Cyrtometopinid trilobites from the upper Volkhov and lower Lynna Formation (lower Darriwilian) of NW Russia

Two cyrtometopinid trilobite taxa from the lower Darriwilian of NW Russia are examined. A new species of the genus Krattaspis is described, extending the stratigraphical range of the genus into the lower Darriwilian (Middle Ordovician). K. paraspinosis n.sp. is the youngest representative of the genus, but is closest to forms assigned to Cyrtometopus and confirms the derivation of Cyrtometopus from Krattaspis. Ontogenetic studies of K. paraspinosis n.sp. and Cyrtometopus clavifrons Angelin, 1854, from the same beds indicates a forward displacement of the eyes with increasing size, which may be contrary to all other known members of the family Cheiruridae.