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Ken Shackleton
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 Posted: Mon Jun 09, 2008 3:15 am Post subject: At the Water's Edge |
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Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
Ken |
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rmacfarl
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| Posted: Mon Jun 09, 2008 3:38 am Post subject: Re: At the Water's Edge |
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On Jun 9, 1:15 pm, Ken Shackleton <ken.shackle...@shaw.ca> wrote:
| Quote: |
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
Ken
|
Haven't read the book but I have read (possibly an S.J. Gould essay?)
that the feet of amphibians are sufficiently different to reptiles
etc. to suggest a possible separate evolution. I think it's that their
front feet only aver have 4 toes & develop differently to the normal
tetrapod feet. All from fallible memory...
Ross Macfarlane |
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Ken Shackleton
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| Posted: Mon Jun 09, 2008 5:23 am Post subject: Re: At the Water's Edge |
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On Jun 8, 10:55 pm, Rupert Morrish <rup...@morrish.org> wrote:
| Quote: |
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book, but I won't let that stop me commenting ;)
I thought the point of tetrapods was that they were our most recent
common ancestor with amphibians? Tetrapods may have diverged into marine
and freshwater species, and thence to amphibians and we reptilomorphs
respectively, but I think that's a technical argument over what you
consider the first amphibians.
|
I was not talking about first amphibians, I was talking about the
tetrapod lineage that led to modern amphibians....the ones that never
got past being amphibians.
It would still be true that our most recent common ancestor with
amphibians would have been a tetrapod, since they were around for a
long time and were quite diverse even when they were all still fish.
Some lived in marine environments and others in fresh water
environments. Zimmer seems to be arguing, on the basis of physiology,
that modern amphibians are descended from a fresh-water tetrapod, and
that amniotes are descended from a different, and most likely marine
lineage.
| Quote: |
Ken
-----------------www.Newsgroup-Binaries.com- *Completion*Retention*Speed*
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Rev Dr' Lenny Flank
Guest
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| Posted: Mon Jun 09, 2008 6:53 am Post subject: Re: At the Water's Edge |
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On Jun 8, 11:15 pm, Ken Shackleton <ken.shackle...@shaw.ca> wrote:
| Quote: |
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
|
A nitpick -- the marine toad, also known as the cane toad, also known
as the giant toad, _Bufo marinus_, is not a marine animal, never
enters water as an adult, marine or otherwise, and breeds only in
freshwater. They are quite common in the dry Australian outback,
where they were introduced. They've also been introduced to Florida.
The name is a misnomer.
They make good pets, by the way. :)
================================================
Lenny Flank
"There are no loose threads in the web of life"
Editor, Red and Black Publishers
http://www.RedAndBlackPublishers.com |
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The Last Conformist
Guest
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| Posted: Mon Jun 09, 2008 9:21 am Post subject: Re: At the Water's Edge |
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On Jun 9, 6:55 am, Rupert Morrish <rup...@morrish.org> wrote:
| Quote: |
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book, but I won't let that stop me commenting ;)
I thought the point of tetrapods was that they were our most recent
common ancestor with amphibians?
|
There are several conflicting definitions of "Tetrapoda" in
circulation. One is indeed (modern) amphibians, amniotes (incl
humans), their last common ancestor and all descendants (this is the
"crown group"). Another common one is the historical definition, ie.
all animals with four feet (plus secondarily reduced forms like
snakes). The later definition covers a larger group than the former -
it's unclear exactly how much larger since there are several large
exinct groups that may or may not be inside the crown.
