Dear Tunicatists,
We were pleased to see the comments from Donna Lapinsky.
Actually, we consider that only some of the mitrates were stem-group
tunicates, since all the others were stem-group acraniates or stem-group
craniates. In addition the strange fossils called cornutes, with gill slits
on the left side of the head only like larval amphioxus, were stem-group
chordates, as were most members of an even more primitive group of fossils
called solutes, which differed from cornutes by having an echinoderm-like
feeding arm anteriorly. The latest common ancestor of extant chordates was
therefore a mitrate and the latest common ancestor of extant deuterostomes
was a solute.
All mitrates were built like tunicate tadpoles with a head and a tail
sharply distinct from each other. (We don't say trunk and tail, unlike
tunicate systematists, since "head" and "tail" give a better notion of the
homologies with craniates.) Mitrates had a tunicate-like pharynx, for
endostylar feeding, inside the head. And they also had some remarkable
tunicate-tadpole-like features, such as a rectum that opened into the left
atrium, for example.
Donna is right to think that we regard the tail as locomotory, but in
crawling it pulled the animal rearwards. This was originally suggested,
purely from the functional morphology of the skeleton, in (Jefferies
1967) but is now confirmed by finding two tracks, together with the
mitrates that caused them. These are preserved on the remarkable piece of
German Devonian slate discussed by Nature (for a longer description see
Sutcliffe, Suedkamp & Jefferies. (2000, Lethaia, 33, 1-12). For all we know,
the mitrates were capable of swimming when very young, like tunicate
tadpoles nowadays, and if they did so they probably swam forwards. A fairly
recent paper (Jefferies & Jacobson. 1998. Integrative Biology, Vol. 1,
115-132) discusses the changes required to get from a mitrate to something
like a hagfish.
Locomotion in mitrates therefore had nothing to do with the rather
tentative crawling of didemnids. It was driven by the tail as would be
expected for primitive chordates.
We could go on at length (as you can imagine) but this is probably enough
for the moment.
Best regards,
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)
