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Drepanoistodus basiovalis (Sergeeva, 1963) is a common conodont species in Middle Ordovician strata of Baltica. For many years it has been widely accepted that the species encompasses a wide range of morphological plasticity. Hence, several different morphotypes that significantly deviate from the holotype have nonetheless been included in the broad species concept. In this study, we performed a detailed taxonomical study on 112 predominantly well-preserved specimens (geniculate elements) from the St. Petersburg region of Russia; 37 of these were selected for morphometric analyses together with 21 well-illustrated specimens from the published literature. The results demonstrate that, among the morphotypes that share some characteristics with D. basiovalis sensu lato, at least five species can be readily distinguished. Hence, three new species – Drepanoistodus iommii sp. nov., D. svendi sp. nov. and D. viirae sp. nov. – are here added to the previously known D. basiovalis and D. contractus (Lindström, 1955). In addition, some specimens were left under open nomenclature and assigned to Drepanoistodus aff. basiovalis and D. cf. suberectus (Branson & Mehl, 1933). In order to objectively compare the Drepanoistodus taxa and test the validity of the new species, we performed a Principal Component Analysis combined with non-parametric (PERMANOVA) tests based on 21 morphological characters.
Due to the fragility of the ophiuroid (brittle star) skeleton, the bulk of the group’s fossil record consists of dissociated ossicles preserved as microfossils. In spite of their great potential as basis for taxonomic and phylogenetic studies, however, ophiuroid ossicles from the Paleozoic have received very little attention so far. Here, we provide an exhaustive taxonomic assessment of such fossils retrieved from sieving residues from the Silurian of Gotland, Sweden. This material was used in a previous study to describe two key taxa that allowed constraining the origin of the extant ophiuroid clade. The remaining taxa belonging to that same lineage are described in the present paper. The evidence at hand suggests that the stem of the extant ophiuroid clade was formed by two genera, Ophiopetagno and Ophiolofsson gen. nov., including six and five species, respectively, and spanning at least the upper Llandovery through upper Ludlow. We conclude that Ophiopetagno and Ophiolofsson represent sister genera that coexisted through most of the Silurian in the shallow tropical seas of Gotland. They underwent repeated body size reductions in correlation with environmental perturbations, with Ophiopetagno paicei eventually giving rise to Muldaster haakei; the first member of the living Ophiuroidea. Herein, we also introduce two new clades, Ankhurida clade nov. and Ophiovalida clade nov., and the following eight new species: Ophiolofsson joelmciveri gen. et sp. nov., O. obituary gen. et sp. nov., O. immolation gen. et sp. nov., O. archspire gen. et sp. nov., O. hendersonorum gen. et sp. nov., Ophiopetagno bonzo sp. nov., O. kansas sp. nov., O. doro sp. nov.; and two probably new species in open nomenclature: Ophiopetagno sp. 1, and Ophiopetagno sp. 2.