Evolutionary trade-off in reproduction of Cambrian arthropods

Trade-offs play a crucial role in the evolution of life-history strategies of extant organisms by shaping traits such as growth pattern, reproductive investment, and lifespan. One important trade-off is between offspring number and energy (nutrition, parental care, etc.) allocated to individual offspring. Exceptional Cambrian fossils allowed us to trace the earliest evidence of trade-offs in arthropod reproduction. dagger Chuandianella ovata, from the early Cambrian Chengjiang biota of China, brooded numerous (<= 100 per clutch), small (empty set, similar to 0.5 mm) eggs under carapace flaps. The closely related dagger Waptia fieldensis, from the middle Cambrian Burgess Shale of Canada, also brooded young, but carried fewer (<= 26 per clutch), larger (empty set, similar to 2.0 mm) eggs. The notable differences in clutch/egg sizes between these two species suggest an evolutionary trade-off between quantity and quality of offspring. The shift toward fewer, larger eggs might be an adaptive response to marine ecosystem changes through the early-middle Cambrian. We hypothesize that reproductive trade-offs might have facilitated the evolutionary success of early arthropods.