Late Cretaceous diversities

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

>> >My apologies to everyone...I did, infact, want to deal at a much
>> >broader level than species.  Families, would probably have been a much
>> >better choice.
>> >
>> >When discussing the "bang or whimper" question of extiction, I wanted
>> >to point out to them that dinosaurs were becoming less abundant and
>> >less diverse by the end of the Cretaceous, and merely wanted to flesh
>> >out the remarks with some specific information.
>>
>> Actually, since you were interested in showning the decrease in diversity,
>> then "Family" is an inappropriate rank to show.  All the common families of
>> dinosaurs that were around in the late Campanian made it to the end of the
>> Cretaceous.
>>
>> The decrease in diversity occured below the level of traditional "families".
>>
>> To put it another way: the family Elephantidae is still doing fine at the
>> family level, but such a statement doesn't reflect the fact that half a
>> dozen species or more of elephantids have died off in the last half million
>> years (and the remaining two aren't in too good a shape, either).
>
>But the dinosaur record is not like the elephant record.  Dinosaur
>abundance at any time in the Mesozoic reflects the quality of the record
>at that time, not the original abundance of dinosaurs.
>
>People suggesting that there was a decrease in abundance are simply not
>recognizing the poor record that exists.  How many intervals (e.g. early
>Maast.) have good collections from more than 4 or 5 local areas around
>the world.  Late Maast. for example has most of the known global
>diversity comming out of a single formation--the Hell Creek.

The higher diversity of the western North American fauna is at least partly
due to more concentrated effort and more exposure there.  However,
Asiamerican faunas are highly endemic, and seem to be much more diverse
taxonomically than the rest of Late Cretaceous faunas (Argentina, India,
Europe, etc.)  Thus, the fact that most diversity comes from western North
America may not simply be a preservational artifact.

>How can we talk about global diversity patterns when most of the record
>comes from a single formation?  The record of dinosaurs is inadequate to
>examine global diversity.

Not entirely true - even in the North American West, the Lance, Laramie,
Denver, lower North Horn, Javelina, Scollard, Frenchman, and basal
Ravenscrag Formations all are fossiliferous ("dinosauriferous"?).  The fact
that the types of most Lancian species come from the Hell Creek simply
reflect that the majority of the initial studies were conducted there.
Since dinosaur species were fairly widespread geographically (like most
large terrestrial vertebrates), it is not surprising that the diversity of
the Hell Creek is similar to the Lance is similar to the Scollard, etc.

>We need to look for changes in local communities through time in areas
>where the communities can be traced through an interval of time.

On this we do agree.  However, the traditional "families" are very broad
ecological units as well as broad taxonomic units.  Ceratopsids and
hadrosaurids can be divided into several different components
(Centrosaurinae vs Chasmosaurinae; Lambeosaurinae vs Brachylophosaurini vs
Hadrosaurini vs Saurolophini vs Edmontosaurini) which probably more closely
reflect ecological units (esp. since their different snout shapes likely
reflect varied diets).  When examined at this level, there are definite
patterns, especially greater survival of long-snouted forms (for whatever
reason, I don't know).

Given that hundreds of ceratopsians skulls have been pulled out of Lancian
rocks, from formations representing more diverse paleoenvironments than
those of the Judithian, how would you explain the absence of centrosaurine
species in the late Maastrichtian?  Since ceratopsids are endemic to
western North America (with two questionable exceptions), this is as strong
an evidence for extinction or decreased diversity as you can find in any
terrestrial vertebrate lineage.  The same case can be made for hadrosaurid
groups.

>> Bob Myers (and others) replied to my posting re specific extinction
>> When discussing the "bang or whimper" question of extiction, I wanted
>> to point out to them that dinosaurs were becoming less abundant and
>> less diverse by the end of the Cretaceous, and merely wanted to flesh
>> out the remarks with some specific information.
>
>I would be interested to know what evidence has been presented that
>dinosaurs were becoming less abundant.  I do not buy it.

As well you shouldn't!  (And I don't know who is suggesting that.)

                                
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.