| Quote: |
Tetrapods may have diverged into marine
and freshwater species, and thence to amphibians and we reptilomorphs
respectively, but I think that's a technical argument over what you
consider the first amphibians. |
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The Last Conformist
Guest
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| Posted: Mon Jun 09, 2008 9:32 am Post subject: Re: At the Water's Edge |
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On Jun 9, 5:15 am, Ken Shackleton <ken.shackle...@shaw.ca> wrote:
| Quote: |
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
|
I don't understand the difference. Obviously, amniotes and
lissamphibians descend from the same lineage if we consider a time
before their last common ancestor, and different lineages if we
consider a time after the LCA. |
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Rupert Morrish
Guest
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| Posted: Mon Jun 09, 2008 9:55 am Post subject: Re: At the Water's Edge |
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Ken Shackleton wrote:
| Quote: |
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
|
I haven't read the book, but I won't let that stop me commenting ;)
I thought the point of tetrapods was that they were our most recent
common ancestor with amphibians? Tetrapods may have diverged into marine
and freshwater species, and thence to amphibians and we reptilomorphs
respectively, but I think that's a technical argument over what you
consider the first amphibians.
| Quote: |
Ken
----------------- |
www.Newsgroup-Binaries.com - *Completion*Retention*Speed*
Access your favorite newsgroups from home or on the road
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Ken Shackleton
Guest
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| Posted: Wed Jun 11, 2008 1:42 am Post subject: Re: At the Water's Edge |
|
|
On Jun 10, 7:05 pm, John Harshman <jharshman.diespam...@pacbell.net>
wrote:
| Quote: |
Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs?
|
It shows Tulerpeton with Amniotes and Greerpeton with Amphibians, they
all come to a common node and the next nodes [back in time] are
Hynerpeton, Ichthyostega, and Acanthostega respectively. There are
several others, but I do not see Westlothiana or Seymouriamorphs in
the diagram. Even though the text states that there were 18 tetrapods
in the analysis, the diagram in the book has about half that number.
It does say that the cladogram presented in the book is a simplified
version of the one that Coates created. |
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Ken Shackleton
Guest
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| Posted: Wed Jun 11, 2008 2:07 am Post subject: Re: At the Water's Edge |
|
|
On Jun 10, 7:40 pm, r norman <r_s_norman@_comcast.net> wrote:
| Quote: |
On Tue, 10 Jun 2008 18:05:19 -0700, John Harshman
jharshman.diespam...@pacbell.net> wrote:
Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs?
I have been without Internet the past few days so I am late joining
this discussion. First, the presence of urea has little to do with a
marine habit except in several special cases: the chondrichthyes
(sharks and relatives) have high urea as does the coelocanth and the
special amphibian, Rana cancrivora. In all these cases, it is used to
build a high osmotic pressure of the body fluids while retaining the
low salt concentration more typical of and associated with freshwater
life. Certainly it has nothing to do with buoyancy. In terrestrial
animals, it is a nice water-soluble form for the elimination of
ammonia produced by metabolism of amino acids. Truly aquatic animals
have large respiratory surfaces in contact with water through which
ammonia can freely leave; terrestrial animals can't lose ammonia that
way and must convert it to some less toxic form. Urea is inexpensive
to produce but needs to be excreted with a lot of water; suitable for
amphibians and mammals. Flying animals don't like to carry around a
lot of heavy water for peeing and animals with impermeable eggs can't
have their embryos using valuable water for urine so these (reptiles,
birds, insects) tend to produce uric acid which can be excreted as a
relatively dry paste.
|
Well....this is why I ask, I have been reading in the book by Zimmer a
few details that run counter to what I read in your POTM:
http://www.talkorigins.org/origins/postmonth/may05.html
Specifically, I was surprised by Zimmer's assertion that the amniotes
are likely descended from a marine or estuarine tetrapod, while the
modern amphibians are likely descended from a freshwater tetrapod.
Now....fresh and marine aside...it still seems clear to me that all
modern amphibians came from a different tetrapod than us
amniotes....would that assessment be on firm ground?
| Quote: |
Now to the details of tetrapod phylogeny. The discussion of
stegocephalian phylogeny on the Tree of Life page
http://tolweb.org/Terrestrial_Vertebrates/14952
as well as other sites all talk about the origin of tetrapods in terms
of shallow or even drying ponds and swamps and such; all fresh water
sites. Estuarine environments are usually brackish water rather than
truly oceanic in salinity. Are there really suggestions that
tetrapods really did develop from truly marine precursors? Certainly
the renal physiology indicates not; the kidneys and body fluid
composition has long been considered to indicate a fresh water mode of
life.
