Stygina, eobronteus (Ordovician Styginidae, Trilobita): Morphology, classification, and affinities of Illaenidae

The fulcrate thorax of Stygina has the articulating furrow and a long artic ulating halfring, no pleural furrow, and the facetted outer portion of the pleura has the doublure terminating outside the fulcrum. In the type specie s the glabella is inflated and the axial furrows partially effaced; in othe r species inflation is not accompanied by such effacement. Distinctions bet ween genera of Stygininae are based on cephalic characters, including the l ength (sag. and exs.) of the preglabellar area; the variability of these ch aracters makes such distinctions difficult to define. Eobronteus has the fo rwardly-expanding glabella, three pairs of glabellar furrows, rostral plate and hypostome typical of Siluro-Devonian Scutelluinae. The relationship be tween the two subfamilies is shown by morphology and by ontogeny. It is dou bted that Styginidae had an origin in common with Illaenidae; the character s suggesting a relationship between Nileus and illaenids are explored. To i mprove the classification of Trilobita, the search for Cambrian ancestors o f Ordovician families needs to be intensified, and characters of the entire axial region should be taken into account. The impendent condition of the hypostome is homeomorphic and is of less significance in classification tha n the natant and conterminant conditions. Effacement of axial furrows of th e cephalon is a trend displayed in Stygininae, and is another homeomorphic character that is not a reliable guide to relationship.