Two early cheirurine trilobites and their later relatives in the Ordovician of Baltoscandia

Ceraurinella ornata (Dalman, 1828) and Laneites ingricus (Schmidt, 1881) are two early representatives of the subfamily Cheirurinae in Baltoscandia. Neither species has been revised since their original descriptions but type and new material provide additional insights on their morphology and their spatial and temporal distribution. They occur in the Middle Ordovician (Volkhovian–Kundan) succession across the Baltoscandian platform, from the St. Petersburg area in Russia, through northern Estonia and central Sweden, Bornholm (Denmark) and the Oslo Region of Norway. Laneites ingricus ranges from the Volkhovian (BIIγ) Megistaspis limbata Zone through the lower Kundan (BIIIα) Asaphus expanus Zone. Specimens of L. cf. ingricus are known in the older Volkhovian (BIIβ) Megistaspis simon Zone and L. aff. L. ingricus (Schmidt) sensu Männil (1958) has been recorded from the lowermost Volkhovian (BIIα) Megistaspis polyphemus Zone in Estonia. Ceraurinella ornata occurs in the Asaphus expansus Zone in Sweden and in the A. raniceps (BIIIβ) to M. gigas (BIIIγ) zones in Estonia. A total of 30 cheirurine taxa are known from the Middle and Upper Ordovician of Baltoscandia. Species diversity (including both formally described species and forms treated under open nomenclature) is greatest in Sweden with 14 occurrences, followed by Estonia (11), Norway (7), the St. Petersburg area (5), Denmark (Bornholm) with two and Finland with a single occurrence. Six genera are recorded: Ceraurinella Cooper, 1953; Ceraurinium Přibyl & Vaněk in Přibyl et al., 1985; Hadromeros Lane, 1971; Laneites Přibyl & Vaněk in Přibyl et al., 1985; Paraceraurus Männil, 1958; and Xylabion Lane, 1971. In Sweden, nearly half of the recorded occurrences are due to Hadromeros spp. in the Upper Ordovician, whereas Paraceraurus accounts for nearly half of the occurrences in the Middle Ordovician of Estonia. A gap in the occurrence of cheirurinae is seen across Baltoscandia between the Kukruse/Dalbyan and the Oandu/Moldåan stages.