A systematic revision of the Upper Ordovician trilobite genus Bumastoides (Illaenidae), with new species from Oklahoma, Virginia and Missouri

Abstract

Species of the Ordovician illaenid trilobite genus Bumastoides Whittington, 1954, have been notoriously difficult to classify because of their high degree of exoskeletal effacement. New species from Oklahoma (B. graffhami sp. nov), Virginia (B. moundensis sp. nov) and Missouri (B. kimmswickensis sp. nov) provide more data for a phylogenetic analysis of the genus. Continuous characters were analysed with finite mixture coding in two separate parsimony analyses. A strict consensus tree of eight equally most parsimonious trees, and a weighted best-fit tree both indicate that many species previously assigned to Bumastoides should be removed from the genus. Both methods yielded a tree with similar topology, supporting the removal of three Stenopareia-like species from Bumastoides. Based on the optimized character distribution, the supporting synapomorphies for Bumastoides are: weak to effaced cranidial axial furrow and lack of glabellar differentiation even by change in convexity, dicuspid inner margin of pygidial doublure, subtrapezoidal pygidium, undifferentiated or minimally expressed pygidial axis, long pygidial doublure, and maximum pygidial width at half-length. The pygidial outline shape, length of pygidial doublure, and degree of independent convexity on the cranidium are characters that may be useful in separating species of Stenopareia and Bumastoides.