RE: Maastrichtian generic distribution?

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> T. Mike Keesey
>
> >       Looks like the dinosaurologists are doing a pretty good
> job of finding
> > these older dinosaurs (and kin).  It certainly seems like Ornithodira
> > established their success relatively quickly, and thenceforth
> displayed what
> > one might expect----a relatively stable succession of faunas gradually
> > replacing one another throughout the Jurassic and Cretaceous.
>
> Well, there is one big exception (even more apparent looking at the genera
> per mega annum figures) -- by the end of the Jurassic, diversity is at its
> highest yet. Then, as the Cretaceous starts, it drops to extremely low
> levels (before building up even higher than before by the end of the
> Cretaceous), indicating a mass J/K extinction (or a paucity of Neocomian
> deposits?).

dingdingdingding... Got it in one.

There are VERY FEW earliest Cretaceous terrestrial deposits anywhere (or,
for that matter, latest Jurassic terrestrial deposits).  So any "decline" in
taxon diversity from a highly fossiliferous time period (like the
Kimmeridgian and early Tithonian) to a more poorly fossiliferous time period
(like the pre-Barremian Cretaceous) must be regarded with at least some
consideration of sampling effects.

Demonstration of a real mass extinction (that is, the geologically rapid
disappearance of disparate lineages of organisms) requires good samples
below, during, and above the proposed extinction event.  Such data ARE
available for the K/T boundary in the American West (for the terrestrial
realm; in many parts of the world for the marine realm).  However, such data
are currently not available for the J/K boundary.  Recall, too, that there
is a gap of more than 10 million years between high-quality terrestrial
records in the Late Jurassic (the Morrison, and to a lesser degree the
Tendaguru, and potentially sites in Zimbabwe, Portugal, etc.) and the
beginnings of a substantial record (i.e., more than a few isolated bones of
one taxon) in the early part of the Early Cretaceous.

> >       Are there any figures on the breakdown of genera (or
> species) numbers
> > for the Maastrichtian in particular.  Is it pretty steady
> throughout this
> > last period of dinosaur evolution until the sudden extinction
> at the end?
>
> If I'm not mistaken, the only known fossil-bearing deposits that go right
> up to the end are in North America -- I don't know if the fossil record is
> good enough to say exactly what was going on world-wide.

Pretty much true: some Chinese sites *might* record the transition, and some
of the Indian intertrappean beds are clearly at the tail end of the
Cretaceous, but neither of these are anywhere near the high-quality record
of the Lancian-to-Puercan rocks of western North America.

Enjoy your roasted maniraptoran!

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
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