neutrino_cannon
Master Poster
- Joined
- Dec 13, 2002
- Messages
- 2,574
This is a relative sort of question. Feathers only evolved once; or at least, that's the most reasonable guess as it would be fantastically unlikely for something as complex as a feather to evolve in exactly the same way twice under independent circumstances. Whichever lineage first evolved feathers could be considered the "Adam and Eve" flock.
(incidentally, pterosaurs were covered in in insulation that may or may not be homologous with feathers. If it is, feathers, or at least hollow proto-feather filaments may go back a lot further than direct fossil evidence indicates at this time)
But evolution doesn't go in straight lines; it prefers a tangled bush, so in that sense, yes, early bird evolution was terribly, terribly complicated.
"lines of neornithine birds" is something of an artificial, human distinction. If you're asking about higher level avian phylogeny... don't ask that question. Seriously, that way madness lies!
In the past decade or so, there have been a lot of big shakeups in the understanding of bird evolution; not just in terms of dinosaurs->birds, but in how the living varieties of birds are related to each other. Consequentially, one of the big "all the world's birds" books you can find at a library from several years ago will extensively and painstakingly list classification schemes for birds that are considered just plain wrong.
The advance of science, or a sinister plot to sell more bird books?
OK, OK, OK. You may not like this; but it's somewhat more complicated than that. Actually, it's horrifically more complicated than that, and all the details are not yet agreed upon. Life, in all its diversity, really has no obligation to make it simple though, so who are we to complain?
Again, all birds have a single common ancestor. All natural clades do. That single avian ancestor has not been discovered yet, and given the probability that any single organism gets fossilized, probably never will be. The closest approximation to an "urvogel" which is currently described is archaeopteryx. No other fossil has yet unseated archie from the coveted position of most primitive known bird, but the recent ones have done a heck of a lot to fill in the gaps.
In any discussion of bird evolution, it behooves us to say exactly what we mean by "bird". In a modern context, that's pretty obvious; living birds are very, very different from other lineages and there are no messy intermediates to complicate things. Birds have feathers; no other group does. Birds have wings, toothless bills, short, stumpy tail bones, hollow bones, and lay eggs. Some other groups have some of those same traits (turtles have toothless beaks and lay eggs), but they're fairly obviously a shared ancestral trait (eggs), or a similar looking trait that evolved independently in each group (toothless beaks). Thus, once you have figured out that bats are freakishly aberrant mammals, and that penguins and whatnot aren't that different from hummingbirds (flight is not a criterion of birdness), defining what a bird is in not hard work; the Potter Stewart Critereon works beautifully. I know a bird when I see one.
Which is what makes the discovery of fossil birds with teeth and feathered dinosaurs such a head-throbbing embuggerance to it all. All those neat categories fall to pieces, and Nature herself laughs maniacally at our attempts to confine her work to pigeonholes. Nature is vindictive that way.
So, what do we know about how Life works? We know that all life shares a common ancestor, and that the various lineages branch outwards from it fractal-like, and continue diverging. We know that populations, when facing selection pressure, or simply from being separated from others, will become genotypically and phenotypically different from the others. We finally know that different lineages, once separated beyond, say, a species level, generally cannot recombine, and that convergence can only throw separate lineages down parallel tracks; it cannot make them combine*.
*bacteria are sorta-kinda exempt from this rule on account of plasmids. Everything else isn't.
Living birds have scales on their legs and lay dry eggs, which makes it most reasonable to assume that they had reptilian ancestors. They also have an elevated metabolism, just like mammals, and while this has caused some speculation, it's a dumb idea. The most recent common ancestor of mammals and birds did not have an elevated metabolism; the two groups evolved it separately.
So, which reptiles? In the fossil record there are, after all, a rather lot of reptiles crawling around, stinking up the place. The fossil record is full of holes, and by the Late Jurassic, when archaeopteryx shows up, replete with feathers and flight, there are no earlier, obvious protobird candidates. It simply isn't satisfying to say that archie was the first bird though; it's too well developed, too different from anything else prior for that to be likely. Its feathers are asymmetrical, its wings are huge. Those sorts of traits don't just sprout overnight. Saying that it was just a glider is silly; nevermind the mind-boggling complexities of aerodynamic biomechanics, archie had way, way, way bigger and more developed wings than any modern canopy glider, and furthermore, was living on an island archepelago where there may not have been any trees. It's simply ludicrous to assert that it wasn't capable of powered flight; although it probably wasn't great at it, by modern standards. Birds (and I'm calling archie a bird because that's what convention dictates) had clearly been evolving for some time before archie was around.
This leaves two possibilities; the fossil record forgot to preserve any bird-ancestors, or that the bird ancestors were before the eyes of the paleontologists all along, and they just didn't notice them. The second possibility is more satisfying, and more likely. The sort of small, generalized reptile likely to evolve into birds would simply have been too common to escape fossilization altogether. Futhermore, the sort of fine-grained sediments needed to preserve fine details like feathers are geologically rare, and an early, bird-ancestral reptile might escape detection as such if it wasn't surrounded in a halo of feathers.
