>Date: Tue, 25 Sep 2001 14:50:35 +0100
>To: [log in to unmask]
>From: Richard Jefferies <[log in to unmask]>
>Subject: Microstructure of tunicate stigmata
>Cc: [log in to unmask]
>
>Dear Tunicatists,
>   Patricio Dominguez has recently completed a preliminary
> computer-aided  microtomographic study of a little  tunicate  mitrate
> called Jaekelocarpus oklahomensis from the Gene Autry Shale of the Upper
> Carboniferous (=Pennsylvanian) near Ardmore, Oklahoma. It is about 300
> million years old and the most recent mitrate known.  Like all mitrates,
> it is constructed like a tunicate tadpole with a head and a tail and we
> have long supposed that there would have been right and left sets of gill
> slits inside the mitrate head, opening  into right and left atria.
>   Jaekelocarpus provides direct evidence of the existence of such paired
> slits, preserved in a skeleton of thin echinoderm-type calcite and
> situated inside the head near the posterior end of the head.  There are
> traces of at least three slits on the left and three on the right. They
> are horizontally elongate and  arranged in a vertical row, in both
> respects  like  modern tunicate stigmata,  and each is about 90 microns
> in depth, consistent with the notion that they carried in life a layer of
> ciliated epithelial cells with the dorsal and ventral cilia of each slit
> meeting, or almost meeting, in the horizontal mid-line of the slit. The
> atrial openings of Jaekelocarpus were paired and situated at the anterior
> end of the head, on either side of the mouth, basically  like a
> post-larval recent ascidian before the left and right atrial openings
> have fused in the mid line.
>   As a preliminary to  writing this up, we find ourselves furiously
> reading. Those who study extant tunicates will know that in all ascidians
> each stigma is surrounded by seven rows (or in fact seven elliptical or
> sausage-shaped rings)  of ciliated cells i.e. in  any section through a
> stigma cut in a pharyngo-atrial plane, there will be a file of seven such
> ciliated cells ventral to the slit and seven dorsal to it. (I cite
> the  lucid paper on the subject by Martinucci, Burighel & Dallai  (pp
> 123-140)  in Lanzavecchia & Valvasori  (eds) (1991) Form and function in
> zoology. Selected Symposia and Monographs, Unione Zoologica Italiana, 5
> 123-140.)
>   Can anybody tell us whether this holds for all pelagic tunicates also,
> because, if so, it was probably true for the latest common ancestor of
> extant tunicates too?
>   We shall be very interested in your comments.
>                                        Best wishes,
>                               Dick Jefferies and Patricio Dominguez

*********************************

Dr. R. P. S. Jefferies,
Department of Palaeontology,
The Natural History Museum,
Cromwell Rd.,
London,
SW7 5BD
Telephone Number:0207 942 5014
(Internationally 00 44 207 942 5014 )
Fax Number: 0207 942 5546
(Internationally 00 44 207 942 5546)