Saturday, April 15, 2006

Discovery of a Limb-Finned Fish

` I have been so unwell lately this 32-day period (!!!) that I haven't even been able to drag myself to a public computer in order to edit and post one of my drafts, though I have roughed-out several.
` Here's the start of a draft concerning something I read about in Nature, which was:

A Devonian tetrapod-like fish and the evolution of the tetrapod body plan
Edward B. Daeschler1, Neil H. Shubin2 and Farish A. Jenkins, Jr3

Abstract

The relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) is well established, but the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes. Here we report the discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose. Although the body scales, fin rays, lower jaw and palate are comparable to those in more primitive sarcopterygians, the new species also has a shortened skull roof, a modified ear region, a mobile neck, a functional wrist joint, and other features that presage tetrapod conditions. The morphological features and geological setting of this new animal are suggestive of life in shallow-water, marginal and subaerial habitats.
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Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103, USA
University of Chicago, Department of Organismal Biology and Anatomy, 1027 E. 57th Street, Chicago, Illinois 60637, USA
Harvard University, Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, 26 Oxford Street, Cambridge, Massachusetts 02138, USA
` There is also an:

Editor's Summary
6 April 2006

When fins became limbs
The transition between fishes and limbed vertebrates, or tetrapods, occurred over 370 million years ago and required changes to virtually the entire body. Sensational fossil finds, and reinterpretations of old ones, have radically altered thinking on this topic in the past 20 years. But the transition itself – the very point where fishes became tetrapods – remains obscure. What fossils there are tend to be incomplete or badly preserved. All that changes with the discovery of remarkable new fossils from the late Devonian of Canada of a near-complete transitional form preserved in the round. It's a fish with fins, but fins that flexed and extended like arms and hands. It has tetrapod-like ribs, a mobile neck and wrist. The impact of this discovery will be felt far and wide in evolutionary biology. On the cover, the fossil as found emerges from under a log as it might have in life in a shallow-water habitat.

` And, as for the limbs (and the name) of the creature itself:

Article
Nature 440, 764-771 (6 April 2006) doi:10.1038/nature04637; Received 11 October 2005; ; Accepted 8 February 2006
The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb
Neil H. Shubin1, Edward B. Daeschler2 and Farish A. Jenkins, Jr3


Wrists, ankles and digits distinguish tetrapod limbs from fins, but direct evidence on the origin of these features has been unavailable. Here we describe the pectoral appendage of a member of the sister group of tetrapods, Tiktaalik roseae, which is morphologically and functionally transitional between a fin and a limb. The expanded array of distal endochondral bones and synovial joints in the fin of Tiktaalik is similar to the distal limb pattern of basal tetrapods. The fin of Tiktaalik was capable of a range of postures, including a limb-like substrate-supported stance in which the shoulder and elbow were flexed and the distal skeleton extended. The origin of limbs probably involved the elaboration and proliferation of features already present in the fins of fish such as Tiktaalik.
Top of page
Department of Organismal Biology and Anatomy, The University of Chicago, Chicago, Illinois 60637, USA
Academy of Natural Sciences of Philadelphia, 19th and Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103, USA
Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA

` My friend Dory also sent me an e-mail of a more 'friendly' article about Tiktaalik in the New York Times. It describes this animal in more image-invoking terms:

"The origin of limbs," Dr. Shubin's team wrote, "probably involved the elaboration and proliferation of features already present in the fins of fish such as Tiktaalik."...

...They found several of the fishes in a quarry, their skeletons largely intact and in three dimensions. The large skull had the sharp teeth of a predator. It was attached to a neck, which allowed the fish the unfishlike ability to swivel its head.

If the animal spent any time out of water, said Dr. Jenkins, of Harvard, it needed a true neck that allowed the head to move independently on the body.

Embedded in the pectoral fins were bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals. The joints of the fins appeared to be capable of functioning for movement on land, a case of a fish improvising with its evolved anatomy. In all likelihood, the scientists said, Tiktaalik flexed its proto-limbs mainly on the floor of streams and might have pulled itself up on the shore for brief stretches.

In their report, the scientists concluded that Tiktaalik was an intermediate between the fishes Eusthenopteron and Panderichthys, which lived 385 million years ago, and early tetrapods. The known early tetrapods are Acanthostega and Ichthyostega, about 365 million years ago.

