[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Large last gasp of pterosaurs



Graydon (oak@uniserve.com) wrote:

<1) we don't have any evidence for it, no strata where the pterosaurs give
way to shore birds, frex.  The few mixed communities we have evidence for
have way more pterosaurs than birds.  (7 Archeopteryx versus how many
pterosaurs?)>

  One needn't invoke shorebirds as an example. But note, only 20 million
years later in the Yixian and related begs of northeastern China, there
are two pterosaur species known, *Dendrorhynchoides* and *Eosipterus,*
compared to some 15 or so species of bird, some ornithurine and some
enantiornithine, and some more basal still. Progress another 20 million
years or so and it's the middle of the Cretaceous, and at Jiufotang and
similar younger sites, the pterosaur count has only marginally increased
with larger forms, yes the birds become just as numerous, with more
enantiornithines, ornithurines, and some even more basal, long-tailed
birds. Beyond this point, the record does not favor bird-laden beds with
pterosaurs to compare to, except the smallest pterosaurs are found much
earlier in the Cretaceous, and the largest bird is non-flying (Campanian
*Hesperornis* where *Ichthyornis* was much smaller). The collected and
observed trend would lend credence to an idea that birds were filling the
small flier niches, and pterosaurs dominated the large fliers. But there
does not seem to be a marked increase in large birds known that would have
removed the azhdarchids and pteranodontids from the picture, as both seem
to have survived into the upper Campanian and some through the
Maastrichtian, this without a well-represented selection of bird-laden
beds to compare to. So I don't think the evidence currently favors
pterosaur/bird competition to explain absence of diversity in either
group, or the forced extinction of one or some groups. One could easily
attempt to use this diversity through the Cretaceous as a means to explain
enantiornithine extinction, as much as pterosaur extinction, prior to the
K/T event.

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

__________________________________
Do you Yahoo!?
Yahoo! SiteBuilder - Free web site building tool. Try it!
http://webhosting.yahoo.com/ps/sb/