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Re: Bird origins
Félix Landry wrote:
> 3) They were IN the transition and they became always more
>birdlike, and one day they became ______! (fill the blank). This hypothesis
>has much stratigraphic problems, so I don't buy it.
Hmm? If the transition beneficial, why could not stages within the
transition be beneficial? If stages in the transition are beneficial, why
could not animals "part-way" through the transition radiate to new taxa?
Remember that these transitions are not always "transitions" at the
time that they are made. Evolution does not start with a basal
maniraptorform and say "now lets evolve flight". We observe what we consider
the "starting point" and the "ending point", cursorial non-avian theropod
and flying bird respectively, and we project the idea of a transition on
that. But "transition" implies a deliberate direction. While there may have
been a direction at some points (e.g. once you're flying, I have little
doubt there is storng selection pressure to fly better), the change does not
necessarily have to have any inherent "direction" or "purpose" (upon that
road lies orthogenisis).
It is just as simple, and probably makes more sense, to consider the
road to flight a series of innovations and minor radiations. A species
evolves which possesses some great new adaptation, is successful. Successful
species may spawn a small radiation. A daughter species comes up with
another great adaptation, and we start again. For whatever reasons, these
adaptations would eventually be exapted for flight. However, just because
members of the earlier radiations were not flying does not mena they were
So what we appear to be seeing with fellas like _Caudipteryx_,
_Protarchaeopteryx_ and _Unenlagia_ (O.K., O.K. brain trust, here's where
you get to taunt us all with the fifty new "dino-birds" that you know about
and we don't). These may have been parts of successful pre-flight
radiations. It is then not too surprizing that they survived to the
Cretaceous. Heck, look at all the other "transitional" forms, like
dromaeosaurs, troodontids, and oviraptorsaurs, which survived to the Late K.
Of course, those groups have a better representation in the fossil record.
On the other hand, the fossil record for terrestrial vertebrates *is*
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"...To fight legends." - Kosh Naranek