Freethought & Rationalism Archive

badger3k · 09-04-2005, 12:35 PM

I hadn't seen a thread, and a search came up with nothing, so I hope this is new for people. Per Ahlberg said a few months back to look for a new publication. Well, it seems it came out.

http://mcdougald.blogspot.com/2005/0...s-of-land.html

and

http://mcdougald.blogspot.com/2005/0...l-is-that.html

My access doesn't include the most recent year of nature, so I can't download the paper now. Have to look for it in stores. I'm not yet qualified to properly understand it all, but I have hopes (and a few textbooks)

.

RBH · 09-04-2005, 02:51 PM

Here's the Abstract of the Sept 1, 2005 paper:

Quote:

Ichthyostega was the first Devonian tetrapod to be subject to a whole-body reconstruction1–3. It remains, together with Acanthostega4, one of only two Devonian tetrapods for which nearcomplete postcranial material is available. It is thus crucially important for our understanding of the earliest stages of tetrapod evolution and terrestrialization. Here we show a new reconstruction of Ichthyostega based on extensive re-examination of original material and augmented by recently collected specimens. Our reconstruction differs substantially from those previously published and reveals hitherto unrecognized regionalization in the vertebral column. Ichthyostega is the earliest vertebrate to show obvious adaptations for non-swimming locomotion. Uniquely among early tetrapods, the presacral vertebral column shows pronounced regionalization of neural arch morphology, suggesting that it was adapted for dorsoventral rather than lateral flexion.

The axial skeleton of the Devonian tetrapod Ichthyostega. Nature 437, 137-140 (01 Sep 2005). Per Erik Ahlberg1, Jennifer A. Clack2 & Henning Blom1,2

RBH

badger3k · 09-04-2005, 09:08 PM

Managed to get a copy of the article - thanks. Now to read it! :thumbs:

Per Ahlberg · 09-05-2005, 12:17 AM

Hi All,

I didn't want to blow my own trumpet, but yes, Ichthyostega is out. It's covered on National Geographic News here, Science Now here and Pharyngula here. The New York Times is supposed to be covering the story on Tuesday. As usual, anybody wanting a pdf is welcome to PM me (don't forget to include your e-mail address!). I'm away for a week from tomorrow at a conference in London, so it may take me a while to get back to you, but all requests will be honoured. Oolon, I seem to have lost your e-mail address, so you will have to drop me a line as well.

Incidentally, there's more coming: two further papers from our lab (both in Nature) dealing with the near-tetrapod Panderichthys, and another from an overseas lab (probably Nature, but possibly Science) that I can't say more about at the moment except to put you all on Major Transitional Alert. It should be an interesting autumn.

Cheers, Per

RBH · 09-05-2005, 07:57 PM

An account of the Nature paper made the NY Times (free registration required).

Quote:

Their research shows that the animal, which was about three feet long with short legs and a long tail, could not have undulated like a fish, but could have arched its spine, something like an inchworm, to move.

"Flexing and straightening of the back really helps you along," said Per Erik Ahlberg, a professor at Uppsala and the lead author of a paper describing the reconstruction in the journal Nature.

"It's the principle of the mammalian gallop," he added. "Ichthyostega, with its short and stumpy legs, obviously didn't gallop. "But it might have been able to effect some inchworm-like shuffle." If so, it probably dragged its hind legs and tail, he said.

Well done!

RBH

Oolon Colluphid · 09-06-2005, 01:02 AM

This is interesting. The model I have had in mind -- not that I've given it much thought -- for tetrapod locomotion is of side-to-side movement, like a salamander or a crocodile... and derived from the lateral tail-swinging of their fishy ancestors. In the same way that whale tail flukes are moved up and down because their ancestors had dorsoventrally-flexing spines for running. Only once they started lifting themselves up off the ground more, I thought, did tetrapods need an up-and-down-moving spine, for running. Per, what's going on here? A basal tetrapod, ancestral (or close to it) to salamanders and crocs, but with an inchworm locomotion?

