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Pterosaur history (was Re: Fwd: Dinogeorge Digest...)



Jeff Hecht wrote:

>I'm curious what proportion of known dinosaur taxa were small at different
>periods.

Just a reminder: as Mike Brett-Surman likes to point out "What dinosaurs are
small dinosaurs?  ALL dinosaurs are small dinosaurs, when they are born at
least."  This is a non-trivial point in terms of ecology, and for that
matter in terms of "extinction ecology" (for lack of a better phrase).

>Preservation biases will throw the ratio off, of course, but was
>there a steady decline in the fraction of small _non-avian_ dinosaurs?

Extraordinarily difficult question to answer, because of wildly different
preservation from formation to formation around the world.  On a gross
level, there are more lineages of small theropods known in (for example)
Judithian-Lancian units in North America than in the Morrison, or in the
Late K units of Mongolia compared to the Jurassic units.

On the other hand, within (for example) the large-bodied dinosaur lineages
of Late K Asiamerica, the latest members (Lancian or Edmontonian) tend to be
the largest known within each lineage: _Tyrannosaurus_, _Triceratops_ &
_Torosaurus_, _Pachyrhinosaurus_, _Pachycephalosaurus_, _Ankylosaurus_,
_Edmontonia_, etc.

Because of preservational biases, though, remember to take these patterns
with big chunks of salt...

>That also raises another question: were all small dinosaurs members of the
>theropod/bird lineage?

That's easier to answer, and the answer is no.  There are a fair number of
small sized members of primitive ornithopods (hypsilophodont grade) in the
Late Jurassic through the Cretaceous.  Whether any of the really small guys
made it to the K-T boundary, though, isn't certain.

>And finally an observation -- birds also appear to have replaced the
>smaller pterosaurs through the Cretaceous, so by the end of the Cretaceous
>the only known pterosaurs were the supergiants.

Be VERY careful of the "only giants made it to the end of the K" pattern:
Buffetaut has pointed out there is a major "Lagerstatten Effect" in
pterosaur studies.  That is to say, the vast majority of our knowledge of
small bodied pterosaurs come from specimens preserved in Lagerstatten
(deposits of spectacular preservation), which are currently unknown in the
later part of the Late Cretaceous.  Our view of giant pterosaurs at the end
may stem from the fact that it is darned difficult to preserve small
pterosaurs in typical environments.

That is not to say that we should assume that the small guys were still
around, either (they don't show up in the Niobrara Chalk, although neither
do azhdarchids, and we know they were around then).  It is just to say that
we should be cautious as to assuming a) the pattern of pterosaur size
changes we see preserved are the real pattern and b) ascribing to this
apparent pattern the difficult-to-test hypothesis of competition with birds.

For the record:
Buffetaut, E. 1995. The importance of "Lagerstatten" for our understanding
of the evolutionary history of certain groups of organisms: the case of
pterosaurs.  pp. 49-52.  II International Symposium on Lithographic
Limestones. Lleida-Cuenca (Spain).  Extended Abstracts.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661