Which one? There has been a ton of posting activity today, even just considering the ones aimed in my direction. I may have missed some in the flood.
Alan Feduccia? Wikipedia states about Caudipteryx : "Feduccia believes these fossils are flightless birds that evolved from a flying ancestor, probably Archaeopteryx."
Hmm, surprising. Feduccia believes that Archaeopteryx is a "full bird" (again, a very minority opinion -- Archaeopteryx has more reptilian features than avian). And Caudipteryx has even more "primitive" reptilian features, so it seems more likely that Feduccia would consider Caudipteryx reptilian and therefore no descendant from a lineage like Archaeopteryx. The Wikipedia article doesn't provide a citation or link to the statement about Feduccia and those fossils, so I can't quickly go check what Feduccia actually said and whether Wikipedia is representing his view correctly.
In any case, a *lot* of other experts in the field have found a lot of fault with Feduccia's conclusions, and Feduccia has been caught relying on obsolete findings when newer, more complete information has superceded it, and so forth, so I don't have a lot of confidence in his reliability on that topic.
[Because cladistic trees aren't based on dates in the first place.]
So what was the point in posting a cladistic tree to show the transistion of dinosaurs into birds if it has nothing to do with dating? Isn't evolution about change over time?
Yes it is, and cladograms show relative time of divergence, *but* they don't provide absolute dates, nor does the dating of particular specimens say much about the date of an evolutionary event (except as an upper bound).
Are you saying that how old fossils actually are has no bearing?
No bearing on the cladogram, right, unless there's a gross violation of consistency.
Scratch that. Let me ask straight out. How old are each of the fossils in the "transitional sequence" (your words) that you posted?
I looked them up back when I originally wrote the post (more than a year ago, IIRC), and they were consistent with the cladogram. But that's not the same thing as saying that they're strictly chronological (and *that* is not the same as saying that they're grossly out of sequence, either).
I'm genuinely curious as to dates and why or why they don't matter.
Okay, let's see if I can explain this clearly enough in text (it's a lot easier in diagrams or on a chalk board...)
Here's the detail section of the cladogram again:
The first thing to remember is that the "lines" from the fossil names down to the "main" line are not "pointers" indicating where the fossil species falls on "the" evolutionary sequence. It's *not* saying that Archaeopteryx is a direct descendant of Caudipteryx, or any other fossil species on the diagram.
Instead, all the (black) lines on the diagram indicate *separate* lineages which have branched off the main lineage being traced in this particular cladogram (the long diagonal line).
So it's actually saying that starting with Theropodia (the "root" of this cladogram on the left), there was a line of descent that eventually ended in modern birds (off the right-hand side of this sub-cladogram), and along the way spun off a lineage which eventually resulted in Sinosauropteryx, and at some time further down the timeline spun off a lineage which eventually resulted in Protoarchaeopteryx, and another branching off which eventually resulted in Caudipteryx, and so on down the line.
The reason that nothing is shown actually being *on* the "main line" is because it's pretty much impossible to determine for certain whether any given fossil is actually *directly* ancestral to another, or merely an offshoot (even if a close offshoot) from "the" directly ancestral form.
This is completely analogous to how, if you're descended from Englishmen and you find a skeleton of long-dead Englishman in a field, it's unwarranted to jump to the conclusion that he's actually your direct ancestor (i.e. your great-great-great-etc. grandfather), when he's more likely to instead be a *relative* of your actual great-great-great-etc. grandfather (i.e., the skeleton and your great-etc.-grandfather were both descendants of a yet-earlier member of the original people who first populated the British Isles).
(Again, I apologize if this isn't real clear when described in words, it's a *lot* clearer when I can sketch things on a blackboard, add lines then erase and move them, etc. If I get good enough making Flash animations maybe I'll whip up an animated "lesson" someday.)
So just to not presume *direct* ancestry when it may not be fully warranted, cladograms are instead drawn as "leaves" at the end of "branches" of family relationship, instead of labeling actual "forks" in the "tree".
