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Re: ABSRD BAND on Sinornithosaurus feathers

Dinogeorge@aol.com wrote:

<< If every stage in the
 evolution of a morphological structure has to be adaptive, then it is
 difficult to fathom how a simple hollow cylinder or a pine needle-like
 branching structure can be functional in an animal. >>

As usual, they overlook the obvious solution to this conundrum: the feathers
of Sinosauropteryx and some of the other Liaoning theropods are derived from
more birdlike feathers such as those seen in the earlier Archaeopteryx.

You could be right. Although personally, I think the feathers of _Sinosauropteryx_ and _Sinornithosaurus_ probably represent the stage of feathers were at (and continued to be, alongside the flight feathers) before they became used for an aerodynamic purpose.

The functional/ecological pathway for the evolution of feathers may have been:

Insulation --> Drag (for controlled descents) --> Drag + Lift + Thrust (for powered flight)

_Sinosauropteryx_, _Sinornithosaurus_, and _Beipiaosaurus_ examplify the first stage. (_Sinornithosaurus_'s ancestors, if semi-arboreal, may have used their feathers for drag as well. But I like the idea that _Sinornithosaurus_ may have been partly arboreal too - those long arms, with a trailing edge of feathers, may have been excellent for co-ordinated branch-to-ground leaping).

Drag (for controlled descents) may have been an additional purpose for the feathers small, semi-arboreal theropods like _Microraptor_. The more elaborate, pennaceous (but symmetrical) feathers on the hands and tail of _Caudipteryx_ (and the tail of _Protarchaeopteryx_) represent the effect of selection for increased drag (and therefore maneuverability) on the evolution of feathers. Lift only became important later in evolution.

The feathers of _Archaeopteryx_ represents a stage in which the feathers on the wings took on a function as generators of lift and thrust (the two evolving concomitantly, on the proximal and distal part respectively of the expanding wing). The long tail, though, was necessary as a balancing organ during running take-offs from the ground (sensu Burgers and Chiappe). The long tail may also have been retained for its use as a drag-inducing device, if _Archaeopteryx_ still employed the branch-to-ground predatory mode of its non-volant ancestors.

Unlike _Archaeopteryx_, the early evolution of modern birds focussed on increased locomotive efficiency in the air. The tail got shorter. The manus lost its prehensile and raptorial function - manipulation of prey might sully the wings, and protecting the integrity of the wing became of paramount importance to metornithines. The teeth were lost (to reduce weight?). Fusions abounded throughout the skeleton for increase skeletal support.

_Archaeopteryx_ only looks primitive by comparison with modern birds. It has the skeleton of a theropod, but the primaries, secondaries and retrices of a modern bird. A lot of people (respected scientists among them) view Archie as a stage of bird evolution in which the skeleton had yet to "catch-up" with the plumage. But Archie's mosaic morphology is best explained by the fact that it was adaptively advantageous to retain the predatory and cursorial abilities of its maniraptoran ancestors. Maniraptorans were the consummate predators of the Mesozoic. Why mess with success? Keep the enhanced predatory capabilities of maniraptorans, and evolve flight to increase the animal's versatility as a predator (e.g. to swoop down from the trees onto prey below; increase the theropod's hunting range; escape from larger predators by flying away.)

The success of the _Archaeopteryx_ "grade" of evolution is shown by the record of very similar birds in the Cretaceous - _Proornis_ and _Rahonavis_ (which lived close to the end of the Mesozoic). Basal avialans didn't go kaput at the end of the Jurassic; in the Cretaceous they lived alongside metornithine (non-basal) avians that sacrificed certain features (retained in Archie and kin) for improved aerodynamic efficiency. Metornithines took a punt that clawed hands, a long tail and long legs could all be dispensed with for the sake of improved flight performance - and it paid off. _Archaeopteryx_ / _Proornis_ / _Rahonavis_ took a different route.



Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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