Although prey selection has been commonly documented in extant animals, evidence of prey selection in deep time is rare by comparison. Here, we collected 147 broken sclerites of trilobites, almost all of which were caused by lethal predation, from the Upper Ordovician in NW China. The damage was mainly distributed in three larger trilobites ( Platyptychopyge , Birmanites and Ordosaspis ), and larger-bodied trilobites were more vulnerable to predation. One plausible explanation is that Late Ordovician predators in this area preferred larger trilobites, because of the greater nutrients and energy provided by larger prey; also, larger prey require less effort to capture because they are less capable of escaping from predation compared with smaller prey, which can more easily hide or escape. Another, lower possibility is survivor bias owing to smaller prey being completely eaten up. Moreover, the large-eyed trilobite Nileus , which has a similar size and morphology to Ordosaspis , had a theoretical preying rate less than 1/20 of that of Ordosaspis , indicating that the eye is an important sensory organ in trilobites. The increased predation pressure may have also contributed to the increased eye diversity and visual system resolution of trilobites since the Ordovician. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6670609 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system