|
I believe that what was meant by marine was a tetrapod that was living
in a marine environment at that time. I do not believe that he was
suggesting that these tetrapods had marine ancestors back to the
earliest vertebrates. |
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John Harshman
Guest
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| Posted: Wed Jun 11, 2008 6:05 am Post subject: Re: At the Water's Edge |
|
|
Ken Shackleton wrote:
| Quote: |
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with |
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs? |
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r norman
Guest
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| Posted: Wed Jun 11, 2008 6:40 am Post subject: Re: At the Water's Edge |
|
|
On Tue, 10 Jun 2008 18:05:19 -0700, John Harshman
<jharshman.diespamdie@pacbell.net> wrote:
| Quote: |
Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs?
|
I have been without Internet the past few days so I am late joining
this discussion. First, the presence of urea has little to do with a
marine habit except in several special cases: the chondrichthyes
(sharks and relatives) have high urea as does the coelocanth and the
special amphibian, Rana cancrivora. In all these cases, it is used to
build a high osmotic pressure of the body fluids while retaining the
low salt concentration more typical of and associated with freshwater
life. Certainly it has nothing to do with buoyancy. In terrestrial
animals, it is a nice water-soluble form for the elimination of
ammonia produced by metabolism of amino acids. Truly aquatic animals
have large respiratory surfaces in contact with water through which
ammonia can freely leave; terrestrial animals can't lose ammonia that
way and must convert it to some less toxic form. Urea is inexpensive
to produce but needs to be excreted with a lot of water; suitable for
amphibians and mammals. Flying animals don't like to carry around a
lot of heavy water for peeing and animals with impermeable eggs can't
have their embryos using valuable water for urine so these (reptiles,
birds, insects) tend to produce uric acid which can be excreted as a
relatively dry paste.
Now to the details of tetrapod phylogeny. The discussion of
stegocephalian phylogeny on the Tree of Life page
http://tolweb.org/Terrestrial_Vertebrates/14952
as well as other sites all talk about the origin of tetrapods in terms
of shallow or even drying ponds and swamps and such; all fresh water
sites. Estuarine environments are usually brackish water rather than
truly oceanic in salinity. Are there really suggestions that
tetrapods really did develop from truly marine precursors? Certainly
the renal physiology indicates not; the kidneys and body fluid
composition has long been considered to indicate a fresh water mode of
life. |
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r norman
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| Posted: Wed Jun 11, 2008 7:50 am Post subject: Re: At the Water's Edge |
|
|
On Tue, 10 Jun 2008 19:07:16 -0700 (PDT), Ken Shackleton
<ken.shackleton@shaw.ca> wrote:
| Quote: |
On Jun 10, 7:40 pm, r norman <r_s_norman@_comcast.net> wrote:
|
<lots of snipping>
| Quote: |
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
I have been without Internet the past few days so I am late joining
this discussion. First, the presence of urea has little to do with a
marine habit except in several special cases: the chondrichthyes
(sharks and relatives) have high urea as does the coelocanth and the
special amphibian, Rana cancrivora. In all these cases, it is used to
build a high osmotic pressure of the body fluids while retaining the
low salt concentration more typical of and associated with freshwater
life. Certainly it has nothing to do with buoyancy. In terrestrial
animals, it is a nice water-soluble form for the elimination of
ammonia produced by metabolism of amino acids. Truly aquatic animals
have large respiratory surfaces in contact with water through which
ammonia can freely leave; terrestrial animals can't lose ammonia that
way and must convert it to some less toxic form. Urea is inexpensive
to produce but needs to be excreted with a lot of water; suitable for
amphibians and mammals. Flying animals don't like to carry around a
lot of heavy water for peeing and animals with impermeable eggs can't
have their embryos using valuable water for urine so these (reptiles,
birds, insects) tend to produce uric acid which can be excreted as a
relatively dry paste.
