We would like to draw your attention to a paper which came out
today in
Nature. The reference is: Dominguez, P., Jacobson, A. G.
& Jefferies, R. P. S. 2002. Paired gill slits in a fossil
with a calcite skeleton.
Nature (London),
417, 841-844.
By using computer microtomography on the little
Carboniferous mitrate
Jaekelocarpus oklahomensis (about 300
million years old and only 4 mm lenght) we have shown that it had gill
slits on left and right inside the head. Moreover, it was a
tunicate because:
1) the gill slits were antero-posteriorly elongate;
2) were arranged in a series one above the other;
3) opened into paired atria which projected anteriorly right and
left of the pharynx;
4) and these atria debouched through a pair of antero-dorsal
openings like those of immediately post-larval
Corella, for
example.
Jaekelocarpus had an echinoderm-like calcite
skeleton made of porous stereom with each plate crystallographically a
single crystal and also a tail which would have flexed mainly in a
vertical plane. Both these features were probably primitive for the
chordates and, since parsimony suggests that they were absent in the
latest common ancestor of living tunicates, they probably imply that
Jaekelocarpus was a stem-group tunicate, not a crown-group
tunicate.
The whole animal was built on the plan of a tunicate
tadpole, with clearly distinct head and tail. (For simplicity of
comparison with vertebrates, we prefer not to speak of "trunk"
and tail.) Indeed this tunicate-tadpole-like body plan must have
been primitive for chordates since it existed in all mitrates and
throughout the bizarre "carpoid" group of fossils.
It is very important that there are
right and left gill slits in
Jaekelocarpus, as in a
post-larval amphioxus, rather than left gill slits only, as in a larval
amphioxus or a member of the fossil group Cornuta (e.g.
Cothurnocystis).
This is not the first known fossil tunicate. Only last
year Prof. Degan Shu and his co-workers, of Xi'an
University in China, recorded the fossil tunicate
Cheungkongella
ancestralis. Since this was fixed down to the substrate rather than
tunicate-tadpole-like, we think it probably belonged to the tunicate
crown group rather than the stem group, although it came from much older
rocks than
Jaekelocarpus. (It is from the Lower Cambrian, perhaps
560 million years old, of southern China.) The reference is: Shu,
D.-G., Chen, L. & Zhang, X.-L. 2001.
Nature, 411,
472-473.
In addition to
Jaekelocarpus, there are several
other known mitrates which, in our opinion, represent
the tunicate stem group. None of them, however, shows the clear
evidence for gill slits revealed by
Jaekelocarpus.
Jaekelocarpus is not new. It was originally described,
with a different interpretation, in 1991 by Dennis Kolata,
Terry Frest and Roy Mapes (1991.
J. Paleontology, 65,
944-855). They were also the first to see the gill slits, though they did
not call them by that name and their material was badly broken. The
material was lent to us by Liz Nesbitt and Ron Eng of the Burke Museum,
University of Washington. The x-ray tomography of the specimens was
carried out by Tim Rowe and his group of the University of Texas at
Austin.
Part of the argument for recognising the slits as gill slits (or
more precisely as stigmata) depends on the monographs of Australian
tunicates produced by Pat Kott. I hope she approves.
Best wishes,
Patricio Domínguez, Antone Jacobson and Dick Jefferies
__
Dr. Patricio DOMINGUEZ-ALONSO
Department of Palaeontology
The Natural History Museum
Cromwell Road, London SW7 5BD, UK
patd@nhm.ac.uk
patricio@geo.ucm.es
padomin@terra.es
Tel 020 7942 5611 (internationally 0044 20 7942 5611)
Fax 020 7942 5546 (internationally 0044 20 7942 5546)