INTRODUCTION
In this short essay I wish to show you a few bits and pieces about Tiktaalik, the fish fossil introduced to the world, last April.
I want to report on a tiny piece Tiktaalik, its forelimb, and thereby hopefully put transitional into a context.
This essay is very simple because there were too, too many new things to learn bones with funny names, animals with funny names, bone orientations with funny names, and funny names with bones. All this, plus a confusion of tons of data and my head began to swim. (And these days I fail to see why too much effort is warranted when the typical YEC response is to block all sensory organs so that the faith can be maintained. This essay is more the result of my own desire to learn and to pass on some of that to those YEC lurkers who may be more open to realistic ideas that, while a bit unsettling, are worthwhile anyway simply because they are substantiated, and grand in their scope.)
The essay is inspired by a set of articles published in Nature last year (1).
BACKGROUND
Today, we have two kinds of fish the lobe finned fish and the ray finned fish.
By far, most fish around today are ray fins. The bones in their front fins look something like those shown in the left hand figure of the first attachment (2).
A few other kinds of fish found today are the lobe fins. These are represented by lungfish and the Coelocanth. The bones in their front fins look those shown in the middle figure of the first attachment.
There is certainly a big difference, but what is interesting is if you compare the second and third figures in that first attachment. The third figure is a sketch of the generalized tetrapod forelimb. Notice how the lobed fin fore-fin has some important characteristics of the tetrapod forelimb? And notice how both the tetrapod limb and the lobe fin look nothing like the limb of the ray finned fish?
Perhaps you can begin to see why evolutionists would suspect that the tetrapod limb is a modified version of the lobe fin?
THE EVIDENCE
It is one thing to see the similarity and conclude relationship, it is another to find evidence for such a relationship.
One type of evidence would be to dig up the remains of animals that in some way, fill in the gap between the lobe finned fish and the true tetrapod.
Fossil evidence shows that fish have been swimming around for the best part of 500 million years. Thus, the bones (3) and the radio isotopes tell us that the ray finned fish were well established in the earths oceans at 440 million years ago. At 425 million years, the lobe finned fish are found, (Coelocanths at 425 Mya and lungfish at 417 Mya (4).) At 340 million years, real tetrapods, capable of living on land are seen.
Limbed tetrapods are not found in the fossil record at 400 Mya, only finned fish are. But they are at 340 Mya. Naturally, the question arises how did those tetrapods come to be if they did not exist earlier?
Roughly 150 years ago, two scientists offered an explanation for how animals like tetrapods could arise from pre-existing animals. Darwin and Wallace independently formulated a theory of evolution based on natural selection. Today, it is the most widely accepted theory of evolution. In the context of their theory, evidence is needed for changes from one kind of animal to another, if the theory is to have credibility. Well, a wealth of fossils show that something weird and wonderful was going on with respect to the lobe finned fish, from some 390 Mya to 360 Mya.
The fossils concerned have names such as Eusthenopteron (5) and (6), Panderichthys (6), Acanthostega (6), and Ichthyostega (6). As you will see from the links, the first is a fish, but it has legs while the last is an amphibian, a tetrapod, but it retains many fish characteristics. In between these two types of animals are many other fossils bearing names such as Elginepeton, Venstastega, and Metaxygnathus (7). These types show all sorts of characteristics that link Eusthenopteron to Ichthyosetega, and they even include animals with 7, 8 or 9 digits on hands and feet.
That is, over a 30 million year period, beginning some 390 Mya, one group of fish, within the lobe finned fish, (as distinct from the ray finned fish), began to take on more and more tetrapod characteristics, to the point that at 340 Mya, amphibians had arisen. In the process odd-ball combinations of digits were being tried. Eventually, the outcome was five digits, as found in tetrapods today.
