摘要：
Trade-offs play a crucial role in the evolution of life-history strategies of extant organisms and have shaped their past evolution.One important trade-off is between the number of offspring and the amount of energy(including nutrition and parental care) allocated to individual offspring.Exceptionally well-preserved fossils from the Cambrian period allow us here to trace the earliest evidence of trade-offs in reproduction of pancrustaceans.We report Chuandianella ovata from the early Cambrian Chengjiang biota of China(～520 Mya) which brooded numerous(≥100 per clutch) but small(? ～0.5 mm) eggs under the valves of its carapace.Waptia fieldensis,a closely related species from the middle Cambrian Burgess Shale of Canada(～508 Mya),showed a similar mode of brood care but,in contrast to C.ovata,carried fewer(≤24 per clutch) but larger(? ～2.4 mm) eggs.The different clutch/egg sizes in the two closely-related waptiid pancrustaceans reveal an evolutionary trade-off between quantity and quality of offspring through the early-middle Cambrian interval,probably as an adaptive response to changes of marine ecosystems.We hypothesize that trade-offs in brood care have played an important role in the evolutionary success of pancrustaceans from the Cambrian through to the present day.
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