Well....this is why I ask, I have been reading in the book by Zimmer a
few details that run counter to what I read in your POTM:
http://www.talkorigins.org/origins/postmonth/may05.html
Specifically, I was surprised by Zimmer's assertion that the amniotes
are likely descended from a marine or estuarine tetrapod, while the
modern amphibians are likely descended from a freshwater tetrapod.
|
The evidence certainly can NOT be the excretion of urea by mammals
since terrestrial amphibians also excrete urea! There must be other
reasons.
However I did find the following:
"Until recently, it was assumed that nearly all early amphibians and
other stegocephalians lived only in freshwater bodies and on dry land
(in a similar manner to extant amphibians[23], which generally cannot
tolerate the marine environment). This assumption was supported partly
by the freshwater paleoenvironmental interpretation of many localities
in which early amphibians, other stegocephalians and their
sarcopterygian relatives were found. However, many of these localities
have recently been re-interpreted as estuarine, deltaic or even as
coastal marine environments [24]. These recent interpretations raise
the possibility that the intolerance of lissamphibians to the marine
environment is a relatively recent specialization of this clade."
Early tetrapod evolution
Michel Laurin, Marc Girondot and Armand de Ricqlès
Trends in Ecology & Evolution 15(3):118-123(2000)
I tend to prefer the old interpretation, but I have to admit that
ideas about how things happened do change. |
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John Harshman
Guest
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| Posted: Wed Jun 11, 2008 8:05 am Post subject: Re: At the Water's Edge |
|
|
r norman wrote:
| Quote: |
On Tue, 10 Jun 2008 18:05:19 -0700, John Harshman
jharshman.diespamdie@pacbell.net> wrote:
Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs?
I have been without Internet the past few days so I am late joining
this discussion. First, the presence of urea has little to do with a
marine habit except in several special cases: the chondrichthyes
(sharks and relatives) have high urea as does the coelocanth and the
special amphibian, Rana cancrivora. In all these cases, it is used to
build a high osmotic pressure of the body fluids while retaining the
low salt concentration more typical of and associated with freshwater
life. Certainly it has nothing to do with buoyancy. In terrestrial
animals, it is a nice water-soluble form for the elimination of
ammonia produced by metabolism of amino acids. Truly aquatic animals
have large respiratory surfaces in contact with water through which
ammonia can freely leave; terrestrial animals can't lose ammonia that
way and must convert it to some less toxic form. Urea is inexpensive
to produce but needs to be excreted with a lot of water; suitable for
amphibians and mammals. Flying animals don't like to carry around a
lot of heavy water for peeing and animals with impermeable eggs can't
have their embryos using valuable water for urine so these (reptiles,
birds, insects) tend to produce uric acid which can be excreted as a
relatively dry paste.
Now to the details of tetrapod phylogeny. The discussion of
stegocephalian phylogeny on the Tree of Life page
http://tolweb.org/Terrestrial_Vertebrates/14952
as well as other sites all talk about the origin of tetrapods in terms
of shallow or even drying ponds and swamps and such; all fresh water
sites. Estuarine environments are usually brackish water rather than
truly oceanic in salinity. Are there really suggestions that
tetrapods really did develop from truly marine precursors? Certainly
the renal physiology indicates not; the kidneys and body fluid
composition has long been considered to indicate a fresh water mode of
life.
What exactly do you mean by "tetrapod" here? If you're talking about the |
crown group, it appears that this group was primitively terrestrial, at
least according to the cladogram shown in Benton's Verttebrate
Palaeontology. |
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The Last Conformist
Guest
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| Posted: Wed Jun 11, 2008 8:51 am Post subject: Re: At the Water's Edge |
|
|
On Jun 11, 3:42 am, Ken Shackleton <ken.shackle...@shaw.ca> wrote:
| Quote: |
On Jun 10, 7:05 pm, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs?
It shows Tulerpeton with Amniotes and Greerpeton with Amphibians, they
all come to a common node and the next nodes [back in time] are
Hynerpeton, Ichthyostega, and Acanthostega respectively. There are
several others, but I do not see Westlothiana or Seymouriamorphs in
the diagram. Even though the text states that there were 18 tetrapods
in the analysis, the diagram in the book has about half that number.