Furthermore, the lengths of the "branches" are not significant in many cladograms. In the above diagram, Sinosauropteryx may actually be a lot more closely related to the "main lineage" than Caudipteryx, or vice versa. Or either could be closely related to the main lineage, or both very distantly related.
Similarly, the time between "branching split-offs" may be longer or shorter between subsequent branchings than between others. In other words, the cladogram (at least this one) is "not to scale". The sizes of its various lines is not meant to represent the absolute time spans involved between or since evolutionary branching of lineages.
So what *is* significant in a cladogram, you ask? Just the *order* of the branchings. This cladogram indicates which lineages split off when. And by virtue of that, it can show which traits arose first, which second, and so on, during the evolution of dinosaurs-to-birds.
To understand how, we have to briefly review how evolution proceeds. Among the many other necessary consequences of how evolutionary processes work, are the following:
1. New traits are added to a single lineage. The same trait does not arise multiple times in separate lineages even if it would be handy. The unique configuration of a bird's wing arose only once during the history of life on Earth. Even when other animals developed wings, and were subject to the same functional requirements, they arose as very different genetic and structural mutations -- bird wings are "constructed" very differently from bat wings, and pteradon wings, and insect wings, and flying-fish wings. Vertebrate eyes are fundamentally different from invertebrate eyes, because they evolved from different "roots".
2. New traits which arise in a lineage are inherited by all of that lineage's subsequent descendants.
3. Lineages "fork". New species split off of old ones, while the "old" one continues to exist in more or less its original form, or evolves off in its own different direction. This is how we've ended up with millions of living species today, from a more limited number of species long ago. These are the "forks" in the branchs of the evolutionary "tree".
Putting these together, it's straighforward to see that it implies a necessary order to the evolutionary "forks". If modern birds have traits A, B, C, D, E, and F, and you find a fossil with traits A, C, and D, but not B, E, or F, then you know for a fact that A, C, and D arose earlier in the evolutionary lineage of birds than did B/E/F. (Of course, it's not quite *that* easy in practice, since traits can also be lost on side lineages, or another trait can falsely appear to be the same as a modern trait, etc. etc., but with proper care such mistakes can be minimized, especially when multiple specimens across the lineage can resolve the finer details and resolve anomolies, and other methods of cross-checking. Creationists like to trumpet the difficulties, but they overlook the safeguards which are used, the cross-checks, the value of expert knowledge based on past experience, etc.)
Similarly, with more fossil finds the order of occurrence of B, E, and F traits can be resolved, and so on. These are the numbered (1-8) traits annotating the main lineage on the above cladogram.
Also note that this works *even though* the fossil find may not be (and in practice almost certainly isn't) *directly* ancestral to the lineage being resolved. Note that although the Dromaeosauridae are a "side" lineage on the cladogram, they *had* to have split off the main lineage before point "6", because if they had split off *after* point 6, they'd have modern-type flight feathers, since flight feathers were added to the main lineage (and thus to all subsequent descendants) at point "6". The Dromaeosauridae are *diagnostic* of the main lineage in the region between "5" and "6", even though they are an offshoot and probably have their own unique added traits which had evolved into that lineage after it split from the "main bird" lineage. Why? Because by rule #2 (traits are passed to descendants), they would still have traits 1-5, but not traits 6-8, just like all the species along the "main line" between 5 and 6 as well. The only thing you *can't* use Dromaeosauridae fossils to infer about the main bird lineage is traits that appear in the Dromaeosauridae but *not* modern birds, because those are necessarily (by rule #2 again) those are necessarily traits which arose in the Dromaeosauridae *after* it had split from the "main line" bird lineage, and thus never appeared in the main line lineage at all.
This is the manner in which I used these fossils in my "bird evolution" post. I used various fossil finds, which have been placed into their proper spots in the bird cladogram, as diagnostic indicators of what would necessarily have been present in the "main line" bird lineage at sequential points in time (again, *even* though those fossils may not have been, and need not have been, actually *on* that lineage, i.e. actually directly ancestral to modern birds). They still provide logically necessary conclusions about what *was* on the main bird lineage (as long as one restricts one's examination to traits which are held in common with modern birds, and not separately derived traits in the fossils).