When all of the fossil data (bone lengths and shapes, muscle attachments, bone orientations, holes for nerves and blood vessels etc.) from these animals are measured they can be objectively set out in the cladogram shown in the second attachment. An alternative diagram can be seen in the third attachment. These days, such diagrams as the third attachment make no claim for direct ancestry. That is, they are not drawn with the view of claiming that Tiktaalik evolved directly from Panderichthys. What is claimed is than an animal existed at such and such a time that had characteristics intermediate between those of Panderichthys and those of Acanthostega. While it is possible that one is directly related to another, more likely, some other animal existed, whose fossilized remains have not been discovered (and probably never will be), that is on the direct line. All that can be said is that the purported transitional (Tiktaalik in this case) is presumed to have characteristics representative of those of an animal on the direct path.
So what was it that was found?
I shall concentrate only on the front fin. The kind of description I provide here can also be given for other parts of the animals body, particularly its skull, and the body parts just behind the skull. That is, while the fins of Eusthenopteron and Panderichthys were beginning to look a bit like legs, while those of Acanthostega were legs, the fins of Tiktaalik were certainly fins but they also had strong internal tetarpod like limb bones and joints. The bones were the humerus, ulna and radius and the joints were a shoulder and elbow. And beyond those two joints were yet others in the smaller bones as shown in the fourth attachment. This same kind of thing can be seen in the skull, (and in other bones) but in a skull sense, rather than a leg/fin sense. That is, the skull has shape and structure somewhat similar to that of a reptile, and less like that of a fish. It has a neck, unlike a fish. But like a fish, it had gills and the remains of scales can be seen.
Fins in fish are used for propulsion, steering, stabilization and moving in the vertical plane. In particular, with respect to the fore-fin, steering, stabilization and vertical movement are the main uses. With the bones of Tiktaalik however, both their morphology and points of muscle contact show that much more than steering, stabilization and movement in the vertical plane were done. Unlike the earlier fish such as Eusthenopteron, this new one appears to have had more flexibility allowing for rotation, flexing and extending and pulling in or pushing away various parts of the limb. So while the fin retained fin rays, suggestive of normal fin activity, the inner bones suggested a limb activity which allowed the fin to be used as both a paddle and a means of supporting the front portion of the body by pushing it up from any substrate the fish may have been resting on.
PREDICTIONS
A complaint often made against evolutionary science, is its supposed inability to predict. I shall address this by offering five comments.
The first is that scientists do know what to look for and they go out in hope that geology and preservation have been kind and that the expected bones are found. Tiktaalik is an example of this.
The second is to ask you to examine the cladogram in the second attachment. If common descent with modification occurred (and occurs), then the cladogram should always hold. That is, any new fossil discovery should not spring nasty surprises such as an animal which cannot be placed into any one grouping. For example, the old canard of a fish with feathers - we know that feathers are diagnostic of birds (and their relative dinosaurs). Likewise, gills are diagnostic of fish (and their close relatives). A fish with feathers, (easily imaginable under the guise of a creator working outside of descent with modification), simply cannot be slotted into one group on a phylogenetic tree. Gills would put it in with fish. Feathers would put it in with birds. However a fish with legs puts the animal in with creatures with four limbs which can be split into two groups fish and tetrapods. And where do the fish with legs go? Right between the two groups. No unresolvable splits occur. One can measure all aspects of a fish and all aspects of a tetrapod and form two distinct groups. Fossil discoveries are then made from which similar measurements are taken and a third group forms quite naturally, between the fish and between the tetrapods. No discoveries occur which cause an unresolvable dilemma.
The third is to consider the cladogram in association with the geologic column. The cladogram tells a researcher what deposits to dig in with the hope of finding an intermediate. Reconsider the fifth attachment. Clearly an animal representing the transition between two groups is best searched for in the deposits between which those two groups are fist found. And if geology has been kind, and one is fortunate, then something relevant should be discovered. Scientists do this, and it works.
The fourth relates to the third. The researchers did exactly this. They wanted to locate an animal undergoing the transition between the fish Panderichthys and the tetrapod Acanthostega. They reasoned that it should be found somewhere in that relatively narrow 10 million year gap shown on the fifth attachment, providing geological and other preservation circumstances have been good. They went looking in the deposits of the right age. Such deposits had been discovered on an island off Canada in 1999 and by 2004, those deposits began to bear fruit. The researchers found an animal they had been looking for.
