The "Encrinurus" variolaris plexus (Trilobita, Silurian): relationships of Llandovery species

Phylogenetically basal, Llandovery parts of the diverse variolaris plexus of encrinurine trilobites havebeen assigned to a grade group, Nucleurus. 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 plexus. Nucleurus Ramsköld,, 1986 is redefined with Aeronian-Telychian core and Rhuddanian-Aeronian stem groups (Nucleurus s.s. and s.l., respectively). Monophyletic groups in the shortest cladograms include Elsarella nov. gen. (Llandovery, east Baltic region) and an unnamed clade (Llandovery-Ludlow?, Baltic region and Japan). Billevittia nov. gen. (Telychian, northern Laurentia) is the closest relative of a diverse post-Llandovery radiation. Previously established east Baltic Llandovery species of the variolaris plexus are revised and illustrated. Most type specimens have been further prepared to show new details, and additional information is provided by new material. New species are Billevittia adraini and Encrinurus ralfi.