THE LOWER ORDOVICIAN TRILOBITE KRATTASPIS: THE EARLIEST CYRTOMETOPINID (CHEIRURIDAE) FROM THE ARENIG OF THE EAST BALTIC by HELJE P A R N A S T E ABSTRACT. The type specimen of Krattaspis viridatus Opik, 1937, one of the earliest known representatives of the family Cheiruridae, is redescribed in order to clear up the confusing characters caused by distortion of the exoskeleton. More than 50 new specimens from different localities in the Baltic-Ladoga Clint area contain three species, the type species and two that are new (K. popovi sp. nov. and K. vitalis sp. nov.); all come from the Arenig Billingen Stage, from different units. A meraspid cephalothorax and a thoracopygon of K. vitalis have been found together with some adults, and show some characters of ontogeny of that taxon. The systematic validity of the subfamily Cyriometopinae Opik, 1937 is discussed. KEY WORDS: trilobite, Cheiruridae, Cyrtometopinae, Ordovician, Arenig, eastern Baltoscandia. THE. earliest Ordovician trilobites known from the northern part of the East Baltic come from the glauconitic sandstone of the Maekiila Member, which forms the upper part of the Leetse Formation, of Arenig age. Trilobites are rare, numbering 55 specimens, and they have been assigned to 18 species, three of which are considered invalid. Balashova (1966, pp. 3-22, pls 1- 2) revised most of this material including that previously described by Schmidt (1904, 1907), Lamansky (1905), 0pik (1937) and Balashova (1961). The majority of specimens came from the Popovka River district close to Pavlovsk, the remainder from localities along the banks of the rivers Tosna, Izhora, Lava, and Volkhov in Ingria (the old province of the St. Petersburg Region between the rivers Narva and Syas) (Text-fig. 1). Seven specimens were recovered from Estonia, including the holotype of the type species of Krattaspis, established by 0pik (1937, pp. 102- 105, text-fig. 26, pl. 13, figs 1- 2) based on one incomplete cranidium with displaced hypostome. New collections of more than 50 specimens of Krattaspis allow a more accurate description to be given and provide more information on morphology and ontogeny of one of the earliest representatives of the Family Cheiruridae (Lane 1971, p. 74, text-fig. 10). STRATIGRAPHIC BACKGROUND The traditional stratigraphicalsubdivision of the glauconitic rocks of the Baltic-Ladoga Clint was provided by Lamansky, who defined a sub-unit BIB (Lamansky 1905, p.4, 161) of the glauconitic sandstone (Griinsand) immediately below the succeeding glauconitic limestone of Bn (Text-fig. 2). He also supposed (1905, pp. 31, 164) that the fauna of BIP resembled that of the succeeding Planilimbata and Limbata limestones but was slightly more primitive. Both Opik (1937, p. 103) and Balashova (1961, p. 130, 1966, 1976) later correlated BIB with the Ceratopyge Limestone in Sweden and Norway, although Tjernvik (1956, p. 184) had considered it to be equivalent to the somewhat younger Megalaspides dalecarlicus Biozone of the Billingen Stage (Group) in Sweden. However, the list of the trilobite species given by the author in Magi et al. (1989) and Aru (1990) shows that the trilobite assemblages in the East Baltic differ from those in Sweden described by Tjernvik (1956; Tjernvik and Johansson 1980). The conodont data of the East Baltic (Magi and Viira 1976; Magi 1984, 1990a, b; Magi et al. 1989; Bergstrom 1988; Dronov et al. 1995; Einasto et al. 1996; Dronov, Fedorov et al. 1997, p. 15; Dronov, Popov et al. 1997, p. 22; Viira unpublished data) show that the Maekiila Member consists of rocks which are diachronous with respect to [Spedal Papers in Palaeontology, 70,2003, pp. 241 - 2517] 0 The Palaeontological Association