Pterosaur distributions (was Re: EARLY EVOLUTION OF 'BIRDS')

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 07:30 PM 11/7/98 -0800, Stan wrote:
>At 11:45 AM 11/7/98 -0000, John V Jackson wrote:
>>  It seems very strange to me though
>>that small pterosaurs can be preserved and found by the dozen 
>>but 20k+ theropods either didn't preserve well or weren't so 
>>easily noticed.  There just possibly *might* be some very wierd 
>>process hiding those "J mani's" ...
>
>It needn't be all that weird.
>
>A) Most Jurassic pterosaurs are found in only two or three lagerstaetten,
>all lakes or lagoons.

Give that man a (non-tobacco) cigar!

For a technical discussion of this aspect, see:
Buffetaut, E.  1995.  The importance of "lagertsta:tten" for our
understanding of the evolutionary history of certain groups of organisms:
the case of pterosaurs.  II International Symposium on Lithographic
Limestones, Lleida-Cuenca (Spain), Extended Abstracts.  pp. 49-52.

(Okay, I admit it: THIS is a hard to find paper.  If Buffetaut didn't give
me a reprint, I wouldn't have known about it.  So, a summary:

The majority of pterosaur diversity is known from a very small number of
formations: the Zorzino limestone, the Blue Lias, the Solnhofen, the
Cambridge Greensand, the Santana, and the Niobrara (the formation in which
_Sordes_ came was left off, but should be added).  These are, not
coincidentally, formations which provide excellent preservation (even the
Greensand has excellent presevation, although not articulation).  Other,
non-lagerstatt deposits (such as the Morrison) yield very few pterosaurs.

(Buffetaut also points out that a lagerstatt is no guarentee of finding
pterosaurs: the Kimmeridgian lithographic limestones of Cerin and the Early
Cretaceous ones of Montsec and Las Hoyas have failed to yeild much, if any,
pterosaur material, although the lie temporally and geographically between
some excellent sites).

>From the conclusion:
"Despite the discovery of new exceptionally preserved fossils, many
chronological gaps remain between the main _Lagersta:tten_, corresponding to
time intervals for which our knowledge of pterosaurs is much less
satisfactory (especially by comparison with the wealth of knowledge provided
by the _Lagersta:tten_).  Geographical gaps also exist (very little is known
about African pterosaurs, for instance, partly because pterosaur
_Lagersta:tten_ have not yet been reported from that continent)."

The importance of geology in paleontology cannot be overemphasized.  We deal
with the tiny fraction of organisms which happened to be buried in deposits
of an appropriate sediment, which were subsequently preserved, with were
subsequently lithified, which were not subsequently metamorphosed or eroded
or buried outside of our reach, which were later exposed, and which were
discovered and identified.

Also, in general, hypotheses should be based on positive observations, not
negative ones.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661