This is a rather in depth discussion going on elsewhere all starting with my blog about Shubin’s Your Inner Fish.
However, there seems to a major misunderstanding and I thought I might bring it up here to correct any misunderstandings people have.
From Berkeley:
Fossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional forms.
Now, let’s parse this out, so we all know what we’re talking about.
Fossils of organisms – so a transitional can be any living or once living thing. (In fact, every living or once living thing is a transitional, but I’ll get to that in a minute.)
that show the intermediate states – So we are looking for something with an intermediate character. Now, that doesn’t mean ‘half a wing’ or ‘half a lung’. What it means is a trait that gives us a clue about the change over from one thing to the other. It is not “No lung, half a lung, complete lung.” It is “no lung, proto-lung, modern lung”. The difference is subtle and creationists will (and are) confuse the issue to make it seem implausible.
between an ancestral form and that of its descendants – note that it doesn’t say “ancestor”. It says “ancestral form”. Kublai Khan is an ancestral form of me, even though he is not my ancestor. He is a human, who lived 800 years ago. However, I have no Asian traits in my genome (that I’m aware of, if anyone wants to spring for my genetic history, that would be awesome (I’ll publish the results here)). “Its” is unfortunately ambiguous here. But I assure that other factors result in there being no requirement that the descendant be directly related (distant cousins are fine).
So, that’s the definition. Note that there is no requirement of time. The transitional fossil or organism doesn’t have to exist in any particular point in the timeline of any species, ancestral or descendant.
You are a transitional form between your parent and your child. If you parent dies, you are still transitional. If you die, you are still transitional. If your child dies… guess what… you are still transitional.
Further a transitional can exist between three current species (or whatever). For example, many genetic traits in humans are polygenic. That means (for our purposes today) that there is a broad range of available values. For example, height. Let’s say that you have three children… fraternal triplets (to remove the time issue). One child is very tall. One child is very short. The other child is just right (meaning you can actually find jeans that fit).
The average height child is transitional between the tall and short child. If you measured the length of bones that are responsible for height (femur, tibia and fibula, veterbrae, etc), you would probably find that the bones of the average child were all intermediate between the tall and short children’s bones. It’s transitional.
Now, let’s look at some examples. Of course, mention of Shubin brings up Tiktaalik. Why is it transitional?
Because the wrist bones have characteristics of both fish fins AND tetrapod wrists.
What about Archeopteryx? The classic feathered dinosaur. Even if you find true birds older than it is and true dinosaurs younger than it is, Archeopteryx is still transitional between birds and dinosaurs. Out of some 23 defining characters between birds and dinos, Archeopteryx has 4 that are definitely bird and 17 that are definitely dinosaur (meaning that these 17 characters are different in birds and dinosaurs, if the thing has teeth it’s not a bird) and the other 3 are variable depending on the species of bird or dino we’re talking about.
It’s intermediate between bird and dinosaur. If conditions and luck are on our side, we might find a fossil that has 5 definite bird like traits and another that has 6 and another that has 7.
Unfortunately, the conditions for fossilization are rare and as rock is recycled, it is difficult to find the right kind of rock, of an appropriate age, with appropriate conditions for fossilization as it became rock, in an area with the species we hope to find.
So this is all about transitional fossils. And, finally, a list of transitional fossils.
Cassandra's Tears 
Reader # 5 checking in. Nice post!
JoeG… OK, so what?
Transitional does not always mean directly ancestral and directly descendant from. In fact, in fossils, it cannot mean that. Apparently, you didn’t read the definition of transitional.
Tiktaalik is transitional in FORM.
Thanks!
Jennifer Clack is the scientist Shubin referenced in his book. And I never said nor implied a transitional means directly ancestral and directly descendended from.
Also your “definition” is nothing more than “It looks like a transitional to me”.
Yes, Joe, I know who Dr. Clack is. Again… so what?
Yes, Joe, that is what you are implying. Your argument, as I understand it, from your blog is that Tiktalik isn’t transitional because there are species that are tetrapods earlier in the fossil record. Yes, I can provide quotes that this is your argument. Unfortunately, as the information I have provided (and everyone else has provided to you, both directly and in an attempt to let you learn for yourself) shows, that has nothing to do with the simple fact that Tiktaalik is a transitional.
