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Re: "Feathery fossil shows birds aren't dinosaurs"
In a message dated 6/23/00 8:29:26 AM EST, email@example.com writes:
<< Despite George's impulsive outbursts (which I will be charitable and say
are due to his strong attachment to his own hypothesis), the fact that I
described _Longisquama_ as not being an archosauromorph (much less a
theropod!!!) has nothing to due with my position on the origin of birds, and
everything to do with the anatomy of the critter itself. >>
First of all, let me say that I couldn't be more pleased that people are
beginning to take a closer, hard look at Longisquama, regardless of whether
it was a primitive bird or not. It would be nice for BCF if it were, but BCF
is a very strong hypothesis that doesn't stand or fall on the basis of a
couple of specimens. I'm attached to it because it explains the origin of
avian flight straightforwardly and in a unified way, without recourse to
miracles or other wildly unlikely events, unlike certain other hypotheses
I've read about.
The ornithologists' position that birds are not dinosaurs is flatly
contradicted by current cladistic analysis; if birds and dinosaurs developed
all those shared characters independently and not from common ancestry, this
would qualify as a miracle. The cladists' position that birds did not evolve
from arboreal archosaurs (if that is still their position) is contradicted by
physics. Where would a ground-dwelling runner without wings obtain the energy
for sustained flight?
Cladistic analysis tells us that birds and dinosaurs are related, but in the
absence of a fossil record it leaves the problem of figuring out what their
common ancestors might have been like up to us. To this end, we must bring to
bear everything we know about the physics of flight in addition to everything
that cladistics tells us. For example, we know, even without having specimens
of them, that the first archosaurs were probably small, lizardlike
animals--because this is the kind of animal that populates the nearest sister
group of the archosaurs, prolacertiforms. We know that many modern small
lizards are adept tree climbers, and we have specimens of gliding forms that
must have been tree-dwellers among the prolacertiforms, so it is not too big
a leap to expect that there were small tree-climbing forms among the early
archosaurs also. Trees were then and are now an abundant natural habitat
freely available to any animals able to take advantage of them.
Now, if the earliest archosaurs include small, arboreal or acronomic forms,
and modern birds include small, arboreal and acronomic forms, what do we need
large, cursorial, ground-dwelling dinosaurs for? BCF proposes that birds
started out as small, arboreal archosaurs and basically stayed that way
throughout their evolution, every so often throwing off side branches and
lineages of other kinds of archosaurs, some of which evolved into giant
forms. When a significant improvement of some kind appears in a species (or
when a species becomes well established in a new niche), this typically gives
rise to a little burst or radiation of species that exploit this improvement
in various ways, sometimes crowding out the predecessor "unimproved" forms.
This probably happened frequently in the lineage that led from the earliest
archosaurs to modern birds (a lineage that cladistics tells us must exist).
By looking at the clades of theropods up this "theropod ladder of being," we
can even crudely reconstruct what some of these incremental improvements were.
The earliest theropods (herrerasaurs and such) retain the broad, four-toed
ancestral dinosaur foot with hallux articulating proximally at the tarsus.
Then ceratosaurs appear, already in the Triassic, with the hallux
articulating distally on the second metatarsus. We can see how an opposable
hallux like this would have been very useful to an animal that climbed in
trees (ceratosaurs also had an opposable pollex on the hands) but is simply
useless to a large, cursorial form such as Coelophysis (large with respect to
birds, that is). Cursorial forms tend to lose their outer digits through
vestigialization; they don't modify them in the highly specific way found in
theropods. The perching foot, developing in conjunction with an increasingly
winglike but still grasping forelimb, would have been a significant
anatomical improvement for any small, arboreal archosaur that required ever
better control over its movements. The strangely positioned hallux of
theropods is a vestige of the opposable hallux of arboreal dinobirds; it
didn't appear miraculously in theropods so that it could one day serve as an
opposable digit for those theropods that became birds.
Next after ceratosaurus on the theropod ladder are tetanurans, characterized
by (among other things) stiffened tails. Imagine how useful a stiffened tail,
fringed with a vane of tail feathers, would be to an animal that leaps from
branch to branch, providing both lift and directional control. More control
would be provided by vanes of feathers on the forelimbs. Incremental
improvements of the forelimbs eventually resulted in maniraptoran theropods,
the forelimbs of whose large, flightless forms are so winglike that they
practically scream that they were once real wings, foldable away when not in
use. Thence follow modern birds.
BCF meets the big problem the ornithologists have with cladistics head on: If
birds are dinosaur descendants, why are all the most birdlike dinosaurs in
the fossil record >after< Archaeopteryx? The answer is that these are
ground-dwelling, flightless maniraptorans, just as ostriches are flightless,
ground-dwelling paleognath birds. There is no conflict or contradiction
between BCF and the fossil record.