Evolution, taxonomy and relationships of the Silurian conodont Genus Pterospathodus

The apparatus of Pterospathodus species comprises Pa, Pb, PC, M and Sa/Sb elements. A more complete first symmetry transition series of Sa-Sc elements occurs in some related species, which are assigned to a new genus, Pranognathus. A sequence through the celloni and amorphognathoides biozones of Estonia has been sampled in detail, allowing documentation of sequential changes shown by Pterospathodus populations. The least conservative element is the Pa, in which gradual trends of increasing size, lateral process development and platform development prevail. Morphological differences between sinistral and dextral specimens in the celloni Biozone include a preferential development of pennate lateral processes on dextral elements; pennate and non-pennate forms previously included in the separate taxa Pterospathodus celloni, P. pennatus and P. angulatus are conspecific and all are assigned to P. celloni (Walliser). Pterospathodus species of the amorphognathoides Biozone: P. amorphognathoides Walliser, P. procerus (Walliser) and P. rhodesi Savage, are distinguished from P. celloni by the presence of platform ledges. There are no major differences between sinistral and dextral elements within any of these species.