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Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE



Matt Troutman: 4/10/98:

>Another weakness of your hypothesis is its interpretation of secondary
flightlessness in maniraptoriforms. A far better interpretation is that
they are avian ancestors not avian descendents.
<

Just for a moment considering the chronological order of appearance in the
fossil record (I?m referring to the non-pre-Archaeopteryx appearance of
maniraptorans), how many paradoxical instances make a piece of evidence?

Jurassic fossils are rarer than Cretaceous fossils; it is also
hypothetically possible that if we assume maniraptorans gave rise to birds,
it may have been early in maniraptoran existence (though the more this is
true, the weaker the case ? see below).

(By chance, only a small fraction of the evidence appears between
Archaeopteryx and the end of the J (assuming the China stuff is K) so the
principle needs only slight re-phrasing to refer to either ?Pre-Archae? or
?Pre-K?).

Say the ratio of maniraptorans living in the K to those living in the J was
A:B;
Say the ratio of the chances of an indiv living in the K ever being found to
that of one in the J is C:D;
The ratio of the chances of a maniraptoran we have found coming from the K
to from the J is (A/B)*(C/D).
The probability of any maniraptoran we have found coming from the K is  1/(1
+ 1/ ((A/B) * (C/D)) ).

The chance of N maniraptoran fossils all appearing after Archaeopteryx is
this expression raised to the power of N.

It is interesting to plug various values into this:

A/B=3.01; C/D=2; N=30;  gives a probability of 0.01  .   Very unlikely, and
bad news for BADD.

However, perhaps a more friendly set of inputs would be if K mani?s were 100
times as numerous as J ones, and the the K was 5 times as fossiliferous as
the J:

A/B=100;  C/D=5;  N=30;  gives a probability of 0.941  . Fine for BADD ?
sort of.  However this combination assumes maniraptorans hardly existed
pre-Archaeopteryx!!  And maybe N should be much bigger than 30.

Perhaps the hypothetical pre-Archae maniraptorans were difficult to find
because they were very small, and/or lived in conditions unhelpful for
fossilisation ? they were living in forest trees for example. . . !!!
Whatever assumptions we make, a BCF-type explanation is the only answer,
using these probability calculations.  Maniraptorans therefore did not give
rise to birds since they all postdate the first bird ? unless they were
effectively early birds already.

To return to the original quote, a far better interpretation is that
maniraptorans *are* avian descendents (though not necessarily pterosaur
descendents).

There have been claims of pre Archae. Jurassic maniraptorans, largely in the
form of teeth, often unusually small.  You may call them maniraptoran but
they will probably be small aerial forms, or larger non-arborial descendents
of them.

JJ