My comments on pterosaur extinction

["David Marjanovic" <David.Marjanovic@gmx.at>]

Competition between Maastrichtian pterosaurs and birds is very hard to
imagine. The known pterosaurs of that time were all enormous soarers,
occupying the niche of overgrown albatrosses. No known birds were in this
niche. Therefore competition for nesting sites is equally difficult to
imagine -- albatrosses don't nest where everyone else does, and I can't
imagine a 12-m-wingspan pterosaur (or one half that size) nesting in a tree
or trying to hide a nest on a shore, things that lots of (small) birds do
today. I'm pretty sure (ha, ha, speculation) that if pterosaurs would have
survived the K-T impact (forget the volcanoes -- see below), they would
still soar above the oceans, and birds would occupy all niches they have
today, except that of albatrosses and suchlike.

Competition of prey... Ambiortimorphae (like *Ichthyornis*) and
Hesperornithiformes didn't survive, neither did they increase in numbers
during the Maastrichtian AFAIK. "Transitional shorebirds" can't have
competed with azhdarchids IMHO, and if we believe our current knowledge of
the fossil record, they were extremely rare.

Competition as a cause for background extinction may be quite common (I
rather doubt it, but it has occurred), but IMHO it is totally unsuitable as
a cause for any mass extinction. Michael J. Benton has shown that in the
Triassic, rhynchosaurs and "thecodonts" did not outcompete "therapsids", and
dinosaurs did not outcompete rhynchosaurs and "thecodonts", rather there
were mass extinctions at the end of the Carnian, at the end of the Triassic
(a chain of craters is known for this one), and before (end of mid-Triassic?
forgot), and each time another group was faster to occupy the empty
ecological niches. The same seems to hold for the K-T -- nonavian dinosaurs
lived right up to the boundary (a *Triceratops* skull [too big to be
reworked] is known from 1.8 m below, hadrosaur footprints [can't be
reworked] are known from 37 cm below), and they did not decline before. For
pterosaurs, the fossil record is even more patchy, but AFAIK the
azhdarchids, if nothing else, survived quite well up to the boundary. And
then disaster stroke, with a plasma jet blowing over North America, an
earthquake of magnitude 25 (Richter scale), global hurricanes, global
wildfires, acid rain, darkness, cold, greenhouse effect and whatnot.

The respiratory system of pterosaurs seems to have been at avian level. All
their bones are paper-thin and full of air. Peter Wellnhofer calls them
"materialized force lines". Air sacs seem to be an ornithodiran synapomorphy
(while the liver-pump system could be crurotarsan or plesiomorphic for
archosaurs or whatever). Changes in air density and oxygen content have
often been advocated as causes for mass extinction; while it is easy to
imagine the global wildfires to consume lots of oxygen, long-term
developments were otherwise, AFAIK state of the art is that oxygen levels
rose in the middle Jurassic and went back to "normal" in the Eocene or
Oligocene or so. And of course this won't explain the survival of mammals,
lizards, amphibians and so on.

The volcanoes -- well. There's a wonderful book from I think 1998, called
Night Comes To the Cretaceous, which says that in the Deccan, the
iridium-containing K-T boundary layer lies in the middle of a sedimentary
layer between two basalt beds, and the dinosaur fossils continue right up to
it and stop there. There were no eruptions tens of thousands of years before
and after the boundary. While global climate and suchlike was probably
affected by the volcanoes, they apparently didn't cause the K-T mass
extinction. _Basta_. =8-)

Someone wrote we have enlarged cerebella just to stay upright. In fact, they
are smaller than expected for an ape of our size -- climbing seems to
require more RAM.

BTW, off-topic: The 6-million-year-old human has been named *Millennium
ancestor* (I hope they wrote it with nn in the original...). Apparently,
there are other people than HP Ken Kinman who think this find is Most
Important Of The Millennium.