PHYLOGENETICS OF THE ORDOVICIAN ACIDIPHORUS-GROUP BATHYURID TRILOBITES

Bathyuridae is one of the most common Ordovician trilobite groups occurring in shallow subtidal environments of Laurentia. Although some 170 valid species have been proposed, no phylogenetic analyses of any members of the group  have previously been attempted. A nearly ubiquitous late early and early middle Ordovician shallow subtidal Laurentian species complex comprises some 26 species  previously classified as Acidiphorus, Platyantyx, Goniotelus, Goniotelina, and Pseudoolenoides. A phylogenetic hypothesis of this plexus is necessary both to evaluate its current classification, and to facilitate studies of diversity, habitat occupancy, and rates of speciation at the time of the onset of the Ordovician radiation.

Parsimony analysis included 21 species  sufficiently well known to code for 46 characters and 136 character states. All characters were unordered. An undescribed species of Peltabellia served as an outgroup. A heuristic search  using 10,000 random addition sequence replicates and tree-bisection-reconnection branch swapping retrieved a single tree of length 168, consistency index .488, and retention index .668.

Previously, all taxa except five species assigned to Pseudoolenoides and one assigned to Platyantyx have been classified in an undivided Acidiphorus, of which Goniotelus and Goniotelina were considered junior synonyms. The present analysis reveals two major clades supported by multiple synapomorphies and high nonparametric bootstrap values. Each contains several plesiomorphic species previously classified as Acidiphorus. One clade centers around Pseudoolenoides; the other contains Goniotelus, Acidiphorus, and an unnamed new genus. Concerning the latter clade, tree topology indicates a restricted Goniotelus is sister to the monotypic Platyantyx. This clade in turn is sister to an Acidiphorus comprised of two species. The type species of Goniotelina  is sister to this combined group.

An explicit phylogenetic hypothesis permits reconstruction of the timing of Acidiphorus-group evolutionary and biogeographic events as well as temporal shifts in habitat occupancy. Further work will involve additional field sampling in an effort to resolve the lesser known and plesiomorphic members of the group.