So, what's the likely suspect look like? As a general rule; small, generalized animals evolve into more specialized ones. Specialist groups tend to be dead ends, and large animals are usually specialists. We want a small reptile. Birds also share details of the scales, skull, heart and ankle with crocodiles, so whatever the bird ancestor was, it was probably fairly closely related to crocodiles. That still leaves a lot of fossil groups; dinosaurs, rauisuchians, ornithosuchians, simiosaurs, pterosaurs, et cetera. It's pretty much a crap shoot at first glance; pterosaurs were covered in hair or feather like integument, and could fly, but why would a group that evolved complicated membranous wings suddenly switch to feathers? Pterosaurs are too specialized to be likely bird ancestors, and they had a few anatomical quirks of their own that birds lack. Simiosaurs had light, birdlike skulls with small teeth and lived in trees (important, since most gliders live in trees as well), but again, they have too many unique traits that birds lack, and their immediate ancestors lack the bird-like traits.
So, using just the Triassic-Jurassic fossil record, you can whittle likely bird ancestors down to dinosaurs, or the crutotarsi, which were some dinosaur-like crocodile relatives, crocodiles, and assorted other archosaurian riffraff.
And, of course, living with the benefits of post 1998 paleontology, we now know that a variety of dinosaurs were wearing feathered coats, but hold the presses; all those dinosaurs with the beautifully preserved feathers come from deposits from after archaeopteryx. Explain that one ya smug Darwinists!
The most plausible explanation is that the lineage of small, two legged, primarily flesh eating dinosaurs called coelurosaurs had been covered in feather the whole time, and the quality of their preservation was such that we just didn't know it. Now, just to forestall any confusion; I'm using "coelurosaur" as a clade, approximately meaning "any animal descended from the last common ancestor of Alex the Parrot and Ornitholestes". The same word had been used to describe all lightweight predatory dinosaurs, but that usage is artificial, and does not describe a clade.
The Coelurosauria includes a lot of animals. Aside from birds, it includes tyrannosaurs (indeed, early Asian tyrannosaurs had feathers, and probably the more familiar late American ones did too), therizinosaurs and a lot of other things that you probably wouldn't consider to be particularly birdy. It also includes things like velociraptors, which are manifestly birdy.
The Coelurosauria includes some of the earlier dinosaurs (although none of the earliest), and also some of the very last non-avian dinosaurs. The Coelurosauria does not include the Ornithiscia, a group of dinosaurs who's name means "bird hipped" and which bore beaks. Convergence happens. Scientists mis-name things. Finally, there were plenty of non-avian dinosaurs around at the KT extinction, and scads that didn't contribute to bird evolution in the slightest, including plenty of sauropods, ceratopsids, thyreophorans, and assorted ornithopod riff raff. They were still around for the same reason that apes are still around today; evolution is not necessarily a replacement process.
Microraptor was just what it sounds like; essentially a scaled down velociraptor, and thus not particularly close to being an ancestor of modern birds. It's more like their uncle; a bygone sideline. Although rear-leg airfoils may have been fairly common in ancient birds, they obviously are not now, and neither are teeth, nor bony tails. That's what's wonderful about early bird evolution.
Neornithine, or "modern" birds are obviously quite different from archaeopteryx. They lack teeth. Their skulls have a few less bones as well, particularly in front of the eyes. Thanks to a tendonous pulley, the flight muscles that elevate the wing actually sit below the axis of the wings. They don't have any bones in their tails, excepting a few stumpy ones that anchor their tail feathers.
During the late cretaceous, the final act for most dinosaurs, and indeed most birds, the skies were populated with all sorts of curious intermediates. Particularly, there was one group of birds that lacked the pulley-system of modern birds, but shared, or convergently developed toothless bills and short tails. These are called the enantiornithines, and they were actually quite common. They went extinct at the same time as the dinosaurs for reasons that are by no means clear.
So, while the dinosaur->bird connection is fairly clear, everything else is not. Why did only one group of birds (the neornithines) survive the KT boundry? How diverse were they at the time of the extinction anyhow?
There's reason to believe that the most recent living ancestor of all living birds diversified out into all the extant clades of the neornithines sometime in the late cretaceous, which would place it flitting about above the heads of T. rex, triceratops, and sharing the skies with the much more common enantiornithines and the very last of the pterosaurs. Of course, there's also reason to believe that the last common ancestor of all living birds, or at least most living birds, evolved much later than that, say, by ten million years or so. Alas, the fossil record of the paleocene, the period immediately after the extinction of most dinosaurs, just plain sucks, and small, hollow bird bones don't preserve that well anyway.
So the matter is still up in the air, and microraptor is, thankfully, irrelevant to it.
