Revision of the Ordovician cheirurid trilobite genus Reraspis with the description of the earliest representative

The morphology of Reraspis Öpik, 1937 is reviewed and compared with other cyrtometopinines. Species previously referred to Reraspis are discussed and the generic composition is revised. Reraspis ranges from the Kunda (lower Middle Ordovician) to the Oandu (middle Upper Ordovician) Stage. It occurs in northern Estonia, Russia, and central Sweden (Dalarna). The earliest representative of the genus is the new species Reraspis orvikui from the Pakri Sandstone of Väike- Pakri Island, NW Estonia. The genus also includes the type species R. plautini (Schmidt, 1881), R. latifrons (Warburg, 1925), and possibly R. kaljoi Männil, 1958. Specimens from the Keila Stage of Estonia, originally referred to as Ceraurinella? cf. latifrons, are rather similar to R. latifrons and may either prove conspecific or closely related. Hemisphaerocoryphe? rosenthali (Schmidt, 1881) and Reraspis? kaljoi Männil, 1958 share several characters with Reraspis and Cyrtometopella Nikolaisen, 1961, but a safe generic assignment requires better knowledge of the latter genus, especially concerning the pygidia. Reraspis? sexermis Öpik, 1937 is better assigned to Xylabion Lane, 1971. The closely similar X. helgoeyense (Nikolaisen, 1961) may constitute a junior synonym. Remipyga? sp. sensu Männil, 1958, (= Xylabion sp. (Männil)) is also very closely related to Xylabion glaber. All three may in turn prove junior synonyms of Xylabion glaber (Angelin, 1854).