THE ENCRINURUS-VARIOLARIS-PLEXUS (TRILOBITA, SILURIAN) - RELATIONSHIPS OF LLANDOVERY SPECIES

Phylogenetically basal, Llandovery parts of the diverse variolaris ple xus of encrinurine trilobites have been assigned to a grade group, Nuc leurus. Parsimony analysis of 24 exoskeletal characters for adequately known species of Nucleurus and allied post-llandovery taxa provides a phylogenetic scheme for reclassifying the base of the variolaris plex us. Nucleurus RAMSKOLD, 1986 is redefined with Aeronian-Telychian core and Rhuddanian-Aeronian stem groups (Nucleurus s.s. and s.l., respect ively). Monophyletic groups in the shortest cladograms include Elsarel la nov. gen. (Llandovery, east Baltic region) and an unnamed clade (Ll andovery-Ludlow ?, Baltic region and Japan). Billevittia nov. gen. (Te lychian, northern Laurentia) is the closest relative of a diverse post -llandovery radiation. Previously established east Baltic Llandovery s pecies of the variolaris plexus are revised and illustrated. Most type specimens have been further prepared to show new details, and additio nal information is provided by new material. New species are Billevitt ia adraini and Encrinurus ralfi.