Morphology, ontogeny and affinities of the Hirnantian triplisiid brachiopod Streptis undifera from Baltoscandia

New silicified topotypic material of the Upper Ordovician Streptis undifera (Schmidt, 1858) from the stratotype of the Porkuni Regional Stage in Estonia provides important data on triplesiid morphology and ontogeny, which has substantial implications for our understanding of the affinity of this group of brachiopods. In particular, the new material shows that the early ontogeny of Streptis includes evidence for a cicatrix attachment and colleplax-like structure in the ventral valve. It is likely also that a lecithotrophic feeding habit evolved in triplesiidines sometime in the Ordovician, which sets them apart from all other known strophomenates. A neotype is selected among the specimens of Streptis undifera from the type locality in Porkuni. In Baltoscandia, Streptis undifera appears first in Norway in late Katian below the strata with the pentamerid brachiopod Holorhynchus. In Estonia, Streptis undifera is an index species of the early Hirnantian and occurs in association with stromatoporoid-coral reefs and related inter-reef deposits, which overly the strata with Holorhynchus. In Baltoscandia, the distribution areas of Streptis and Holorhynchus are more or less identical in spite of some differences in age.