Claspers in the mid-Cambrian <i>Olenoides serratus</i> indicate horseshoe crab–like mating in trilobites

Abstract Sexual reproduction represents a fundamental aspect of animal biology, but the diversity reproductive strategies among early Paleozoic metazoans remains obscure. Direct evidence comes from exceptionally preserved egg masses in Cambrian and Ordovician euarthropods such as waptiids trilobites, anatomical or behavioral adaptations for mating these taxa are all unknown. We demonstrate presence functionally specialized appendages mid-Cambrian (Wuliuan) Burgess Shale trilobite Olenoides serratus. New material O. serratus preserves significantly modified reduced endopodites underneath seventh thoracic first pygidial tergites. Comparison with extant indicates that limbs functional analogs to claspers, which used by sexually mature males grasp females prior during mating. The claspers were most likely male hold onto posterior spines female amplex, similar strategy observed horseshoe crab Limulus polyphemus. new appendicular data provide direct sexual dimorphism based on non-biomineralized ventral anatomy. Our findings illuminate biology trilobites indicate complex behaviors originated explosion.