Someone pass the sunscreen.  The beach is going to be fabulous tomorrow.

Marlo
Florida

Gary Rosenberg wrote:

> When I say one phylum can't give rise to another, I'm talking about pattern
> not process. Again, it is a problem of ranks. Lets take the Tridacnidae
> (giant clams) and the Cardiidae (cockles). When you see these names, you
> picture them as two separate groups (C=Cardiidae; T=Tridacnidae):
>
> T1  T2  T3  C1  C2  C3  C4
>  \  /   /    \   \   \  /
>   \/   /      \   \   \/
>    \  /        \   \  /
>     \/          \   \/
>      \           \  /
>       \           \/
>        \          /
>         \        /
>          \      /
>           \    /
>            \  /
>             \/
>             /
>            /
>
> But it turns out that giant clams are modified cockles, based on anatomical
> and molecular evidence. The diagram below (simplified from work of Jay
> Schneider) is the currently accepted pattern:
>
> C1  C2  C3  C4  T1  T2  T3
>  \   \   \   \   \   /  /
>   \   \   \   \   \ /  /
>    \   \   \   \   \  /
>     \   \   \   \   \/
>      \   \   \   \  /
>       \   \   \   \/
>        \   \   \  /
>         \   \   \/
>          \   \  /
>           \   \/
>            \  /
>             \/
>             /
>            /
>
> This means that the giant clams arose from within the cockles. The rules of
> phylogenetic classification do not allow me to call C1 through C4 the
> Cardiidae while excluding T1 to T3, because then I have excluded some of
> the descendants from the taxon. Instead I must include them all in
> Cardiidae. The rules of classification also don't allow a taxon of a
> particular rank to contain other taxa of the same rank. I can call T1-T3
> Tridacninae (a subfamily), but Tridacnidae is a synonym of Cardiidae.
>
> I could call the whole group Cardiiacea, which allow me to use Tridacnidae
> for the giant clams, but that's just playing with ranks. Also, there are so
> many levels in the tree of life that there aren't enough ranks to go
> around. And whose to say that a class of vertebrates is the same kind of
> thing as a class of mollusks. Better to abandon ranks altogether and just
> deal with taxa.
>
> Now, returning to macroevolution. Why is that for so many years the giant
> clams were considered so distinct that they had their own superfamily,
> Tridacnacea? Because they seem so different in morphology. What allowed
> that morphology to evolve? A symbiosis with zooxanthellae than provides the
> giants clams with extra fuel. The symbiosis is the hypothesized
> macroevolutionary event.
>
> >There weren't any Chordates in the Cambrian.
>
> Cathaymyrus is the oldest known chordate at 535 million years old and
> Pikaia from the Cambrian Burgess Shale is slightly younger
> <http://www-geology.ucdavis.edu/~GEL3/Cathaymyrus.html>; Yunnanozoon from
> the lower Cambrian of China
> <http://www.lu.org/symposium/cambrian_chordate.html> is also thought to be
> an early chordate.
>
> Gary
>
> ---------------------------------------------------------------------
> Gary Rosenberg, Ph.D.                     [log in to unmask]
> Malacology & Invertebrate Paleontology    gopher://erato.acnatsci.org
> Academy of Natural Sciences               http://www.acnatsci.org
> 1900 Benjamin Franklin Parkway            Phone 215-299-1033
> Philadelphia, PA 19103-1195 USA           Fax   215-299-1170



--
Marlo
Merritt Island, Florida
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