Re: At the Water's Edge

r norman wrote:
On Tue, 10 Jun 2008 20:05:38 -0700, John Harshman
<jharshman.diespamdie@xxxxxxxxxxx> wrote:

r norman wrote:
On Tue, 10 Jun 2008 18:05:19 -0700, John Harshman
<jharshman.diespamdie@xxxxxxxxxxx> wrote:

Ken Shackleton wrote:
On Jun 9, 9:29 am, John Harshman <jharshman.diespam...@xxxxxxxxxxx>
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.

I was talking about its origin, that is, from what aquatic group did
it derive and what kind of water did that group live in? Or if partly
aquatic and partly terrestrial, still what kind of water?

What do you mean by "its"? It seems to me that environment has been fairly plastic in the evolution of tetrapods (and other stegocephalians), and that the primary "ancestry is destiny" claim of Zimmer's book is therefore dubious, as well as making the proper referent ancestor ambiguous.

.

Relevant Pages

  • Re: At the Waters Edge
    ... He discusses our aquatic tetrapod ... adaptation to reduce water loss. ... our tetrapod ancestor developed the amniote egg prior to leaving the ... If you take the environment fossils are found ...
    (talk.origins)
  • Re: At the Waters Edge
    ... He discusses our aquatic tetrapod ... adaptation to reduce water loss. ... If you take the environment fossils are found ... What are the groups directly rootward of the amphibian-amniote common ancestor on that tree? ...
    (talk.origins)
  • Re: At the Waters Edge
    ... He discusses our aquatic tetrapod ... adaptation to reduce water loss. ... I assume that modern amphibians do ... On page 99 of the book he presents a cladogram that was produced by ...
    (talk.origins)
  • Re: At the Waters Edge
    ... He discusses our aquatic tetrapod ... adaptation to reduce water loss. ... I assume that modern amphibians do ... On page 99 of the book he presents a cladogram that was produced by ...
    (talk.origins)
  • Re: At the Waters Edge
    ... He discusses our aquatic tetrapod ... adaptation to reduce water loss. ... I assume that modern amphibians do ... perhaps our ancestors' eggs came ashore before they did. ...
    (talk.origins)