Evolutionary trends and their functional significance in chasmopine trilobites

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Abstract
In response to environmental pressures, speciation within the Chasmopinae [Ordovician] involved the attempt to minimize the anterior blind spot. This was attempted in 4 different ways: by positioning the eyes progressively closer to the glabella, in the Chasmops-Toxochasmops lineage; on the sides of the cephalon, in Bolbochasmops and Rollmops; by elevating and reducing the size of the eyes, in Scopelochasmops and by development of very large eyes in species of Chasmops and Toxochasmops. Development of different eye morphologies resulted in complementary cephalic morphological changes, involving frontal lobe convexity, position and orientation of glabellar furrows, size of hypostome and overall cephalic shape. The strong selection pressure acting on eye form in order to minimize the anterior blind spot suggests that the Chasmopinae may have led a predatory mode of life.