Re: K-T extiction (fwd)

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

>Since the gradual extinction crowd has been active on the list I thought
>I should put out a bit from the other camp.  I find that at this point in
>time the two sides are essentially talking at each other, rather than
>trying  to convince each other.  This is much like I remember as a student
>when plate tectonics was becoming accepted.  Conversion from one camp to
>the other occurs when an individual decides the weight of evidence is
>overwhelming--and the conversion is almost instantaneous.  Some never
>change.

[Incidentally, I'm not in any "gradualist camp".  I believe that the K-T
impact was a probable reason for the terminal extinctions.  However, there
are definate changes in the biota during the Campano-Maastrichtian which
may have some bearing on the over-all nature of the extinction.]
>        To the extinction.  I present my view--the background has been
>published--I will not attempt to document it here.
>
>1.  Was there an extinction event--some gradualists deny it.
>        There was a huge extinction.  Sepkoski data base shows it as one
>of the big 5.  On land, just consider what the land was like in the
>earliest Paleocene.  There were no large bodied herbivores.  There were
>no large bodied carnivores (except some stream dwellers that might get
>onto land around water--but they would have no food source on land.)
>Mammals were tiny, mostly insectivores and omnivores.  This community
>was unlike anything before or after.  It reflects the enormity of the event.

True.  I think we all agree on most aspects of this.

>
>2.  Dinosaur communities died out suddenly and there is no evidence they
>were in decline.
>        The record is too poor and spotty to look at global diversity
>over the last 20 my but we can look at communities over shorter time
>spans.  My group found that over the last 2 my of the K, dinosaur communities
>were essentially stable.  There was no evidence of any sort of decline in
>the communities as would be expected if a gradual extinction were in
>progress.

Depends on the definition of diversity.  Obviously those of us working on
terrestrial vertebrates cannot determine with decent accuracy the various
percentages of each taxon in most faunas.   However, recent advances in
global work on Campano-Maastrichtian faunas allow a preliminary study of
the fate of lineages.

The major weaknesses in most recent work on the catastrophic extinction of
the faunas are problems of scale - too little time and too broad a
phylogenetic base.  It is true that the Ceratopsidae and Hadrosauridae (the
best documented families of large Asiamerican herbivores) both make it to
the K-T boundary.  However, based on the excellent western interior faunas
of North America, we can recognize that within these families, various
smaller taxonomic units terminate before the end of the Maastrichtian.

For example, within the Ceratopsidae there are two major groups: the
short-snouted Centrosaurinae and the long-snouted Chasmosaurinae.  Both
lineages are represented by several species during the late Campanian.  By
the early Maastrichtian only one species (Pachyrhinosaurus canadensis) of
centrosaurine has been documented, while several species of chasmosaurine
are known.  By the late Maastrichtian, with hundreds of ceratopsids skulls
collected from Saskatchewan to Texas, no centrosaurines are known.
Instead, all known late Maastrichtian ceratopsids are chasmosaurines
(Torosaurus latus, Diceratops hatcheri, Triceratops horridus, and
Triceratops sp. 2 of C. Forster's work).

A similar situation exists for hadrosaurines (the "non-crested"
hadrosaurids).  While the Maiasaurini, Gryposaurini, and Saurolophini (and,
by phylogenetic position, the ancestors of the Edmontosaurini) are all
present in the late Campanian, only the Saurolophini and Edmontosaurini are
known from the early Maastrichtian and only the Edmontosaurini for the late
Maastrichtian.

So, many lineages present in the late Campanian to early Maastrichtian have
not been documented from the hundreds of skeletons known from the late
Maastricthian.  Within the late Maastrichtian, only descendants from a few
of these lineages are known.  However, Dr. Sheehan is correct that there is
no evidence for a gradual decline in the these species during their 2-3
million year duration.

>>3.  What does the sea level decline mean?  Not much--3~ seas went in and out
>numerous times in Mesozoic--with no extinction events (I assume the
>Triassic event was not caused by sea level flux.)  Look at Greg
>Retallack's paper in the lastest GSA Bull.  There was no drastic change
>in soils during Hell Creek.  Flora in Hell Creek changed modestly, but
>there was a huge event at the time of the impact (followed by disaster
>floras world wide).  The point is the sea level change  did not seem to
>have much effect on the land climate or biota.  And in the marine realm it
>was not much of an event either.

