Dear Karen,
It seems to me that the occurrence of paired atrial openings in the
stem group of the tunicates (necessarily among fossils since stem groups
are by definition extinct) is a better indication of the primitive adult
condition than the occurrence of paired atrial openings or paired atria in
the juvenile stages of ontogeny in extant forms, or the occasional presence
of paired atrial openings in the adults of extant forms. We do not have to
disagree, however, since all three lines of evidence give the same answer.
Indeed, fossil evidence shows that paired atria and paired atrial openings
are present not in stem tunicates only, but also in stem craniates and stem
acraniates, which implies that the paired condition would have existed in
the adult of the latest common ancestor of extant chordates.
There is still another approach. I have written previously to the
Tunicate List to argue that the otic vesicles of vertebrates are homologous
with the atria of tunicates. Now the otic vesicles of vertebrates are
always paired, so the primitive condition for vertebrates and tunicates was
also most likely paired.
By the way, you asked me a few weeks ago for a copy of our recent
paper in Nature (Dominguez, Jacobson & Jefferies; 2002). Thankyou for your
interest. I shall certainly send you one when I receive them, which has not
yet happened.
Best
wishes,
Dick
Jefferies
>Dear Richard,
>
>Of course, everyone who studies ascidians knows that paired openings is a more
>primitive condition than the single one. This was demonstrated many years
>ago by Berrill, Brien and others. Of course this is an explanation why the
>duplication of atrial siphons may happen in adult ascidians (and why this
>does not occur with the branchial siphon).
>
>In my previous email I stated that paired atrial siphons is an
>abnormality. We can explain in the terms of ascidian development why this
>abnormality may occur. But in any case, for the adult ascidians, this is
>an abnormality, isn't it?
>
>I also stated that it is hardly possible to take this abnormality as an
>argument for any kind of comparison. If we find duplicate siphons in one
>or two specimens (from many thousands) we CANNOT conclude (solely from
>this fact!) that primitive condition for tunicata was duplicate siphons.
>Just like we cannot conclude that primitive condition for snakes was
>two-headed dragon. But we CAN use our knowledge of ascidian larval
>development to state - "yes, the primitive condition is paired openings".
>To state this we do not need to involve additional facts - like duplicated
>siphons or fossil records.
>
>
>
>
>--
>Best wishes,
> Karen
>mailto:[log in to unmask]
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