Extinction theory (was Extinction - specific info)

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

>In particular, I'm interested in specific species that thrived (?) in
>the early Cretaceous, but went extinct BEFORE the end of the
>Cretaceous, and any theories as to why that particular species didn't
>survive.

A clarification here - the Early Cretaceous (a formal unit) ended 32
million years (and a LOT of evolution) before the K-T boundary.  If I
interpret you correctly, what you want is information on the
next-to-the-last dinosaur assemblages, which still lived late in the Late
Cretaceous.

>This is part of a training course, so I don't need alot of depth, but
>I do want accurate, and updated information.

First, some points about extinction:  there actually has been relatively
little attention paid to "background extinction" (the general turn over of
species through time, as opposed to mass extinctions).  As such, there are
fewer scenarios about specific causes of extinction of particular species.

Extinction is closely associated with speciation.  Some species go
"extinct" when they evolve into a new species (traditionally called
"chronospeciation").  However, many evolutionary biologists do not regard
this as "speciation" or "extinction", calling it "pseudoextinction".

Some general patterns can be observed in the "background" extinction.
Elisabeth Vrba, a mammal paleontologist and evolutionary theorist, has
shown what she calls the "turnover-pulse" in the record of life (esp.
terrestrial vertebrates, but other groups as well).  In the fossil record,
pulses of speciation (first appearance of new species) and extinction (last
disappearance of old species) occur simulanteously.  Additionally, these
evolutionary changes occur at the same times as climatic and other
environmental changes occur.  At other time periods, the species remain
basically unchanged, with few new species or extinctions.  This pattern is
called the "turnover-pulse", "turnover" being either speciation or
extinction.  The idea is that changes in the environment are such that some
species cannot survive (and so go extinct), some must migrate out of a
region or die, and in some cases, smaller populations of species become
isolated from the main population.  In the last case, genetic drift and
isolation and slightly different environmental conditions between the small
group and the ancestral population can lead to former become so different
from their ancestors that they become new species.

This pattern has been recognized in the dinosaur fossil record of the Late
Cretaceous North American Western Interior, where the primary changes are
rise and fall of the Interior Seaway, the rise of the Rockies (and related
changes in water drainage), and (possibly) bolide impacts.  There seems to
be little change within the early Campanian, late Campanian, early
Maastrichtian, and late Maastrichtian faunas (called Aquilian, Judithian,
Edmontonian, and Lancian by most paleontologists).  Most of the turnover
occurs between faunas.  One idea is that the rise of the Interior Seaway
would isolate populations on different parts of the coastal region, and
that when the seaway went back, the populations in each region would have
diverged from the other isolated groups, forming new species.

In the next posting, I'll list the latest Maastrichtian dinosaurs from
around the world.  Any dino not listed can be assumed to have gone extinct
before then.

                                
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.