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Re: K-T mammals



----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Friday, May 10, 2002 6:15 AM

> Perhaps more true than you know.  It is very possible that
> some/many/most/all modern orders made it through.

This implies that the fossil record of even the best localities is very,
very bad. Maybe it is... but some molecular clock estimates would cause me
to expect pretty modern placentals and birds in the Yixian Formation,
instead of (or at least in addition to) *Eomaia* and *Yanornis*. If those
are found -- considering the excellent preservation there, I think a very
large percentage of all species that lived there are preserved -- then I'll
obviously consider myself corrected. Too bad we don't have more such
localities... LK Antarctica would be very interesting for birds. :-)

> The molecular studies
> say most modern orders diverged well before the K/T.

At least some such studies rely on wildly guessed calibration points, and
all suffer from impressive error margins. I've written a long mail
concerning the problematic dates in

Tara Paton, Oliver Haddrath & Allan J. Baker: Complete mitochondrial DNA
genome sequences show that modern birds are not descended from transitional
shorebirds, Proc. R. Soc. Lond. B 269, 839 -- 846 (published online 25 March
2002)

to the Killed Threads list -- because pretty long and rather off-topic, I'll
send it offlist to everyone who wants. (Just the most extreme guess -- the
oldest emu is 25 Ma old, _therefore_ -- they don't mention any other
reason!!! -- they fix the emu-cassowary divergence at 35 Ma and calibrate
their tree with it. They assume the bird-croc divergence was 245 Ma ago, but
then that weird turtle pretends to be the sister group of the crocs and to
have diverged from them only 225 Ma ago.

> A study by Foote _et al_ in Science a while ago (ref. if
> needed) found that it was unlikely that gaps in the record meant missing
> fossils.  A new study in _Nature_ about a month ago found that if you
> include the likely number of missing species (i.e., we know that only a
> small proportion of species actually fossilize) then the
> (statistical) fossil record of mammals is pushed back to agree with
> molecular data (I'd love to hear informed opinion on this study).

I have yet to find these papers... I think I've copied at least one, so it
should lie next to me :-]
        Do these papers take mass extinctions into account at all? IIRC they
assume _constant diversification_, which is a very simplistic extrapolation
IMHO. How can we know the percentage of species that have fossilized (and
have been found [and published])?

> Add to
> this the claim of Archibald that most placental
> "extinction" is actually pseudoextinction--the result of rapid
> speciation--then you have a pretty good claim for saying that mammals did
> not have such a bad day.

Then you still talk exclusively about the Eutheria of North America, not all
mammals of the world. (Archibald himself talks much about the great
reduction of metatherian diversity in the book you probably mention.)
Mammals are expected to fare better on average anyhow in a mass extinction,
being small and independent of green plant matter for months. (And if I
don't interpret too much into the cladogram in the *Eomaia* paper, then
maybe none of those -- Leptictida and Palaeoryctidae -- are in the
crown-group Placentalia, at least the crown-group Laurasiatheria, and left
any descendants beyond the Oligocene.)

> And just to throw one more log on the fire, an article in the latest
> _Discover_ mag is written by Horner's cowriter, Edwin Dobb, about
> Horner and Clemens finding that dinosaurs were in big
> trouble measurably _before_ the event.  I know it's not a peer reviewed
> paper...

Either they have some brand new evidence that contradicts the last few years
in their entirety, or Dobb is talking about what happened 10 or 15 years
ago, which I doubt... ~:-|
        The last hadrosaur footprint occurs 37 cm below the boundary layer,
anyway. A bit to the east, the last ammonite is within the last 10 cm.