Oolon Colluphid · 09-06-2005, 01:07 AM

Ah, I see. Evolutionary dead end, a locomotive try-out that didn't last the course.

The creationists will now be able to say it's not transitional because it's a dead-end side-branch (again)...

Per Ahlberg · 09-06-2005, 02:01 AM

Yes, Ichthyostega is a bit of an odd beast. We were very surprised when we realised how "mammal-like" the vertebral column is. One point that perhaps didn't come through strongly enough in the article is that the tail appears to have had normal lateral flexibility; it is only the presacral vertebral column (i.e. from the hips forwards) that appears unable to flex laterally. We suspect that the large fan-shaped neural arches above the hips, and the expanded ribs jut behind the pelvis, relate functionally to this distinction between a stiff presacral backbone and a laterally flexible tail. In a swimming salamander or crocodile, the lateral undulations start somewhere around the mid-trunk and grow gradually in amplitude as they travel backwards. In Ichthyostega, however, the undulations could only start behind the hips, and could only be powered by tail muscles, not tail and trunk muscles in concert. The expanded neural arches and ribs at the tail base probably served as attachments for enlarged tail muscles that gave the tail enough propulsive "oomph". I imagine tail-propelled locomotion in Ichthyostega looked rather funny, with the quite short tail swishing frantically from side to side like one of those wind-up bath toys.

As for the presacral column, the specialized features we see in Ichthyostega do not appear in the lineage leading to modern tetrapods. It is informative to compare Ichthyostega with the slightly later and phylogenetically "higher" Pederpes from the Early Carboniferous. In many ways Pederpes has a rather Ichthyostega-like skeleton - for example, it too has flanged ribs - but the neural arches do not show "mammal-like" regionalization and their zygapophyses (the joint surfaces that connect them) are broad and horizontal, indicating a laterally flexible backbone.

Cheers, Per

Martin B · 09-06-2005, 02:42 AM

Quote:

Originally Posted by Per Ahlberg

Yes, Ichthyostega is a bit of an odd beast. We were very surprised when we realised how "mammal-like" the vertebral column is.

Ah HA!!!!!!!!!!!!!11111!!!!1!1! See? It's not a transitional form!!!!1! This is a clear violation of the nested hierarchy! These superficial similarities with mammals speak volumes about a common designer but fly in the face of evolution!

On a more serious note: I have found some great images that I think help to illustrate the "mammal-likeness" of Ichthyostega. What one can see is that Ichthyostega is effectively modifying a similar construction seen in Acanthostega (very bottom) to "imitate" the condition that mammals exhibit. If any creationists out there wish to word-twist (and it's only a matter of days/hours before AiG has a raving article posted...), they should take a look at what is meant by "mammal-like".

Per Ahlberg · 09-06-2005, 02:49 AM

Martin, is that a thylacine? Looks like a dog, but it seems to have epipubic bones (which support the pouch in marsupials).

P.

09-04-2005, 12:35 PM	#1
badger3k Veteran Member Join Date: Jul 2004 Location: Killeen, TX Posts: 1,388	Ichthyostega - Per's Latest I hadn't seen a thread, and a search came up with nothing, so I hope this is new for people. Per Ahlberg said a few months back to look for a new publication. Well, it seems it came out. http://mcdougald.blogspot.com/2005/0...s-of-land.html and http://mcdougald.blogspot.com/2005/0...l-is-that.html My access doesn't include the most recent year of nature, so I can't download the paper now. Have to look for it in stores. I'm not yet qualified to properly understand it all, but I have hopes (and a few textbooks) .

