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Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)




A few words regarding this confused babble...

The model of feather development proposed by Prum, and illustrated
in fig. 6 of this paper, also makes no adaptive sense. 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.

And again:

It is therefore
foolhardy to fly with wings made from interelocking filaments, not knowing a priori whether the interlocking structures are strong enough or structurally sound enough to serve functionally as wings.

OK, I think I know what they're getting at. Suchs assertions (which come thick and fast from the ABSRD/BAND camp) stem from the misguided premise that the purpose of feathers FROM THE VERY BEGINNING was FLIGHT. In other words, feathers first evolved as aerodynamic structures, and every modification thereafter improved the ability of the feather (and, by extension, the wing composed of these feathers) to provide the animal with lift.


Well, not necessarily. Most likely, not at all. The branching structures we in _Sinornithosaurus_, _Microraptor_ etc may have initially served in some function that had nothing to do with aerial locomotion: thermoregulation or display have been suggested. (By Richard Prum, among others. Gosh, how 'bout that.) These branching, non-aerodynamic feathers may have served as wonderful heat-trapping structures. There's a novel idea!

The function of incipient structures is something that confounded Darwin, and it still bedevils evolutionary biologists and paleontologists today. It doesn't befuddle scientists as much as it used to though. The notion of "exaptation" - that anatomical structures are evolved for one purpose, and later in evolution are selected for a totally different purpose - has been around for quite a while. A certain property of the structure in question pre-adapts this structure to performing another function. (If memory serves, Gould and Vrba actually coined the term "exaptation" in 1982; but the very concept is older than that).

I like the notion that the feathers evolved first for insulation, then took on a function in promoting drag in small facultatively arboreal theropods that leaped from tree branches onto prey below. Essentially this is the "Pouncing Proavis" model of Garner, Taylor and Thomas (except that it makes better sense to put the proavian up in the branches of trees, like _Microraptor_, rather than having to jump from rocks.) The original purpose of the "proto-wing" was drag. Having drag-inducing devices positioned far from the body's center of gravity (such as on the hands and the end of the tail) allows improved maneuverability. This is an excellent adaptation for a leaping predator which first sights the prey from the vantage of a tree branch, then endeavors to (1) take prey by surprise and (2) land as close as possible to the prey.

Here we come to another flaw in the Feducciary model. ABSRD/BAND proponents are certain that the ancestors of birds were gliders. Gliders come as two types. In the first type, the aim is to increase lift (i.e. high lift-to-drag ratio) by developing a wide lift surface across the body. This is what we see in modern gliding mammals - such as "flying" squirrels, "flying" possums (phalangers), and "flying lemurs (the colugo - not a true lemur), which have a patagium stretched between the fore- and hindlimbs contiguous with the body wall. The added lift allows these animals to extend the duration of aerial leaps - useful for getting from one tree to the next by avoiding the ground below.

In these creatures, gliding efficiency (measured as the horizontal distance travelled) is increased. This is probably the rationale behind the putative gliding structures of an array of arboreal Permo-Triassic reptiles: kuehneosaurids, coelurosauravids, _Sharovipteryx_, and perhaps _Longisquama_ (assuming the appendages were paired and could be spread out laterally to catch the air). In all these creatures the gliding surface extends from the body. (_Megalancosaurus_ was probably not a glider, as Ruben has claimed - see Silvio Renesto's work on this genus.) ABSRD/BAND supporters are currently scouring the Permian and Triassic fossil record for gliding reptiles that show such adaptations. Ignoring (for a moment) that there is overwhelming support for the origin of birds from among theropods, their quest is doomed to failure on biomechanical grounds as well. I'll mention why later.

The other type of gliding is called "gliding" but strictly speaking it isn't true gliding. It's often referred to as "parachuting" or a "controlled fall". The aim is to increase drag, not lift (although this may become an important component later). By having drag-creating surfaces, the animal improves its maneuverability in descents from an elevated surface (such as a tree branch) to a lower surface (such as the ground). In parachuters, the drag-creating surface is positioned FAR FROM THE BODY. The maximizes maneuverability in the air, but results in poor gliding efficiency.

