Freethought & Rationalism Archive

Godless Dave · 01-24-2003, 05:17 AM

Is there a generally accepted ancestor for all land animals? I need it for some idiot at Christian Forums, not that it's going to do any good.

MrDarwin · 01-24-2003, 06:05 AM

Try doing a google search on "tetrapod evolution". You'll find lots of good stuff. Here's one good example.

As the oldest known tetrapods, Acanthostega and Ichthyostega are pretty good candidates for the ancestors of all 4-legged land creatures. If somebody complains that they're "just amphibians", point out that these creatures were completely aquatic, and in fact share many characteristics with slightly earlier creatures that most people would call "fish". Basically, they are fish with legs (and even the legs, bearing hands with too many fingers, are somewhat transitional).

Tetrapods apparently evolved in the water, only emerging as terrestrial creatures after evolving legs--rather counterintuitive, but that's sure how it's looking right now.

Godless Dave · 01-24-2003, 06:13 AM

Thanks Mr D!

Monkey · 01-24-2003, 07:43 AM

Wern't Eusthenopteron, Panderichthys and Elginerpeton all earlier Devonian tetrapods than Ichthyostega and Acanthostega?

MrDarwin · 01-24-2003, 08:05 AM

Panderichthys had fins so is considered a fish. Regarding some interesting creatures like Elginerpeton ,

Quote:

The skull of this animal exhibits a mosaic of elpistostegelian fish and tetrapod features. The hip and limbs share features with Ichthyostega while the shoulder is more similar to those of Hynerpeton or Tulerpeton. Unfortunately, no feet (or fish-like fins) were recovered. Consequently, its unclear whether Elginerpeton is more closely allied with the Panderichthys or with other early tetrapods.

In other words, except for their legs these early tetrapods are pretty much indistinguishable from the lobe-finned fish that were swimming around at the time; if the fossils don't have legs (or fins) you can't say with confidence which group to put it in.

pz · 01-24-2003, 08:42 AM

These are from Zimmer's book.

Eusthenopteron and Acanthostega:

An overview of tetrapod phylogeny:

Blinn · 01-24-2003, 08:43 AM

A great book on this topic which Mr. Darwin brought up earlier is "Gaining Ground" by Jennifer Clack.

Oolon Colluphid · 01-24-2003, 02:13 PM

Quote:

Originally posted by Zetek
A great book on this topic which Mr. Darwin brought up earlier is "Gaining Ground" by Jennifer Clack.

... Jenny Clack, who has worked on various of these fossils with sometime Infidel Per Ahlberg.

You there Per? This thread needs you!

Cheers, DT

Per Ahlberg · 01-28-2003, 02:41 AM

Errm, yes. Hi Simon, Godless Dave and everyone! I happen to be in the neighbourhood for a change, so here goes -

The key points about the tetrapod family tree, nicely shown in Carl Zimmer's figure, are the crown group, crown group node and stem group. The crown group comprises all living tetrapods (mammals, birds, reptiles and amphibians) and their fossil kin (such as dinosaurs), down to the last common ancestor of the living lineages. This 'last common ancestor' is represented by the crown group node. (In Carl's figure this is the node uniting the amniotes + Tulerpeton branch and the amphibians + Greererpeton branch.) Below the crown group node we find the stem group, represented in Carl's figure by the sequence of animals from Hynerpeton down to Eusthenopteron. The stem group consists entirely of extinct forms that 'map out' the lineage leading up to the crown group. In a collective sense they are 'ancestral' to the crown group, though the poor sampling of the early tetrapod fossil record makes it doubtful whether we have actually found any of the direct ancestors yet.
Returning to Godless Dave's question, I suppose that in the strictest sense you're asking for the common ancestor that occupies the crown group node. No luck, I'm afraid: there are a number of Carboniferous tetrapod groups that clearly fall very close to this node (colosteids, baphetids, Crassigyrinus, Eucritta...) but we can't put a finger on one particular animal. If we're looking for the transition from water to land, it is beautifully documented by the sequence Eusthenopteron-Panderichthys-Acanthostega-Ichthyostega. New fossils are continuing to come to light such as Pederpes, a Tulerpeton-like form from the Early Carboniferous of Scotland that Jenny Clack described in July last year, or Sinostega, the first Devonian tetrapod from Asia, that I described with three Chinese colleagues on 19 December (both papers in Nature). There's also new material of Ventastega to report, including an as yet undescribed near-complete skull. Still, I doubt whether it will convince your phylogenetically challenged C.F. contributor...

