Stratocladistic analysis and taxonomic revision of the character-poor New Zealand crassatellid bivalves Spissatella and Eucrassatella

Accurate, phylogenetically informed taxonomy is required for any group of organisms to be used in biostratigraphy, palaeoecological and palaeoenvironmental studies, or palaeoclimatology. We use stratocladistic methods combined with morphometric data to produce a phylogeny of two closely related genera of New Zealand crassatellid bivalves—Spissatella Finlay, 1926 and Eucrassatella Iredale, 1924— and find that the inclusion of the stratigraphic character improves congruence of tree to data, as measured using retention indices. According to the resulting phylogeny, New Zealand taxa currently included in the genus Eucrassatella do not form a monophyletic group: ‘Eucrassatella’ australis (Hutton) occupies a basal position with regards to the rest of the taxa, and the remaining species of Eucrassatella, including the Australian type species E. kingicola (Lamarck), form a derived group that is sister to Spissatella. A full revision of the New Zealand species of Spissatella and Eucrassatella is presented, incorporating the results of this new phylogenetic analysis. Two species (‘Spissatella’ subobesa and the Australian ‘S.’ maudensis) are transferred to the genus Eucrassatella; the genus Spissatella is redescribed and the species assigned to it are reviewed. Spissatella maxwelli n. sp. is described and Triplicitella n. gen. is proposed for the oldest New Zealand species of crassatellid, ‘Eucrassatella’ australis.