This is what "flying" frogs do (e.g. _Rhacophorus_). Contrary to what I have read in at least one book, the webbed hands and feet of flying frogs do not act as gliding surfaces. When jumping from a tree branch, the webbed hands and feet DO NOT increase the horizontal distance travelled in the air by the frog. In fact, the webbed surfaces may actually decrease gliding efficiency. (This has been determined experimentally using both models and living frogs - and both "flying" frogs and closely related non-flying frogs). The webbed surfaces, which are positioned far from the body, promote drag, allowing greater maneuverability during descents (good for dodging leaves and tree branches on the way down) and precision in landing.

Sifakas (_Propithecus_) are arboreal mammals (lemurid primates) have a mat of hair on the trailing edge of their arms, which may serve a similar purpose when jumping from tree to tree. (Feduccia has discussed these as a modern analog for a proavian - not realising that it undermines his arboreal glider model for the origin of flight in birds.) Interestingly, in both flying frogs and sifakas there are NO obvious SKELETAL adaptations for this lifestyle. The drag-creating surfaces are developed entirely from the integumentary structures: skin or hair. This is why the notion of looking for Permo-Triassic reptiles that show both parachuting adaptations and bird-like characters is so wrong-headed (apart from the 99 other reasons).

Let's look at _Archaeopteryx_. It shows primaries on the manus, secondaries on the ulna, but no remiges on the humerus. No tertiaries have been identified in any _Archaeopteryx_ specimen: the inner arm was devoid of flight feathers. Also its tail is long and covered in a pairwise arrangement of retrices. Although the tail feathers are asymmetrical, the long wide tail is not much good for generating lift. Excellent for creating drag though.

Let's look at _Caudipteryx_. It has primaries on the manus, but no pennaceous feathers (remiges) on the forearm or humerus. It has retrices (paired and laterally-splayed as in _Archaeopteryx), but these are restricted to the distal half of the tail. What's more, the symmetrical feathers are excellent for producing drag when oriented perpendicular to the air flow (as they would be during leaps to the ground).

_Protarchaeopteryx_ also has retrices at the end of the tail. The forelimbs do not show evidence of feathers though (I'm not certain if this is due to absence in the living animal, or lack of preservation.)

The adaptive advantage of stiff, vaned, non-asymmetrical feathers is clear: drag and consequent maneuverability. This was the primordial force which eventually led to powered flight. This explains why the incipient flight surface (evident in the "proto-wings" of _Caudipteryx_) evolved distally to proximally in both the forelimb and tail. The incipient flight surface did not first appear close to the body wall (at the armpit) nor equally along the entire length of the forelimb+manus - the very arrangements one would expect in a glider trying to maximize lift.

_Microraptor_ shows us that small maniraptorans (deinonychosaurs, no less) could climb trees. They may not have spent their entire time there: they were facultatively arboreal, comfortable both in trees and on the ground. Same for _Archaeopteryx_. The above model for the origin of avian flight is much like that proposed by Sankar Chatterjee in _The Age of Birds_, without the incipient lift surfaces or the unnecessary diving stage.

_Caudipteryx_ may not have been at all arboreal, but its ancestors probably were. Basal oviraptorosaurs and basal deinonychosaurs (like _Microraptor_) could have been sem-arboreal; Sereno gives the Oviraptorosauria and Deinonychosauria as sister groups, so the common ancestor may have been scansorial and at least partly aboreal. The obvious terrestriality of _Caudipteryx_ and oviraptorids, and troodontids (assuming a monophyletic Deinonychosauria) and velociraptorines, would be secondary under this scenario. Chatterjee also suggests that the opisthopubic pelvis and semilunate carpal may also be scansorial/arboreal adaptations; both ideas deserve further investigation. (I prefer to regard the semilunate carpal as a pre-adaptation for tree-climbing.) With this scenario in mind, I would like to take a closer look at the claw geometry of _Microvenator_.

Further, the major anatomical innovations between basal paravians (of Sereno) or eumaniraptorans (of Holtz) and _Archaeopteryx_ probably occurred entirely in the integument. There is not much difference between the skeletons of _Sinornithosaurus_ and _Archaeopteryx_, but I'm sure a helluvalot of change occurred in the arrangement and structure of the feathers.

Finally:

It is quite obvious that the cladists are influenced primarily by their cladograms, rather than by the structural similarities between avian feathers and Longisquama feathers.

Guilty as charged. What audacity paleontologists have, claiming birds evolved from theropods simply because they're closely related. Cheek!


Anyway, that's my thoughts. Apologies for the long-winded post - considerably more than 2c worth.


Tim

------------------------------------------------------------

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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