Cheers, Per

MrDarwin · 01-28-2003, 07:18 AM

Per, it's great to have you here! As you point out, none of this will convince the hard-core creationists, who will ignore the most convincing series of intermediate and transitional forms if we cannot say "species A begat species B, which begat species C, etc." But the recent fossils relating to tetrapod (and bird, and arthropod) evolution are so exciting that I think we need to call attention to them, over and over, to win over those fence-sitters who might otherwise be convinced by the continuing creationist chorus of "there are no transitional fossils!"

01-24-2003, 05:17 AM	#1
Godless Dave Contributor Join Date: Feb 2002 Location: With 10,000 lakes who needs a coast? Posts: 10,762	Common tetrapod ancestor Is there a generally accepted ancestor for all land animals? I need it for some idiot at Christian Forums, not that it's going to do any good.

01-24-2003, 08:42 AM	#6
pz Veteran Member Join Date: Aug 2001 Location: Morris, MN Posts: 3,341	Pictures! These are from Zimmer's book. Eusthenopteron and Acanthostega: An overview of tetrapod phylogeny:

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01-24-2003, 06:05 AM	#2
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	Try doing a google search on "tetrapod evolution". You'll find lots of good stuff. Here's one good example. As the oldest known tetrapods, Acanthostega and Ichthyostega are pretty good candidates for the ancestors of all 4-legged land creatures. If somebody complains that they're "just amphibians", point out that these creatures were completely aquatic, and in fact share many characteristics with slightly earlier creatures that most people would call "fish". Basically, they are fish with legs (and even the legs, bearing hands with too many fingers, are somewhat transitional). Tetrapods apparently evolved in the water, only emerging as terrestrial creatures after evolving legs--rather counterintuitive, but that's sure how it's looking right now.

01-24-2003, 06:13 AM	#3
Godless Dave Contributor Join Date: Feb 2002 Location: With 10,000 lakes who needs a coast? Posts: 10,762	Thanks Mr D!

01-24-2003, 07:43 AM	#4
Monkey Junior Member Join Date: Jan 2003 Location: Perth, Western Australia Posts: 93	Wern't Eusthenopteron, Panderichthys and Elginerpeton all earlier Devonian tetrapods than Ichthyostega and Acanthostega?

01-24-2003, 08:43 AM	#7
Blinn Veteran Member Join Date: Aug 2001 Location: Ohio, USA Posts: 1,162	A great book on this topic which Mr. Darwin brought up earlier is "Gaining Ground" by Jennifer Clack.

01-28-2003, 02:41 AM	#9
Per Ahlberg Senior Member Join Date: Feb 2002 Posts: 582	Errm, yes. Hi Simon, Godless Dave and everyone! I happen to be in the neighbourhood for a change, so here goes - The key points about the tetrapod family tree, nicely shown in Carl Zimmer's figure, are the crown group, crown group node and stem group. The crown group comprises all living tetrapods (mammals, birds, reptiles and amphibians) and their fossil kin (such as dinosaurs), down to the last common ancestor of the living lineages. This 'last common ancestor' is represented by the crown group node. (In Carl's figure this is the node uniting the amniotes + Tulerpeton branch and the amphibians + Greererpeton branch.) Below the crown group node we find the stem group, represented in Carl's figure by the sequence of animals from Hynerpeton down to Eusthenopteron. The stem group consists entirely of extinct forms that 'map out' the lineage leading up to the crown group. In a collective sense they are 'ancestral' to the crown group, though the poor sampling of the early tetrapod fossil record makes it doubtful whether we have actually found any of the direct ancestors yet. Returning to Godless Dave's question, I suppose that in the strictest sense you're asking for the common ancestor that occupies the crown group node. No luck, I'm afraid: there are a number of Carboniferous tetrapod groups that clearly fall very close to this node (colosteids, baphetids, Crassigyrinus, Eucritta...) but we can't put a finger on one particular animal. If we're looking for the transition from water to land, it is beautifully documented by the sequence Eusthenopteron-Panderichthys-Acanthostega-Ichthyostega. New fossils are continuing to come to light such as Pederpes, a Tulerpeton-like form from the Early Carboniferous of Scotland that Jenny Clack described in July last year, or Sinostega, the first Devonian tetrapod from Asia, that I described with three Chinese colleagues on 19 December (both papers in Nature). There's also new material of Ventastega to report, including an as yet undescribed near-complete skull. Still, I doubt whether it will convince your phylogenetically challenged C.F. contributor... Cheers, Per

01-28-2003, 07:18 AM	#10
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	Per, it's great to have you here! As you point out, none of this will convince the hard-core creationists, who will ignore the most convincing series of intermediate and transitional forms if we cannot say "species A begat species B, which begat species C, etc." But the recent fossils relating to tetrapod (and bird, and arthropod) evolution are so exciting that I think we need to call attention to them, over and over, to win over those fence-sitters who might otherwise be convinced by the continuing creationist chorus of "there are no transitional fossils!"

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