This guy's an idiot who hasn't even bothered to *look* at the available fossils before spewing his propaganda. But then that's par for the course for anti-evolution creationists -- I suppose it's because people who actually care to look at the evidence don't stay anti-evolution creationists for long. (And what's your excuse for not knowing any better than to parrot his junk? If you don't know enough about the field to personally be able to verify what you post -- then don't post it.)
So "the attempts to find any transitional forms have all failed", eh? What a moron:
He's *also* an idiot when he writes, "Archaeopteryx has been shown to have fully developed flight feathers (thus, no half-bird)". Yes, Archaeopteryx had fully developed flight feathers (although *not* configured as well as in modern birds), HOWEVER, McIntosh sort of "forgets" to mention that what makes Archaeopteryx a "half bird (and half reptile)" in a sense is the fact that while it had feathers like birds, many of its *other* features are clearly reptilian (and other features are birdlike). This is *exactly* the kind of mosaic of "mix-and-match" features that one would expect to find in a transitional form during the period when one was evolving into the other. But the creationists close their eyes, point at just the feathers, say "must be just a bird, nothing to see here", and then cover their ears and sing "la la la I can't hear you"...[from a previous post of mine:] The cladogram for the evolution of flight looks like this:
(Note -- each name along the top is a known transitional fossil; and those aren't all that have been discovered.) Here's a more detailed look at the middle section:
Fossils discovered in the past ten years in China have answered most of the "which came first" questions about the evolution of birds from dinosaurs.
We now know that downy feathers came first, as seen in this fossil of Sinosauropteryx:
That's a close-up of downy plumage along the backbone. Here's a shot of an entire fossil
Sinosauropteryx was reptilian in every way, not counting the feathers. It had short forelimbs, and the feathers were all the same size. Presumably, the downy feathers evolved from scales driven by a need for bodily insulation.
Next came Protarchaeopteryx:
It had long arms, broad "hands", and long claws:
Apparently this species was driven by selection to develop more efficient limbs for grasping prey. One of the interesting things about this species is that the structure of the forelimb has been refined to be quite efficient at sweeping out quickly to grab prey, snap the hands together, then draw them back towards the body (mouth?). The specific structures in question are the semilunate carpal (a wrist bone), that moves with the hand in a broad, flat, 190 degree arc, heavy chest muscles, bones of the arm which link together with the wrist so as to force the grasping hands to spread out toward the prey during the forestroke and fold in on the prey during the upstroke. Not only is this a marvelously efficient prey-grabbing mechanism, but the same mechanism is at the root of the wing flight-stroke of modern birds. Evolution often ends up developing a structure to serve one need, then finds it suitable for adaptation to another. Here, a prey-grasping motion similar in concept to the strike of a praying mantis in a reptile becomes suitable for modifying into a flapping flight motion.
Additionally, the feathers on the hands and tail have elongated, becoming better suited for helping to sweep prey into the hands.
Next is Caudipteryx:
This species had hand and tail feathers even more developed than the previous species, and longer feathers, more like that of modern birds:
However, it is clear that this was still not a free-flying animal yet, because the forelimbs were too short and the feathers not long enough to support its weight, and the feathers were symmetrical (equal sized "fins" on each side of the central quill). It also had very reduced teeth compared to earlier specimens and a stubby beak:
But the elongation of the feathers indicates some aerodynamic purpose, presumably gliding after leaping (or falling) from trees which it had climbed with its clawed limbs, in the manner of a flying squirrel. Feathers which were developed "for" heat retention and then pressed into service to help scoop prey were now "found" to be useful for breaking falls or gliding to cover distance (or swooping down on prey?).
Next is Sinornithosaurus:
Similar to the preceding species, except that the pubis bone has now shifted to point to the back instead of the front, a key feature in modern birds (when compared to the forward-facing publis bone in reptiles). Here are some of the forearm feathers in detail:
Long feathers in detail:
Artists' reconstruction:
Next is Archaeopteryx:
The transition to flight is now well underway. Archaeopteryx has the reversed hallux (thumb) characteristic of modern birds, and fully developed feathers of the type used for flight (long, aligned with each other, and assymetrical indicating that the feathers have been refined to function aerodynamically). The feathers and limbs are easily long enough to support the weight of this species in flight. However, it lacks some structures which would make endurance flying more practical (such as a keeled sternum for efficient anchoring of the pectoral muscles which power the downstroke) and fused chest vertebrae. Archaeopteryx also retains a number of clearly reptilian features still, including a clawed "hand" emerging from the wings, small reptilian teeth, and a long bony tail. After the previous species' gliding abilities gave it an advantage, evolution would have strongly selected for more improvements in "flying" ability, pushing the species towards something more resembling sustained powered flight.
Next is Confuciusornis:
This species had a nearly modern flight apparatus. It also displays transitional traits between a reptilian grasping "hand" and a fully formed wing as in modern birds -- the outer two digits (the earlier species had three-fingered "hands") in Confuciusornis are still free, but the center digit has now formed flat, broad bones as seen in the wings of modern birds.
Additionally, the foot is now well on its way towards being a perching foot as in modern birds:
It also has a keeled sternum better suited for long flight, and a reduced number of vertebrae in the tail, on its way towards becoming the truncated tail of modern birds (which while prominent, is a small flap of muscle made to look large only because of the long feathers attached).
From this species it's only a small number of minor changes to finish the transition into the modern bird family.