The fifth comment concerns my mention of Darwins and Wallaces theory of evolution by natural selection. Clearly, one test of the predictive nature of evolution is to look for certain bones in deposits of a particular geological age, based on relationships shown in phylogenetic trees. The other test is much harder to do and involves much speculation. The question arises as to what was being selected for and why. Evidence to answer questions of this nature does not fossilize. Evidence that fish were developing limbs to move from pool to pool in a drying environment does not fossilize. (What does fossilize is evidence that times were becoming drier.) That they were developing limbs in response to the opening of a new food niche as insects moved onto land following the colonization of land by plants that does not fossilize. (What does fossilize is evidence that plants were on land and that insects had just colonized the land.) Nevertheless, tetrapods live on land and fish live in water, while amphibians inhabit an intermediate environment. So researchers speculate that a shallow water environment would be the place where this transition from fin to limb was being driven. And certainly fossils discovered to date, appear to be from animals inhabiting shallow water environments.
So, not only did the scientists who discovered Tiktaalik know to look in sediments of a certain age, but they speculated that the best sediments to search in would be those of shallow water environments. The formations searched were those deemed, by geologists, to be from ancient meandering streams, that is, shallow water environments.
THINK ABOUT THIS
Tiktaalik is a fish. Refer back to that cladogram in the second attachment. When all bones are measured, holes counted, positions recorded, muscle attachment points considered, and so on, then Tiktaalik is closer to Elpistostege, a fish with legs, than it is to the tetrapods Acanthostega and Ichthyostega. That is why the animal is grouped where it is on the diagaram. Elpistostege has for a long time been recognized as an intermediate between fish and tetrapods, even though it was clearly a fish. Unfortunately, Elpistostege has only been known from a few very partial skeletons. Tiktaalik fills in those details missing from Elpistostege. Like all fish, Tiktaalik has scales and fin rays on its fins. It has gills. But unlike fish, the animal has a neck and a reptilian like head with a long snout and eye sockets on top. Inside its fins, the bones clearly follow the tetrapod pattern of jointed bones. It has a flattened body.
And these kinds of discoveries keep rolling in.
REFERENCES and NOTES
(1) Per Erik Ahlberg and Jennifer A. Clack, A firm step from water to land, Nature, 440 6-April-2006, p747-749
Edward B. Daeschler, Neil H. Shubin & Farish A Jenkins Jr, A Devonian tetrapod-like fish and the evolution of the tetrapod body plan, Nature, 440 6-April-2006, p757-763.
Neil H. Shubin, Edward B. Daeschler & Farish A Jenkins Jr, The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb, Nature, 440 6-April-2006, p764-771.
(2) Diagrams taken from George C. Kent and Robert E. Carr, Comparative Anatomy of the Vertebrates, 9th Ed., McGraw Hill, 2001.
(3) This part taken from Richard Dawkins, The Ancestors Tale: A Pilgrimage to the Dawn of Life, Weidenfelf & Nicolson. A problem arises when one consults another source. These times can vary somewhat, illustrating the pace of progress. Within the space of a few months an article can be published which pushes the origin of one group or another back further in time.
(4) Mya is just a shorter way of writing millions of years ago which is another way of saying before you and I were born :teeth:
(5) Here is a good link:-
http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.860.html#Eusthenopteron.
The authors of the link begin:-
[box]Eusthenopteron: the "Fish with Legs"
What can we say about Eusthenopteron that has not already been said, much better, by someone else? Eusthenopteron is the gold standard for osteolepiforms, the Morganucodon of sarcopterygians, and, in fact, probably the best known Paleozoic vertebrate of any taxon, bar none.[/box]
Note the title? The animal is a fish. But it has legs.
(6) Here is another link (Go to the end of the page and view the last diagram. Then contemplate it.):-
http://people.eku.edu/ritchisong/RITCHISO//342notes5.htm
(7) Mark Ridley Evolution, 3rd edition, Blackwell Science Ltd, 2004. See figure 18.9
In this short essay I wish to show you a few bits and pieces about Tiktaalik, the fish fossil introduced to the world, last April.
