Article: Protocystites menevensis—a stem-group chordate (Cornuta) from the Middle Cambrian of South Wales
R. P. S. Jefferies, M. Lewis and S. K. Donovan
Protocystites menevensis Hicks, 1872, from the Hypagnostus parvifrons Zone of the Middle Cambrian, near St David's, Dyfed, South Wales, is reconstructed and redescribed. It proves to be a cornute, and therefore a stem-group chordate, representing a plesion intermediate between that of Ceratocystis perneri (the most primitive known chordate) and that of Nevadaecystis americana. For purposes of reconstruction, tectonic distortion of the fossils was corrected by means of a computer program. The positions of oesophagus, stomach, and intestine are suggested in P. menevensis on the basis of skeletal evidence. The locomotory cycle of the animal, which probably crept rearwards over the sea floor pulled by its tail, is deduced.It is argued that, on a practical definition, every plesion is fundamentally paraphyletic. The term 'more crownward' is used to signify that a plesion is more closely related to the relevant crown group than is some other plesion. The term 'nodal group' is proposed for all those members of a group which possessed all the autapomorphies of the crown group but none of the autapomorphies of any of the subgroups of the crown group.A comparison between stem chordates and the echinoderms shows that echinoderm 'dorsal' is homologous with chordate ventral and vice versa, so in echinoderms the use of the terms 'dorsal' and 'ventral' should be abandoned.