The First Quantitative Evidence of Ontogenetic Growth in the Early Cambrian Ecdysozoan Worm Cricocosmia jinningensis Reveals the Earliest Energy Reallocation.

Ontogeny is both biologically pivotal and ubiquitous among extant ecdysozoans. Ontogenetic data from the fossil record offer direct evidence for resolving questions of developmental evolution within this clade. However, such data remain exceedingly scarce, especially for ecdysozoan worms. Here, our study provides the first comprehensive analysis of the growth pattern of the early Cambrian ecdysozoan worm Cricocosmia jinningensis from the Chengjiang biota. Cricocosmia exhibits isometric growth of the trunk and introvert scalids, allometric growth of the lateral sclerites, and a constant number of sclerites through ontogeny. More importantly, investment in sclerites reverses from negative to positive allometry at the juvenile-adult transition, a pattern quantitatively documented here for the first time in the fossil record, pushing back the evidence of such a functional shift to 518 million years ago. This unique growth pattern provides the earliest direct evidence for the evolution of developmental patterns of ecdysozoans and demonstrates an energy reallocation strategy that closely matches the defensive demands and locomotion strategy of different life stages.