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The expression "transitional form" is a slippery one.
Tiktaalik is transitional in the sense that it possesses some, but not all, of the features found in all tetrapods but not in their lobefish ancestors. Crudely, it has radius, ulna, and wrist bones, and what may to my non-expert eye be digits (an expert would look at the anatomy in much more detail, of course, correlating numerous points and ridges) but with only one bone per digit, unlike the jointed digits found in Acanthostega.
Inference; the LAST COMMON ANCESTOR (LCA) of Tiktaalik and tetrapods was later than the LCA of lobefish and tetrapods, but earlier than the LCA of Acanthostega and tetrapods. This, and no more than this, is what is meant by saying that Tiktaalik is intermediate between lobefish and tetrapods, but that Acanthostega is closer to tetrapods than Tiktaalik is.
The Polish footprints show, either that there were in the mid-Devonian two completely separate lines of descent leading to tetrapods (extremely unlikely), or that Tiktaalik is at least 20 million (if I recall the numbers correctly) years more recent than its LCA with tetrapods. And why not? There are ways in which a modern frog is intermediate between lobefish and amniotes (notably, it has limbs, but needs to lay eggs in water), although it is IIRC around 350 million years more recent than its LCA with amniotes including you and me.
The confusion arises because we think of evolution as a ladder, whereas it is really a bush. "Transitional" is defined in terms of features, but is often misunderstood to imply ancestry or at least sequence; hence your perceptive question. Creationists, of course, are unable or more probably unwilling to escape the confusion.
There is another trap lurking, BTW. The temptation is to think of the frog as less evolved than us. WRONG. We and the frog have been evolving for exactly the same length of time since our LCA.
I hope this helps.
Paul S. Braterman,
Professor Emeritus, University of North Texas
Honorary Senior Research Fellow in Chemistry, University of Glasgow
psiloiordinary wrote:This was from Paul by email;The expression "transitional form" is a slippery one.
Tiktaalik is transitional in the sense that it possesses some, but not all, of the features found in all tetrapods but not in their lobefish ancestors. Crudely, it has radius, ulna, and wrist bones, and what may to my non-expert eye be digits (an expert would look at the anatomy in much more detail, of course, correlating numerous points and ridges) but with only one bone per digit, unlike the jointed digits found in Acanthostega.
Inference; the LAST COMMON ANCESTOR (LCA) of Tiktaalik and tetrapods was later than the LCA of lobefish and tetrapods, but earlier than the LCA of Acanthostega and tetrapods. This, and no more than this, is what is meant by saying that Tiktaalik is intermediate between lobefish and tetrapods, but that Acanthostega is closer to tetrapods than Tiktaalik is.
The Polish footprints show, either that there were in the mid-Devonian two completely separate lines of descent leading to tetrapods (extremely unlikely), or that Tiktaalik is at least 20 million (if I recall the numbers correctly) years more recent than its LCA with tetrapods. And why not? There are ways in which a modern frog is intermediate between lobefish and amniotes (notably, it has limbs, but needs to lay eggs in water), although it is IIRC around 350 million years more recent than its LCA with amniotes including you and me.
The confusion arises because we think of evolution as a ladder, whereas it is really a bush. "Transitional" is defined in terms of features, but is often misunderstood to imply ancestry or at least sequence; hence your perceptive question. Creationists, of course, are unable or more probably unwilling to escape the confusion.
There is another trap lurking, BTW. The temptation is to think of the frog as less evolved than us. WRONG. We and the frog have been evolving for exactly the same length of time since our LCA.
I hope this helps.
Paul S. Braterman,
Professor Emeritus, University of North Texas
Honorary Senior Research Fellow in Chemistry, University of Glasgow
Just for the record, Paul didn't post his qualifications on the forum. What was posted (by Psi) was a copy of an email, from before SO joined the forum, which contained Paul's email sig. An email containing an answer to SO's question that evidently didn't satisfy him, btw.Christine Janis wrote:Christine Janis
Professor of Biology, Brown University
Fellow of the Paleontological Society
(if Paul Braterman is going to list his qualifications to answer, then so shall I).
SkepticalOne wrote:Excellent. This is what I was after, Christina. You would perhaps be surprised at how unsuccessful I was at finding an answer like that through various Google searches. I think I understand what you're saying, but I want to clarify a bit. Let us leave out the phylogenies for a moment as they represent a specific scientific tool designed for a specific purpose as you said.
So Tiktaalik's morphology makes it appear transitional (regardless of whether it is ancestral) between fish and tetrapods. The incompleteness of the fossil record makes it very unlikely that Tiktaalik is actually on the direct line to tetrapods so it is being used to represent the animal that actually is our direct ancestor. You are saying that Tiktaalik's mosaic morphology indicates its relationship to fish and tetrapods and if it is related in this way then it implies the existence of the ancestral animal that must have looked similar. Do I have that right?
Also, Christina, while I appreciate the detailed response, I think you should not be so quick to judge the motives of those posting here unless they reveal them. I wanted a scientific answer to a question about science, not to try to express any metaphysical position.
I'm not interested in what holes you think you have found in evolutionary biology. I'd like, instead, for you to provide your creationist position.
SkepticalOne wrote:
So Tiktaalik's morphology makes it appear transitional (regardless of whether it is ancestral) between fish and tetrapods. The incompleteness of the fossil record makes it very unlikely that Tiktaalik is actually on the direct line to tetrapods so it is being used to represent the animal that actually is our direct ancestor. You are saying that Tiktaalik's mosaic morphology indicates its relationship to fish and tetrapods and if it is related in this way then it implies the existence of the ancestral animal that must have looked similar. Do I have that right?
SkepticalOne wrote: Also, Christina, while I appreciate the detailed response, I think you should not be so quick to judge the motives of those posting here unless they reveal them. I wanted a scientific answer to a question about science, not to try to express any metaphysical position.
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