Evolutionary processes leading to the trilobite suborder Phacopina

A new Ordovician genus, Gyrometopus (fam. Diaphanometopidae, subfam. Gyrometopinae nov. ), is established and the type species Nileus? lineatus Angelin, 1854, redescribed. The origin of the Phacopina was associated with at least two morphological discontinuities: fixation of the number of thoracic segments and fusion of free cheeks. The characters of Gyrometopus (number of thoracic segments, general morphology of the cephalon and pygidium, structure of the thoracic pleurae) agree with those of the Phacopina except that the genus possesses a functional rostral shield. Thus Gyrometopus fulfills most of the requirements of an immediate ancestor of members of the Phacopina but has not bridged one of the morphological discontinuities. The evolutionary processes associated with bridging morphological discontinuities are discussed.