I want to report on a tiny piece Tiktaalik, its forelimb, and thereby hopefully put transitional into a context.
This essay is very simple because there were too, too many new things to learn bones with funny names, animals with funny names, bone orientations with funny names, and funny names with bones. All this, plus a confusion of tons of data and my head began to swim. (And these days I fail to see why too much effort is warranted when the typical YEC response is to block all sensory organs so that the faith can be maintained. This essay is more the result of my own desire to learn and to pass on some of that to those YEC lurkers who may be more open to realistic ideas that, while a bit unsettling, are worthwhile anyway simply because they are substantiated, and grand in their scope.)
The essay is inspired by a set of articles published in Nature last year (1).
BACKGROUND
Today, we have two kinds of fish the lobe finned fish and the ray finned fish.
By far, most fish around today are ray fins. The bones in their front fins look something like those shown in the left hand figure of the first attachment (2).
A few other kinds of fish found today are the lobe fins. These are represented by lungfish and the Coelocanth. The bones in their front fins look those shown in the middle figure of the first attachment.
There is certainly a big difference, but what is interesting is if you compare the second and third figures in that first attachment. The third figure is a sketch of the generalized tetrapod forelimb. Notice how the lobed fin fore-fin has some important characteristics of the tetrapod forelimb? And notice how both the tetrapod limb and the lobe fin look nothing like the limb of the ray finned fish?
Perhaps you can begin to see why evolutionists would suspect that the tetrapod limb is a modified version of the lobe fin?
THE EVIDENCE
It is one thing to see the similarity and conclude relationship, it is another to find evidence for such a relationship.
One type of evidence would be to dig up the remains of animals that in some way, fill in the gap between the lobe finned fish and the true tetrapod.
Fossil evidence shows that fish have been swimming around for the best part of 500 million years. Thus, the bones (3) and the radio isotopes tell us that the ray finned fish were well established in the earths oceans at 440 million years ago. At 425 million years, the lobe finned fish are found, (Coelocanths at 425 Mya and lungfish at 417 Mya (4).) At 340 million years, real tetrapods, capable of living on land are seen.
Limbed tetrapods are not found in the fossil record at 400 Mya, only finned fish are. But they are at 340 Mya. Naturally, the question arises how did those tetrapods come to be if they did not exist earlier?
Roughly 150 years ago, two scientists offered an explanation for how animals like tetrapods could arise from pre-existing animals. Darwin and Wallace independently formulated a theory of evolution based on natural selection. Today, it is the most widely accepted theory of evolution. In the context of their theory, evidence is needed for changes from one kind of animal to another, if the theory is to have credibility. Well, a wealth of fossils show that something weird and wonderful was going on with respect to the lobe finned fish, from some 390 Mya to 360 Mya.
The fossils concerned have names such as Eusthenopteron (5) and (6), Panderichthys (6), Acanthostega (6), and Ichthyostega (6). As you will see from the links, the first is a fish, but it has legs while the last is an amphibian, a tetrapod, but it retains many fish characteristics. In between these two types of animals are many other fossils bearing names such as Elginepeton, Venstastega, and Metaxygnathus (7). These types show all sorts of characteristics that link Eusthenopteron to Ichthyosetega, and they even include animals with 7, 8 or 9 digits on hands and feet.
That is, over a 30 million year period, beginning some 390 Mya, one group of fish, within the lobe finned fish, (as distinct from the ray finned fish), began to take on more and more tetrapod characteristics, to the point that at 340 Mya, amphibians had arisen. In the process odd-ball combinations of digits were being tried. Eventually, the outcome was five digits, as found in tetrapods today.