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09-04-2005, 09:08 PM	#3
badger3k Veteran Member Join Date: Jul 2004 Location: Killeen, TX Posts: 1,388	Managed to get a copy of the article - thanks. Now to read it! :thumbs:

09-05-2005, 12:17 AM	#4
Per Ahlberg Senior Member Join Date: Feb 2002 Posts: 582	Hi All, I didn't want to blow my own trumpet, but yes, Ichthyostega is out. It's covered on National Geographic News here, Science Now here and Pharyngula here. The New York Times is supposed to be covering the story on Tuesday. As usual, anybody wanting a pdf is welcome to PM me (don't forget to include your e-mail address!). I'm away for a week from tomorrow at a conference in London, so it may take me a while to get back to you, but all requests will be honoured. Oolon, I seem to have lost your e-mail address, so you will have to drop me a line as well. Incidentally, there's more coming: two further papers from our lab (both in Nature) dealing with the near-tetrapod Panderichthys, and another from an overseas lab (probably Nature, but possibly Science) that I can't say more about at the moment except to put you all on Major Transitional Alert. It should be an interesting autumn. Cheers, Per

09-06-2005, 01:02 AM	#6
Oolon Colluphid Contributor Join Date: Sep 2000 Location: Alibi: ego ipse hinc extermino Posts: 12,591	This is interesting. The model I have had in mind -- not that I've given it much thought -- for tetrapod locomotion is of side-to-side movement, like a salamander or a crocodile... and derived from the lateral tail-swinging of their fishy ancestors. In the same way that whale tail flukes are moved up and down because their ancestors had dorsoventrally-flexing spines for running. Only once they started lifting themselves up off the ground more, I thought, did tetrapods need an up-and-down-moving spine, for running. Per, what's going on here? A basal tetrapod, ancestral (or close to it) to salamanders and crocs, but with an inchworm locomotion?

09-06-2005, 01:07 AM	#7
Oolon Colluphid Contributor Join Date: Sep 2000 Location: Alibi: ego ipse hinc extermino Posts: 12,591	Ah, I see. Evolutionary dead end, a locomotive try-out that didn't last the course. The creationists will now be able to say it's not transitional because it's a dead-end side-branch (again)...

09-06-2005, 02:01 AM	#8
Per Ahlberg Senior Member Join Date: Feb 2002 Posts: 582	Yes, Ichthyostega is a bit of an odd beast. We were very surprised when we realised how "mammal-like" the vertebral column is. One point that perhaps didn't come through strongly enough in the article is that the tail appears to have had normal lateral flexibility; it is only the presacral vertebral column (i.e. from the hips forwards) that appears unable to flex laterally. We suspect that the large fan-shaped neural arches above the hips, and the expanded ribs jut behind the pelvis, relate functionally to this distinction between a stiff presacral backbone and a laterally flexible tail. In a swimming salamander or crocodile, the lateral undulations start somewhere around the mid-trunk and grow gradually in amplitude as they travel backwards. In Ichthyostega, however, the undulations could only start behind the hips, and could only be powered by tail muscles, not tail and trunk muscles in concert. The expanded neural arches and ribs at the tail base probably served as attachments for enlarged tail muscles that gave the tail enough propulsive "oomph". I imagine tail-propelled locomotion in Ichthyostega looked rather funny, with the quite short tail swishing frantically from side to side like one of those wind-up bath toys. As for the presacral column, the specialized features we see in Ichthyostega do not appear in the lineage leading to modern tetrapods. It is informative to compare Ichthyostega with the slightly later and phylogenetically "higher" Pederpes from the Early Carboniferous. In many ways Pederpes has a rather Ichthyostega-like skeleton - for example, it too has flanged ribs - but the neural arches do not show "mammal-like" regionalization and their zygapophyses (the joint surfaces that connect them) are broad and horizontal, indicating a laterally flexible backbone. Cheers, Per

09-06-2005, 02:49 AM	#10
Per Ahlberg Senior Member Join Date: Feb 2002 Posts: 582	Martin, is that a thylacine? Looks like a dog, but it seems to have epipubic bones (which support the pouch in marsupials). P.

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