Revision of Hyolitha from the Ordovician of Estonia

Topotype specimens of the Middle Ordovician Hyolithes acutus Eichwald, 1840, which is the type of the genus that lent its name to a family, order, class, and even phylum according to some, and H. latus  Eichwald,, 1860 allow that genus and those species to be firmly established on a sound, morphologic basis. In addition, preservation of the types of Hyolithes striatus  Eichwald,, 1860 is sufficiently good to warrant reassignment to Dorsolinevitus Syssoiev , 1958. In contrast, the type of H. insularis  Eichwald,, 1860 is incompletely preserved, and this species is not recognizable beyond the type material. The concept of the family Hyolithidae is revised to more closely conform to the morphology of Hyolithes, with authorship herein ascribed to Syssoiev (1958) rather than to Nicholson (1872). The stratigraphic distribution of these taxa suggests that Hyolithes as defined herein first appears in the Middle Ordovician, but extends into at least the Lower Devonian, as suggested by two species from the Barrandian region of the Czech Republic. Their geographic distribution further re-enforces the notion of two distinct paleobiogeographic provinces based on hyoliths, a Mediterranean province and Baltic province, with almost no mixing of hyolith faunas during the Ordovician.