When all of the fossil data (bone lengths and shapes, muscle attachments, bone orientations, holes for nerves and blood vessels etc.) from these animals are measured they can be objectively set out in the cladogram shown in the second attachment. An alternative diagram can be seen in the third attachment. These days, such diagrams as the third attachment make no claim for direct ancestry. That is, they are not drawn with the view of claiming that Tiktaalik evolved directly from Panderichthys. What is claimed is than an animal existed at such and such a time that had characteristics intermediate between those of Panderichthys and those of Acanthostega. While it is possible that one is directly related to another, more likely, some other animal existed, whose fossilized remains have not been discovered (and probably never will be), that is on the direct line. All that can be said is that the purported transitional (Tiktaalik in this case) is presumed to have characteristics representative of those of an animal on the direct path.
So what was it that was found?
I shall concentrate only on the front fin. The kind of description I provide here can also be given for other parts of the animals body, particularly its skull, and the body parts just behind the skull. That is, while the fins of Eusthenopteron and Panderichthys were beginning to look a bit like legs, while those of Acanthostega were legs, the fins of Tiktaalik were certainly fins but they also had strong internal tetarpod like limb bones and joints. The bones were the humerus, ulna and radius and the joints were a shoulder and elbow. And beyond those two joints were yet others in the smaller bones as shown in the fourth attachment. This same kind of thing can be seen in the skull, (and in other bones) but in a skull sense, rather than a leg/fin sense. That is, the skull has shape and structure somewhat similar to that of a reptile, and less like that of a fish. It has a neck, unlike a fish. But like a fish, it had gills and the remains of scales can be seen.
Fins in fish are used for propulsion, steering, stabilization and moving in the vertical plane. In particular, with respect to the fore-fin, steering, stabilization and vertical movement are the main uses. With the bones of Tiktaalik however, both their morphology and points of muscle contact show that much more than steering, stabilization and movement in the vertical plane were done. Unlike the earlier fish such as Eusthenopteron, this new one appears to have had more flexibility allowing for rotation, flexing and extending and pulling in or pushing away various parts of the limb. So while the fin retained fin rays, suggestive of normal fin activity, the inner bones suggested a limb activity which allowed the fin to be used as both a paddle and a means of supporting the front portion of the body by pushing it up from any substrate the fish may have been resting on.
PREDICTIONS
A complaint often made against evolutionary science, is its supposed inability to predict. I shall address this by offering five comments.
The first is that scientists do know what to look for and they go out in hope that geology and preservation have been kind and that the expected bones are found. Tiktaalik is an example of this.
The second is to ask you to examine the cladogram in the second attachment. If common descent with modification occurred (and occurs), then the cladogram should always hold. That is, any new fossil discovery should not spring nasty surprises such as an animal which cannot be placed into any one grouping. For example, the old canard of a fish with feathers - we know that feathers are diagnostic of birds (and their relative dinosaurs). Likewise, gills are diagnostic of fish (and their close relatives). A fish with feathers, (easily imaginable under the guise of a creator working outside of descent with modification), simply cannot be slotted into one group on a phylogenetic tree. Gills would put it in with fish. Feathers would put it in with birds. However a fish with legs puts the animal in with creatures with four limbs which can be split into two groups fish and tetrapods. And where do the fish with legs go? Right between the two groups. No unresolvable splits occur. One can measure all aspects of a fish and all aspects of a tetrapod and form two distinct groups. Fossil discoveries are then made from which similar measurements are taken and a third group forms quite naturally, between the fish and between the tetrapods. No discoveries occur which cause an unresolvable dilemma.
The third is to consider the cladogram in association with the geologic column. The cladogram tells a researcher what deposits to dig in with the hope of finding an intermediate. Reconsider the fifth attachment. Clearly an animal representing the transition between two groups is best searched for in the deposits between which those two groups are fist found. And if geology has been kind, and one is fortunate, then something relevant should be discovered. Scientists do this, and it works.
The fourth relates to the third. The researchers did exactly this. They wanted to locate an animal undergoing the transition between the fish Panderichthys and the tetrapod Acanthostega. They reasoned that it should be found somewhere in that relatively narrow 10 million year gap shown on the fifth attachment, providing geological and other preservation circumstances have been good. They went looking in the deposits of the right age. Such deposits had been discovered on an island off Canada in 1999 and by 2004, those deposits began to bear fruit. The researchers found an animal they had been looking for.