>From paleoclimate work here at the USGS and elsewhere, it is known that
shallow seas have a very significant effect on climate.  The Maastrichtian
regression is the most severe of late Mesozoic.  Some lines of evidence
suggest that the terrestrial environment of the late Maastrichtian differed
from those of immediately earlier time.  For one, the angiosperms
(flowering plants) only become the dominant foliage during this interval
(previously, they had been diverse species-wise but rare as individuals).
The recomposition of the terrestrial herbivore fauna may have been related
(for one thing, the major differnces between the various lineages of
ceratopsids and hadrosaurines are snout shape, aka feeding adaptations).
Some Asian lineages of dinosaurs show up in North America at this time, and
possible North American immigrants in Europe.  These biotic changes may
have been associated with the regression, although a considerable body of
work must still be done on this.

>>4. Was there a gradual or step-wise extinction.  The Signor-Lipps effect
>explains virtually all known instances of supposed gradual and step-wise
>extinction. The poorer the record the longer the supposed "gradual
>extinction" occurs.  People still talk about a 10 my long decline of
>dinosaurs.  But dino record is basically hopeless for this kind of change.

The Signor-Lipps effect would explain such events, it is certain.  However,
does such an effect apply on times scales longer than the species
durations?  Also, a step-wise extinction would also produce a step-wise
extinction pattern, so other evidence must be brought into bear for either
hypothesis.

Also, "hopeless" is a little strong...

>5.  Birds--idle speculation with almost no record.

The hypothesis that birds (and amphibians) suffer from radical changes in
environmental conditions is far superior to the idea that impacts cause
extinction.  The former has been observed many times, the latter has yet to
be directly observed (although I do belive it has happened).  So, it is not
"idle speculation".

Also, the Mesozoic record of birds is rapidly getting better.  There will
be a book out in the next year (or maybe 1996?) on the current state of
Mesozoic paleoornithology.

>6.  Loss of sunlight (from dust and Sulfur gases) and other associated
>effects of impact caused collapse of food chains based directly on primary
>production.

"May have caused" not "caused".  This is still a hypothesis (although a
pretty strong one).

[various mass death scenarios deleted.  I believe they are likely, but have
yet to be sufficiently tested.]

>        There is virtually no evidence for gradual extinction.
>Inoceramids and rudists declined or died out before.

And various members of the the late Late Cretaceous dinosaur lineages.  And
some plant groups.

>This is not
>something foreboding the extinction.  Groups of animals, especially
>highly specialized groups die out through out geologic time these groups
>died, but were not part of the extinction event.

They certainly weren't part of the K-T extinction, but they were part of a
general step-wise decline in global lineage diversity.

>        There was a huge extinction event both on land and in the oceans
>which, to the level of resolution allowed by the fossil record, was
>1) geologically instantaneous, and 2) at the impact layer.

Yes.

>
>        But this will not convince a gradualist in the near future.  Nor will
>a
>gradualist convince me.  We are involved in a kind of scientific
>exchange that differs from most science.  To move from one camp to the
>other requires a huge reevaluation of a scientists basic understanding of
>nature.  This is not an issue of normal science, where new data is just
>an addition to our understanding.   We are in the sort of controversy
>that happened during the plate-tectonic revolution.  For those of you
>viewing this process from the outside, my guess is that in 20 years
>the abrupt, impact extinction will be accepted--but only time will tell.
>Those of us involved in this debate are simply talking at each other at
>this point in time.

[jump on soap box]

I disagree.  Any of the extinction hypothesese proposed should be testable
and falsifiable.  It was the superior data and explanatory power of plate
tectonics that won over previous ideas.

I am willing to hear new ideas.  I am sorry to hear that there are
professionals in our community who will not examine ideas which do not
conform to their beliefs.

[jump off soap box]

Bye for now.  Hope this clears up my position.

                                
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.