Abstract

Illaenus Dalman, 1827 has long served as a wastebin for effaced trilobite species and the problem has been compounded in North America by the addition of Thaleops Conrad,1843 and Nanillaenus Jaanusson, 1954. This study takes the first step toward defining the relationships of Laurentian taxa within this group using cladistic methods. Parillaenus Jaanusson, 1954 forms a monophyletic group based on pygidial morphology that is distinct from Illaenus. Ouranalysis indicates that Illaenus is paraphyletic and requires major revision. Thaleops is a large group of species whose monophyly is supported by an unambiguous apomorphy: the presence of long articulating facets on the pygidium that are oriented at a high angle to the anterior margin of the pygidium. All but one basal member of the clade possesses a second apomorphy, posteriorly positioned palpebral lobes. Nanillaenus is not monophyletic and is synonymised with Thaleops. Several Laurentian species originally identified asIllaenus are reassigned to Thaleops. Seventeen previously named species are revised and re-illustrated, four of which (I. arcturus Hall, 1847; I. clavifrons Billings, 1859; I. punctatus Raymond, 1905; I. americanus Billings, 1859) are restricted to the type material. In addition, four new species,T. anusacerbissima, T. jaanussoni, T. mobydicki and T. laurentiana, are described from the Middle and Upper Ordovician Bromide and Viola Springs formations of Oklahoma and the Trenton Group of New York.