> regression
> of the WIS.

   Why?  There had been transgression and regression
prior to that, in the Campanian, when diversity seems
to have peaked. Logically, a regression should have
favored the more inland types like lambeosaurs and
saurolophines but there appear to have been few if any
left in the Lancian.

> However, yes, within the final million years or so
> covered by the Lance and Hell Creek, you don't see
> any
> taxa go extinct 

  The final disappearance of nodosaurs and lambeosaurs
may have occurred then. Compare the Scollard with the
Hell Creek.

>(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,

Triceratops was present in the Alamosaurus fauna? It
may have been coeval with one or more Alamosaurus
bearing units which yield T. rex but AFAIK not itself
in that environment.

> so this diversification
> presumably
> happened about the same time as centrosaurines and
> lambeosaurines disappear. Hmmm....... now that's
> interesting....
> > 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
> Tom! 
> Denver.
> > 
> >             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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