The fifth comment concerns my mention of Darwins and Wallaces theory of evolution by natural selection. Clearly, one test of the predictive nature of evolution is to look for certain bones in deposits of a particular geological age, based on relationships shown in phylogenetic trees. The other test is much harder to do and involves much speculation. The question arises as to what was being selected for and why. Evidence to answer questions of this nature does not fossilize. Evidence that fish were developing limbs to move from pool to pool in a drying environment does not fossilize. (What does fossilize is evidence that times were becoming drier.) That they were developing limbs in response to the opening of a new food niche as insects moved onto land following the colonization of land by plants that does not fossilize. (What does fossilize is evidence that plants were on land and that insects had just colonized the land.) Nevertheless, tetrapods live on land and fish live in water, while amphibians inhabit an intermediate environment. So researchers speculate that a shallow water environment would be the place where this transition from fin to limb was being driven. And certainly fossils discovered to date, appear to be from animals inhabiting shallow water environments.
So, not only did the scientists who discovered Tiktaalik know to look in sediments of a certain age, but they speculated that the best sediments to search in would be those of shallow water environments. The formations searched were those deemed, by geologists, to be from ancient meandering streams, that is, shallow water environments.
THINK ABOUT THIS
Tiktaalik is a fish. Refer back to that cladogram in the second attachment. When all bones are measured, holes counted, positions recorded, muscle attachment points considered, and so on, then Tiktaalik is closer to Elpistostege, a fish with legs, than it is to the tetrapods Acanthostega and Ichthyostega. That is why the animal is grouped where it is on the diagaram. Elpistostege has for a long time been recognized as an intermediate between fish and tetrapods, even though it was clearly a fish. Unfortunately, Elpistostege has only been known from a few very partial skeletons. Tiktaalik fills in those details missing from Elpistostege. Like all fish, Tiktaalik has scales and fin rays on its fins. It has gills. But unlike fish, the animal has a neck and a reptilian like head with a long snout and eye sockets on top. Inside its fins, the bones clearly follow the tetrapod pattern of jointed bones. It has a flattened body.
And these kinds of discoveries keep rolling in.
REFERENCES and NOTES
(1) Per Erik Ahlberg and Jennifer A. Clack, A firm step from water to land, Nature, 440 6-April-2006, p747-749
Edward B. Daeschler, Neil H. Shubin & Farish A Jenkins Jr, A Devonian tetrapod-like fish and the evolution of the tetrapod body plan, Nature, 440 6-April-2006, p757-763.
Neil H. Shubin, Edward B. Daeschler & Farish A Jenkins Jr, The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb, Nature, 440 6-April-2006, p764-771.
(2) Diagrams taken from George C. Kent and Robert E. Carr, Comparative Anatomy of the Vertebrates, 9th Ed., McGraw Hill, 2001.
(3) This part taken from Richard Dawkins, The Ancestors Tale: A Pilgrimage to the Dawn of Life, Weidenfelf & Nicolson. A problem arises when one consults another source. These times can vary somewhat, illustrating the pace of progress. Within the space of a few months an article can be published which pushes the origin of one group or another back further in time.
(4) Mya is just a shorter way of writing millions of years ago which is another way of saying before you and I were born :teeth:
(5) Here is a good link:-
http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.860.html#Eusthenopteron.
The authors of the link begin:-
[box]Eusthenopteron: the "Fish with Legs"
What can we say about Eusthenopteron that has not already been said, much better, by someone else? Eusthenopteron is the gold standard for osteolepiforms, the Morganucodon of sarcopterygians, and, in fact, probably the best known Paleozoic vertebrate of any taxon, bar none.[/box]
Note the title? The animal is a fish. But it has legs.
(6) Here is another link (Go to the end of the page and view the last diagram. Then contemplate it.):-
http://people.eku.edu/ritchisong/RITCHISO//342notes5.htm
(7) Mark Ridley Evolution, 3rd edition, Blackwell Science Ltd, 2004. See figure 18.9
![[serious]](/data/avatars/m/160/160873.jpg?1431539545){kind=avatar}
