Article: A new species of stem-group chordate from the Upper Ordovician of Northern Ireland

A new scotiaecystid, Scotiaecystis collapsa sp. nov. is described from the Killey Bridge Beds, lower Cautleyan Stage, Ashgill Series, near Pomeroy, Co. Tyrone, Northern Ireland. It is most closely related to S. curvata Bather. The interrelations of cornutes are studied through a cladistic analysis using PAUP (Phylogenetic Analysis Using Parsimony) involving twenty-one species and thirty-nine characters. Three equally parsimonious trees are obtained and their information content summarized in the form of a consensus tree. By the addition of the mitrates (primitive crown-group chordates) this consensus tree is resolved. As in previous studies, the genus Cothurnocystis forms an uncharacterizable group with some species more crownward than others. The genus Thoralicystis is also paraphyletic. The Scotiaecystidae are an extinct monophyletic group more crownward than C. elizae Bather and less crownward than the Phyllocystidae, Amygdalotheca, and Reticulocarpos. The scotiaecystids exhibit a departure from a primitively deposit-feeding mode of life towards suspension feeding. The hind tail of S. collapsa is unusual for it is not truncated at the end as is often the case in other cornutes and is flexible in both the horizontal and vertical planes. The family Phyllocystidae is redefined to contain Phyllocystis and Chauvelicystis, and the family Cothurnocystidae redefined to include only Cothurnocystis elizae Bather, C. courtessolei Ubaghs, and C. primaeva Thoral.