My definition is very strict (unlike any of your ID definitions). The attributes and characters used in various examinations of being transitional can be measured (again, unlike anything ID has so far produced). If you measure bone shapes, for example, and find one with a large heavy knob at one end and one with just a slight bulge at that end, then somewhere, there should be a transitional that is measurably between those two. It’s that simple. Tiktaalik is transitional because on one hand you have fish fins and the other hand (pardon the pun) you have fully formed hands. Tiktaalik is transitional because its hand fits in between the two.
As described in Shubin’s book, even 4-year olds can see that a fossil has attributes of two different lineages, then it’s transitional.
Fish-> fishapods-> tetrapods, the fishapods would be the transitional
But the evidence now has:
Fish-> tetrapods-> fishapods, how can fishapods be the transitional form?
Because it has NOTHING TO DO WITH THE TIME LINE, Joe.
It has to do with the morphology.
Yes, it is that simple. Transitional DOES NOT MEAN that it is a descendant of one group and the ancestor of another group. I thought I made that clear in my post above Joe. Perhaps you should read it.
Transitional means that FEATURES are INTERMEDIATE between two groups, Joe.
Joe,
“Fish-> tetrapods-> fishapods, how can fishapods be the transitional form?”
It’s simple. You don’t understand what you are talking about. The rest of the world understand this, but not you apparently. Just give up, you won’t be able to get it. It’s not worth it anyway.
Joe, I’m uncertain as to what your issue really is. Science, you see, works. Evolutionary theory works. In simplepersonspeak, really cool stuff is being done by really smart people who are using evolutionary theory. What possible benefit does your brand of whateveritis convey? What possible reason could those really smart people have to abandon what works and has worked for a long time and embrace your whateveritis? Why do you care, really? Science marches on and your whateveritis amounts to less than a wad of discarded chewing gum on its boots.
For the sake of argument, wouldn’t a definitive transitional form have to be within the timeline of adjacent species? In not, while it may display shared features, it could also have been an evolutionary deadend and led to no further species. We have no true way of knowing that it was actually transitional for some species or set of species. Or do we?
Sophist, nope. That’s the thing I was most trying to get across in this post. It’s not transitional because of it’s timeline. It’s transitional because of its morphology.
Sure, Tiktaalik, because of the discoveries in Poland, probably was an evolutionary dead end. However, for however long it existed, it retained those traits that make it transitional.
Consider the timeline argument in terms of this simple fact. There may be 50 million years of geologic history compressed into a foot of rock. Even if you find two fossils of the same species side by side, there’s no way to say whether they were siblings or separated by 45 million years.
Remember, it’s not transitional because it’s in the same lineage, it’s transitional because of the morphology (the anatomy).
I can see the difficulty here. As a matter of semantics, transitional implies something came after, and so by definition a dead end could not be ‘transitional.’ It would have to be assured that features that emerged in one species would arise again in others, those not being dead ends, in order for the forms to show up in later species. The problem I see with this definition is it implies a deterministic nature of forms and that could ilict considerable controversy.
Sophist says
It may imply that, but it doesn’t MEAN that. You’re thinking about it backwards. You are assuming that the intermediate traits only appear in organism (or species) in the path to the ‘end result trait’.
That isn’t the case. There is no ‘end result trait’ or ‘best trait’ or ‘purposeful trait’ or anything else. Neither the organisms, nor their genes, nor anything else knows what traits will evolve next. We can only see these in terms of our 20/20 hindsight.
Humans are a transitional species. We’re not sure to what, but we are. Every living thing, every species is transitional. I am the transitional between my child and my parent. My house cat is transitional between a civet and a lion. T. Rex is transitional between birds and Marasuchus.
The T. Rex didn’t know that birds were the coming thing and was trying to get to be like a bird.
As far as the traits, think of it like a tree. I’ll try to do this in graphic form on in the blog, but I can’t here. So anyway, you have a big oak tree. At the first branch, everything on the left branch has bones and everything on the right does not. In those terms (as shown in Your Inner Fish) a lampray is transitional (it doesn’t have bones, but it does have teeth which are precursors of bones). If we follow the bone lineage, we have another early branch that separates sharks, rays and skates from everything else. These creatures have heavy cartilage, but it never actually forms ‘bone’. In that sense, sharks are transitional between lamprays and humans. I could on, but I hope you get the point. This is important when we consider that lamprays, sharks, and humans are all alive today. So, yes, a modern shark is transitional between a modern human and a modern lampray… for that character (which is an important caveat to this whole discussion. Again, I hoped that I made that clear with the Archeopteryx example).
I hope that helps.
very clear. Given the fact that no species is static then all are transitional, understood.
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