PHYLOGENY AND EVOLUTIONARY HISTORY OF LICHID TRILOBITES

Lichid trilobites are a speciose (431 species in 63 genera) clade of Paleozoic arthropods known from the early Ordovician to the mid Devonian. Previous phylogenetic analysis of the group consisted of 48 morphological characters coded for 36 taxa, however genera were coded as the operational taxonomic unit formed from a conglomeration of character states of constituent species, resulting in chimera terminals that do not accurately represent any given species. Our analysis comprises 178 characters currently coded for 55 individual species, with some genera represented by multiple species exemplars. The characters are distributed across the cephalon (52%), hypostome (14%), thorax (4%), and pygidium (30%). We analyze the dataset under both maximum parsimony and Bayesian inference, retrieving broadly concordant gross tree topologies, although the exact interrelationships of species within clades varies. Both topologies agree in suggesting a deep split within lichids into two clades, one (clade A) comprising Lichinae, Echinolichinae and the majority of Tetralichinae with the other (clade B) consisting of Platylichinae, Trochurinae and the majority of Homolichinae. Both analyses also show abundant para- and polyphyly within the current taxonomic framework, with Lichakephalidae and Platylichinae resolving as paraphyletic and Tetralichinae and Homolichinae resolving as polyphyletic. These results differ from those of previous analyses, which assumed a monophyletic Lichakephalidae and resolved a monophyletic Platylichinae. The new tree topology shows that a number of lichid clades survived the end Ordovician mass extinction, with the majority of Ordovician-only families forming paraphyletic lineages at the bases of clades A and B. Both clades subsequently undergo radiations in the Silurian, with Trochurinae and Dicranopeltinae becoming the predominant lichid clades of the time. Clade A largely peters out during the middle Silurian, with only a few genera of Echinolichinae persisting into the Early Devonian, however Clade B undergoes another radiation of Trochurinae in the Early Devonian that form the bulk of lichid diversity until their extinction in the Middle Devonian.