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Re: Vegavis gen. nov. - new anseriform in today's Nature



On Fri, Jan 21, 2005 at 11:34:25AM -0500, John Bois scripsit:
> On Thu, 20 Jan 2005, Graydon wrote:
> 
> > On Thu, Jan 20, 2005 at 02:13:07PM -0500, John Bois scripsit: [snip]
> > > Birds should have been affected as badly as non-avian dinosaurs.
> > > The more species we find that are ancestral to extant species,
> >
> > There's a slippery bit of logic in there -- recent findings are not
> > of ancestry; they're of common clade membership.  'Something like
> > this lived in enough numbers to continue to breed'.
> 
> Right, but it does show that the ancestors of ducks were present, that
> the genes in them survived into modern ducks. 

Doesn't show that at all; it shows that the point of common ancestry
between modern ducks and their next closest non-duck relative is back in
the K somewhere.

It doesn't tell us what proportion of the descent of that ur-duck made
it over the boundary, nor do we have any way to put the known fossils
into the pathway between the hypothesized ur-duck and a modern duck.

> If substantiated, it also means that the chicken/duck split was
> pre-K/T--mass survival.

Sure.

But you're looking at this as though there was no loss of diversity, and
we can be sure there was a substantial one, both through finding
Cretaceous birds that don't have post-K/T descendants, on the one hand,
and through noting that the extant neornithine bird clades represent a
small fraction -- and not the most populous fraction in the Cretaceous
-- of the Cretaceous bird diversity.

Rather like the extant mammal clades represent a small fraction and not
the most numerous fraction of the Cretaceous mammal clades.  Placentals
aren't what an alien observer at 70 MYbp would have expected to be the
dominant form of large terrestrial life by the time of our present.

> > The evidence for the catastrophe is irrefutable -- dead ocean marine
> > sediment layers, pan-planetary iridium layer at the K/T boundary,
> > ash-and-fern layers at the boundary, and a mass extinction.
> 
> And I'm not trying to refute it.  But I remain skeptical of the power
> attributed to it.  More to the point, to the extent that it spared
> birds, it probably spared some non-avian dinosaurs in some nooks and
> crannies.

I think you've got 'spared birds' where you should have 'a few bird
species survived'.  There was a massive loss of bird diversity.

> Also, if a diverse fauna of neornithines is established before K/T, we
> need a mechanism for killing off enantiornithines.  A good contender
> for their demise is compettion/predation with neornithines.  A bad
> contender is an all-encompassing instantaneous event--that is, unless
> someone is proposing a differential selective mechanism.

People are generally proposing 'dumb luck'; a very few neornithine
species made it over the boundary, along with maybe *one* Gondwanan
ratite species.  (Or maybe more, but not a dozen.)

Every available evidence from the Cretaceous indicates that neornithines
were not ecologically dominant at that time.  Maybe some small portion
of the then-extant neornithine species were more suited to survive, or
maybe they just got lucky.  It's important to remember that everything
to do with a mass extinction event does not have a neat casual reason;
dumb luck *matters*.

> > Why you think some small, flying, opportunistic feeders -- do you have
> > any idea what ducks will eat? -- surviving makes the mass extinction
> > less encompassing I don't understand.  Some things survived; they tended
> > to be -- among automatic endotherms -- small, opportunistic feeders with
> > some means of avoiding dire conditions.  (Flight or (probable)
> > hibernation plus burrowing.)
> 
> Flying didn't help pterosaurs,

Motor-gliders, rather than continuous flappers, ill equipped to cope
with a month of global storms.

> nor enantiornithines; nor would it protect any organism from the
> proposed conditions e.g., heat radiation from re-entering ejecta.

Hence the 'dumb luck' part.  Sometimes, something would be diving, or in
a wet marsh, or behind a big tree and then escape the resulting fire.

It would then have to find food and a mate and food for offspring, and
the dire conditions in which to do so would last for a long time;
hundreds of generations.

> And why would one exempt na dinosaurs (especially juveniles) from
> opportunistic feeding.

Because the dire ecological consequences of the event lasted for, at a
minimum, a thousand years.  Juvenile non-avian dinosaurs would have to
reach sexual maturity in a landscape devoid of sufficient food to do so.

Note that *nothing* terrestrial with an adult body mass over 10 kg made
it.  That says a great deal about food availability.

> And it is not just ducks, but chickens now (whatever they were in the
> Cretaceous).

More or less what they are now -- jungle fowl.  And even modern domestic
chickens will eat anything, up to and including attempts on the toes of
anyone so incautious as to wear sandals near them.

Also note that it's plenty if *one* proto-chicken and *one* proto-goose
species make it over the boundary, to explain the present observed
diversity.