Tiktaalik, Dr. Shubin said, is "both fish and tetrapod, which we sometimes call a fishapod."
` Irritatingly, though, for a science article, there was something which is completely irrelevent to the subject:
Dr. Shubin's team played down the fossil's significance in the raging debate over Darwinian theory, which is opposed mainly by some conservative Christians in this country, but other scientists were not so reticent. They said this should undercut the argument that there is no evidence in the fossil record of one kind of creature becoming another kind.
` I suppose it would be fine to leave it at that. But, no... weight is actually lent to the conservative Christians. (Or is it? Well, I suppose attention is power.) What irks me mildly is that the reporter obviously is not even familiar with science, creationism, nor the proper usage of nonscientific terms like 'major kinds'. This term was made up, of course, because no discernable border has ever been found between any so-called 'kind' wherever anything is known about it.

Dr. Novacek responded: "We've got Archaeopteryx, an early whale that lived on land, and now this animal showing the transition from fish to tetrapod. What more do we need from the fossil record to show that the creationists are flatly wrong?"

` No! Stop arguing and acting arrogant! You're just drawing more attention to creationism! But then... he was probably prompted to by the reporter.

Duane T. Gish, a retired official of the Institute for Creation Research in San Diego, said, "This alleged transitional fish will have to be evaluated carefully." But he added that he still found evolution "questionable because paleontologists have yet to discover any transitional fossils between complex invertebrates and fish, and this destroys the whole evolutionary story."

` This display of Great Reasoning was brought to you by; Duane Gish! One of my favorite creationists of all times. That's right, folks! As long as you ignore Cathaymyrus diadexus, Pikaia, Yunnanozoon, Haikouella, conodont animals, Myllokunmingia, and Haikouichthys, it would look as if there were no proto-vertebrates of any kind! On top of this, when you factor in the point that we have no evidence that suggest that those fossils shouldn't exist - even though they clearly do - we'll just assume that Duane's way is right.
` Scientific reasoning, on the other hand, states that lack of evidence means you need to have an open mind until the 'gap' in one's knowledge is found. So far, no fossils (or genes, or quirks of nature) have been found that go against common descent in general, and so there is no reason to think that anything will yet be found to contradict it.
` What was I saying? Oh yes; Duane also likes to ignore the existence of living animals such as cephalochordates, and especially tunicates, which are animals with spinal cords (but not spines) whose brains degenerate as adults. These worm-like beasts are the most closely-related animals to those of us with backbones.
` I am not terribly knowledgeable about all of that which links vertebrates with vertebrate-like animals, though it is clear that there is a continuum between them.

` Ironically, I may be accused of 'Tunicating' (losing my brain) by some creationists for this kind of thinking. Well, I'm sticking to my guns. And since I did not publish this post until just now, I have discovered yet another Nature update on tetrapod evolution:

Editor's Summary
13 April 2006

Stranger on the shore
Last week's Nature cover featured a fossil that marked a world-changing event, the development of limbed vertebrates from finned fish. A glimpse of how that may have led to the 'conquering of the land' now comes from an unexpected source: the eel-catfish found in the muddy swamps of tropical Africa. It has the remarkable ability to forage and capture prey on land. This animal's capacity to bend its head downwards when feeding seems to be the secret of its adaptability.
` In other words, this is direct proof that fish - at least rubber-necked ones - will sometimes strike out on land in pursuit of prey. Since this is a fairly useful technique, one would expect that fish would occasionally employ it when given the opportunity.
` If this behavior continued for long enough, it would probably facilitate the evolution of limbs as in Tiktaalik because having a little extra mobility on shore would help a land-lubbing animal to find food and thus have a better chance of reproducing than variants with less mobile fins.

` I just realized that this is a lovely mess of words. I seriously don't have much time to edit more carefully, but that's the way it goes. Tune in next time!

6 comments:

Galtron said...

A jointed fish fin! I never thought I would live to see the day of that discovery!
...That day came about a week ago, but no matter. The thing with Duane was pretty funny.

S E E Quine said...

` Well excuse me for posting this days after my original attempt.

` No cookies for you.

Galtron said...

At least I still have cake.

Aaron said...

Interesting post Spoony. I wonder how the religious fanatics explain documented evolution of species.

The triumphant return of AaronKCMO is at hand. An explainatory post is upcoming.

S E E Quine said...

` Well, they don't, really. But that doesn't matter because their followers will swallow anything.

` Welcome back to the blogging world! I'm surprised I haven't disappeared myself!

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