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Re: "Feathery fossil shows birds aren't dinosaurs"



In a message dated 6/23/00 8:29:26 AM EST, tholtz@geol.umd.edu 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.