Phylogenetic support for the monophyly of proetide trilobites

Abstract

The monophyly of the order Proetida, the only trilobite group to survive the end-Devonian mass extinction, has been regularly questioned since its erection almost three decades ago. Through analysis of a novel phylogenetic data set comprising 114 characters coded for 55 taxa, including both traditional members of the Proetida along with a number of other trilobite groups, the monophyly of proetide trilobites is rigorously tested for the first time. Proetida is shown to be monophyletic, united by the initial compound eye formation in early protaspids occurring at the lateral margin rather than the anterior margin, and the form of the protaspid glabella being tapering with a pre-glabellar field. A number of adult characters, including the possession of a quadrate or shield-shaped hypostome with angular posterior margins, the hypostome median body being divided by a deep groove that entirely traverses the median body, the presence of an enlarged thoracic spine on the sixth tergite and a tergite count of between 7 and 10, also define the basal node. Hystricurid and dimeropygoid trilobites are shown to resolve at the base of the group, while the remaining proetide taxa are divided between large proetoid and aulacopleuroid clades. Some taxa previously allied with Aulacopleuroidea, such as rorringtoniids and scharyiids, are retrieved as basal members of the Proetoidea.