Now, back to your question about the dates of the fossils...
Pulling numbers out of thin air (I'm too tired to look them up right now) for way of example, let's say that the point where the Caudipteryx lineage "forks" from the main line was 200 million years ago. And the time that the Archaeopteryx lineage "forks" is 140 million years ago. Does this mean that an Archaeopteryx fossil must necessarily be younger than a Caudipteryx fossil? Nope!
The Caudipteryx "fork" is only the fork point of Caudipteryx's last common ancestor with modern birds. It is *not* the time at which the species Caudipteryx actually arose. Caudipteryx may have been the end result of *another* 90 million years of evolution after the "fork". Similarly, Archaeopteryx might have arisen a mere 10 million years after "forking" from the main bird line (i.e., Archie's common ancestor with modern birds would have been rather Archie-like already, even if it wasn't exactly an Archaeopteryx like in the fossils which have been found).
Consequently, Caudipteryx fossils might be 110 million years old (200 mya fork - 90 my divergence), while Archaeopteryx fossils might be *130* million years old! (140 mya fork - 10 my divergence). Yes, that's right, Archaeopteryx as a species might be *older* than a fossil whose lineage split from the bird lineage *longer* ago than did Archie's lineage!
This is why dates of fossils is not extremely meaningful for cladograms. It's also why you can (via traits) *still* reconstruct the necessary *order* of each fossil's forking from the main lineage (albeit not the *absolute* dates of that split) via the traits which are and are not in common between various fossils, even apart from any concern about the dates of the fossil finds. In fact, the relative dates of the fossils may be misleading (for the reason I gave above).
That's not to say the fossil dates are entirely useless *after* you've constructed the cladogram, however. After the cladogram has been properly constructed from a careful analysis of the distribution of their traits, the dates of the fossils place a useful "lower bound" on the time of the actual divergence of lineages. With enough dates, and enough additional forks (and re-forks) in the cladogram, you can use the dates to get a pretty accurate estimate of the actual divergence dates.
Now to answer a few obvious questions, this method seems to be based on a number of assumptions, how do we know they're correct? One of the strongest ways is that for lineages where there are living representatives of various branches of the tree (such as cladograms of vertebrate lineages), we can use DNA analysis as a *very* powerful independent cross-check. In the many cases where that has been done, cladistic analysis has been shown to be a valid method.
Also, even for lineages (such as for birds) where all but the most modern offshoots are extinct, additional cladograms can be constructed for various "side branches" of the lineage, making not just a "forking line" like the one above, but an entire complex tree, such as this big-ass cladogram of all dinosaur families. If cladistic analysis was based on faulty assumptions, it would be impossible to create a coherent cladogram (i.e., one where all of the above "construction rules" are strictly followed) of that size with that many specimens, without the structure breaking down rather quickly (i.e., having the trait combination relationships indicate that something belonged at two different branches of the tree). Instead, coherent trees without contradiction are constructed even for the largest collections of specimens, which is very strong evidence that the assumptions and methods are valid.
Cladograms can even be constructed "universally" for all forms of life and remain coherent, yet more support for the theory of the evolutionary origin of all life from a common ancestral stock. Here is a stunning "super cladogram" of 3000 representative species from all Earth life (you'll need a PDF viewer, which may already be installed on your computer -- if not download it free here). Source: David M. Hillis, Derrick Zwickl, and Robin Gutell, University of Texas. It looks like a dense "fuzz" when displayed without magnification, but use the "zoom in" feature and you'll find an *incredible* amount of detail. To see full detail all at once, you'll need to print it out in "wall poster size" FIVE FEET across. Even as detailed as this is, it's only about 1/3000th of all the known species...
In any case, the point is that if cladistics was not a valid form of analysis (and indeed, if evolutionary origins were not true, since cladistics is built upon evolutionary principles), such cladograms wouldn't be possible to build coherently. But they can be, lending further support to evolutionary theory (i.e., life has the patterns of properties which evolution would be expected to produce).