(Hey, who said there are no transitional fossils? Oh, right, a lot of dishonest creationists. And there are a lot more than this, I've just posted some of the more significant milestones.)
There's been a very recent fossil find along this same lineage, too new for me to have found any online images to include in this article. And analysis is still underway to determine exactly where it fits into the above lineage. But it has well-formed feathers, which extend out from both the "arms" and the legs. Although it wasn't advanced enough to fully fly, the balanced feathering on the front and back would have made it ideally suited for gliding like a flying squirrel, and it may be another link between the stage where feathers had not yet been pressed into service as aerodynamic aids, and the time when they began to be used more and more to catch the air and developing towards a "forelimbs as wings" specialization.
So in short, to answer your question about how flight could have developed in birds, the progression is most likely some minor refinement on the following:
1. Scales modified into downy feathers for heat retention.
2. Downy feathers modified into "straight" feathers for better heat retention (modern birds still use their body "contour feathers" in this fashion).
3. Straight feathers modified into a "grasping basket" on the hands (with an accompanying increase in reach for the same purpose).
4. Long limbs with long feathers refined to better survive falls to the ground.
5. "Parachute" feathers refined for better control, leading to gliding.
6. Gliding refined into better controlled, longer gliding.
7. Long gliding refined into short powered "hops".
8. Short powered flight refined into longer powered flight.
9. Longer powered flight refined into long-distance flying.Note that in each stage, the current configuration has already set the stage for natural selection to "prefer" individuals which better meet the requirements of the next stage. Evolution most often works like this; by taking some pre-existing ability or structure, and finding a better use for it or a better way to make it perform its current use.
Also from a prior post of mine:
For frick's sake, bvw, next time when you attempt to post something on a topic in biology, at least try to scrape the bottom of the creationist barrel for a *biologist* who might help you cling to your anti-evolution notions -- when you have to look as far afield as a "Reader in Combustion Theory" to find *anyone* who can still deny the obvious evidence for bird evolution, it should be a red flag to even the most die-hard creationist...Archaeopteryx [...] was not a dinosaur. It was a bird.
It's always funny listening to creationists try to explain Archaeopteryx. The reason it's so funny is that half of them declare it to be "obviously" just a bird -- and the other half declare it to be "obviously" just a reptile.
So it's a bird, eh? Well that explains the wings and feathers and so on. But how then do you explain these clearly reptilian features?
Premaxilla and maxilla are not horn-covered. This is posh talk for "does not have a bill."[The above condensed from All About Archaeopteryx by Chris Nedin, which has far more information and quotes from primary research.Trunk region vertebra are free. In birds the trunk vertebrae are always fused.
Pubic shafts with a plate-like, and slightly angled transverse cross-section. A Character shared with dromaeosaurs but not with other dinosaurs or birds.
Cerebral hemispheres elongate, slender and cerebellum is situated behind the mid-brain and doesn't overlap it from behind or press down on it. This again is a reptilian feature. In birds the cerebral hemispheres are stout, cerebellum is so much enlarged that it spreads forwards over the mid-brain and compresses it downwards.
Neck attaches to skull from the rear as in dinosaurs not from below as in modern birds. The site of neck attachement (from below) is characteristic in birds, _Archaeopteryx_ does not have this character, but is the same as theropod dinosaurs.
Center of cervical vertebrae have simple concave articular facets. This is the same as the archosaur pattern. In birds the vertebrae are different, they have a saddle-shaped surface: "The most striking feature of the vertebrae is the simple disk-like facets of their centra, without any sign of the saddle-shaped articulations found in other birds" (de Beer 1954, p. 17).
Long bony tail with many free vertebrae up to tip (no pygostyle). Birds have a short tail and the caudal vertebrae are fused to give the pygostyle.
Premaxilla and maxilla bones bear teeth. No modern bird possess teeth.
Ribs slender, without joints or uncinate processes and do not articulate with the sternum. Birds have stout ribs with uncinate processes (braces between them) and articulate with the sternum.
Pelvic girdle and femur joint is archosaurian rather than avian (except for the backward pointing pubis as mentioned above).
The Sacrum (the vertebrae developed for the attachment of pelvic girdle) occupies 6 vertebra. This is the same as in reptiles and especially ornithipod dinosaurs. The bird sacrum covers between 11-23 vertebrae!
Metacarpals (hand) free (except 3rd metacarpal), wrist hand joint flexible. This is as in reptiles. In birds the metacarpals are fused together with the distal carpals in the carpo-metacarpus, wrist /hand fused.
Nasal opening far forward, separated from the eye by a large preorbital fenestra (hole). This is typical of reptiles, but not of birds.
Deltoid ridge of the humerus faces anteriorly as do the radial and ulnar condyles. Typical of reptiles but not found in birds.
Claws on 3 unfused digits. No modern adult bird has 3 claws, nor do they have unfused digits.
The fibula is equal in length to the tibia in the leg. This again is a typical character of reptiles. In birds the fibula is shortened and reduced. [When you eat a chicken drumstick, the fibula is the toothpick-like sliver of bone you find lying alongside the large "legbone", which is the tibia. Ich.]
Metatarsals (foot bones) free. In birds these are fused to form the tarsometatarsus.
Gastralia present. Gastralia are "ventral ribs," elements of dermal bone in the ventral wall of the abdomen. Typical of reptiles, they are absent in birds