The trilobita as a natural group

It has been claimed that olenellids, long regarded as trilobites, are more closely related to chelicerates than to the rest of the trilobite clade (Lauterbach 1980). This was based on an interpretation of the homologies of segmental arrangement in the thorax, and of the thoracic axial spine. It is shown that there are more synapomorphies uniting accepted trilobites with the olenellids, than there are uniting olenellids with chelicerates. At least seven characters serve to define Trilobita as a natural, monophyletic group. These include the presence of a pygidium, the unique optical system, the presence of eye ridges, circum‐ocular ecdysial sutures, and the construction of the hypostome. Olenelloids include the most primitive of the trilobites, retaining three (possibly four) primitive characters, including the permarginal suture, flange‐like thoracic articulation and highly expressed segmentation on the larval cranidia. If facial sutures are primitively absent in the Trilobita there is no obvious olenelloid autapomorphy, and they may constitute a paraphyletic group.