[no subject]

["Tyrberg Tommy" <tommy.tyrberg@aerotechtelub.se>]

On Tue, 18 Dec 2001, John Bois wrote: 
The article makes a good case for pre K/T diversity of neornithes. I
take issue with the bipolar division proposed for birds at the K/T.
Cracraft is saying that the   bolide hit affected North Hemisphere and
that since neornithes were largely Gondwana species, and since the
Southern hemisphere wasn't hit as hard, they inherited the Earth (rather
than enantiornithes). My difficulty is that flight enhances dispersal.
This makes it likely that succesful usurping of Southern niches should
be followed by the same for northern niches. I know the fossil record
doesn't indicate this yet. But, if and when it does, it will also
indicate that neornithes were more effective competitors, not the lucky
survivors of a fluke event. In any case, why, if a considerable
diversity of birds survived in the south, didn't some of their non avian
brethren share their good fortune. I mean, presumably there is a great
range of tolerance among the putative bird survivors; and certainly a
similar range existed for non avian dinosaurs--was there no overlap, or
was extinction event so precise so as to intersect the precise division
between the two taxa--that is a surgical strike, indeed! 

Things aren't quite that simple. Even rather narrow water gaps are
actually rather strong barriers to bird dispersal, especially for
landbirds. The avifauna on either side of Wallace's line or the
Mocambique channel are very different. Note for example that no
woodpecker has yet made it to either Australia or Madagascar despite
ample empty niche space. Even more striking is how few successful
dispersals have occurred from Africa to Madagascar in 65 million years
considering that the Mocambique channel is not very wide and has a
considerable number of islands. The avifaunas of North and South America
are also quite distinctive and originally northeren and southern
elements are easily identifiable, even after a few million years
admixture. 
Also in the Maastrichtian the continents were actually unusually
isolated. New Zealand/New Caledonia, Madagascar and India seem to have
been completely isolated. South America/West Antarctica/East
Antarctica/Australia were probably only connected by isthmian links
which would have acted as filters. Africa was apparently only weakly and
intermittently in contact with Laurasia and the same seems to be true
for South America. Eurasia and America were in firm contact, but only at
high latitudes while Europe was an archipelago. Altogether there would
seem to have been excellent opportunities for landbirds to differentiate
on the different continental blocks.
Actually there are some evidence for Late Cretaceous neornithines in the
north. In addition to the "transitional charadriiforms" both
presbyornithids and lithornithids have been reported, though not well
published as yet. If the identifications are correct this implies that
palaeognaths and galloanseres had a cosmopolitan distribution in the
Maastrichtian.
On the other hand a Gondwanan origin for the passeriforms and the
"higher non-passeriforms" seems eminently probable based both on modern
distribution and fossil evidence. Indeed the only reasonable way to
explain the peculiar extant distribution of the main groups of suboscine
and oscine passerines seems to be in terms of differentiation on the
various Gondwanan blocks and repeated dispersal events (probably no more
than three or four) with subsequent differentiation in the north. 
These would be:
Old world suboscines quite early in the Tertiary (either from Madagascar
to the mainland, or by rafting north on Greater India)
Probably two dispersals of oscines from Australia to Laurasia (in the
Eocene and Oligocene?) (there has been some additional crossings in both
directions during the Pliocene/Pleistocene)
A few South American suboscines into North America from the Pliocene
(less important)
As for the survival of birds at the K/T,  the fossil record is
compatible with a scenario where _no_ birds, either enantiornithine or
neornithine survived in the north and where only a few (perhaps no more
than 10-15 species) of neornithines survived at middle to high latitudes
in the south. Note that there was definitely enanthiornithines in the
south as well. They are well attested for South America and the
Maastrichtian birds from Madagascar (Vorona and Rahonavis) are anything
but neornithine. 
As for why only neornithines survived there is evidence that
enantiornithines were at least partly poikilothermic and this may have
prevented them from colonizing high latitudes, at least in winter
(Chixculub probably happened during northern summer).

Tommy Tyrberg