The phylogeny of cyclopygid trilobites

A new phylogeny of cyclopygid trilobites (Superfamily Cyclopygacea Raymond, 1925) is presented, based on an analysis of 47 genera and 63 characters. The majority-rule consensus supports a paraphyletic Taihungshaniidae Sun, 1931, basal to two clades united by the loss of pygidial spines: Nileidae Angelin, 1854, united by a hypostomal notch in the cephalic doublure, a sub-ocular socle and a pointed posterior margin of the hypostome, excluding <i>Borthaspidella</i> Rasetti 1954, <i>Lakaspis</i> Kobayashi, 1937, and <i>Petrbokia</i> Přibyl and Vaňek, 1965; and Cyclopygidae Raymond 1925, united by hypertrophied eyes and the loss of a pestle-shaped glabella, including <i>Illaenopsis</i> Salter, 1866a, a supposed nileid. Nileidae contains two clades, Parabarrandiinae Hupé, 1955 and Nileinae Angelin, 1854. Character transformations within cyclopygids indicate increased adaptations to a pelagic life habit and deeper depth of life habit, including increased eye size and their migration to the lateral margin of the cephalon, improving the field of vision, reduction in the number of thoracic segments, decrease in body size, overall improved streamlining, an increased length/width ratio and changes in the glabellar shape.