It does say that the cladogram presented in the book is a simplified
version of the one that Coates created.
|
That's an usual result - most analyses find Tulerpeton and
Greererpeton comfortably basal to the crown group. If true, it would
imply among other things that five-toed feet were arrived at
independently in Amniota and Lissamphibia.
Regarding whether it supports Zimmer's scenario, well, arguably.
Hynerpeton, Ichthyostega and Acanthostega are all from freshwater
environments, as is Greererpeton. This suggest that the LCA of
amniotes and lissamphibians was freshwater and that the early
ancestors of lissamphibians stayed there. Tulerpeton, however, was
found in a marine context. This might be taken to imply that the
earliest proto-amniotes passed through a marine stage. However,
Tulerpeton does look a lot like the freshwater denizens like
Acanthostega, and was found along terrestrial-looking plant remains.
This suggests that perhaps it was a freshwater dweller after all, that
somehow ended up washed out to sea, in which case the cladogram offers
no support to Zimmer's scenario.
(Well, that's unless he shows something marine between amniotes and
Tulerpeton.) |
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John Harshman
Guest
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| Posted: Wed Jun 11, 2008 9:46 am Post subject: Re: At the Water's Edge |
|
|
Ken Shackleton wrote:
| Quote: |
On Jun 10, 7:05 pm, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@pacbell.net
wrote:
Ken Shackleton wrote:
Hello All;
I am reading "At the Water's Edge" by Carl Zimmer. Interesting book, I
am about half-way through it. He discusses our aquatic tetrapod
ancestors at length and he makes the claim that modern amphibians are
likely descended from a fresh-water tetrapod and that the amniotes
[including us] are descended from another tetrapod which came from a
marine or estuary environment.
I found this rather strange, but part of the evidence that he cites is
the fact that we produce urea as a way of dealing with the ammonia
produced by our metabolic processes. He says that this is a marine
adaptation to reduce water loss. I assume that modern amphibians do
not produce urea [except for a marine toad] and simply urinate more
[than amniotes] to get rid of ammonia?
Another point that I believe is more speculative is the notion that
our tetrapod ancestor developed the amniote egg prior to leaving the
water, and that the egg was laid on land [buried?] where there were,
as yet, far fewer predators than would be found in the water at that
time. So, perhaps our ancestors' eggs came ashore before they did.
I had thought [assumed?] that all land vertebrates [amphibians
included] had descended from the same tetrapod lineage....anyone else
read the book? Comments?
I haven't read the book. But if it doesn't have any cladograms in it,
that would be problematic. If you take the environment fossils are found
in and optimize them on a cladogram, that's evidence. Using the tree in
Benton's Vertebrate Palaeontology, I get equivocal results. There are
freshwater, estuarine, and terrestrial tetrapods on both sides of the
split. I think tetrapods have gone back and forth from land to water
throughout their evolution. And of course one grades into the other.
On page 99 of the book he presents a cladogram that was produced by
Michael Coates [I am not familiar with him]. He based the cladogram on
76 traits in eighteen different tetrapods. I have not found a
reference to the original work.
Coates is one of the big names in early tetrapod phylogeny, along with
Jenny Clack and a few others, so that's good. But does the cladogram
support the theory? And would that be Coates 1996, The Devonian tetrapod
Acanthostega gunnari (etc.)? What are the groups directly rootward of
the amphibian-amniote common ancestor on that tree? Where does it put
Westlothiana and seymouriamorphs?
It shows Tulerpeton with Amniotes and Greerpeton with Amphibians, they
all come to a common node and the next nodes [back in time] are
Hynerpeton, Ichthyostega, and Acanthostega respectively. There are
several others, but I do not see Westlothiana or Seymouriamorphs in
the diagram. Even though the text states that there were 18 tetrapods
in the analysis, the diagram in the book has about half that number.
It does say that the cladogram presented in the book is a simplified
version of the one that Coates created.
Apparently, early tetrapod phylogeny is controversial. (Greererpeton is |
a colosteid, by the way, in case you want to compare trees.) There's a
nice review on the ToLWeb page:
http://tolweb.org/Terrestrial_Vertebrates/14952 |
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