The oldest known larva and its implications for the plesiomorphy of metazoan development

There has been a century-long debate in evolutionary developmental biology about whether the ancestral metazoan was a larva or an adult. Two competing hypotheses have been proposed: the “terminal addition” theory, which assumes the primitiveness of larvae, and the “intercalation” theory, which assumes the primitiveness of adults. A consensus has not yet been reached, but the “terminal addition” theory appears to be more widely accepted. However, in contrast to the majority of larvae among living metazoans, all currently known fossil invertebrate embryos such as Markuelia and Olivooides are direct developers. Here, we describe Eolarva kuanchuanpuensis gen. et sp. nov., the oldest known larva, from the early Cambrian (~535Ma) of South China. Eolarva kuanchuanpuensis lacks a mouth or any other type of feeding apparatus, which is non-feeding or lecithotrophic. It possesses a distinct body plan and might represent a cnidarian-grade animal. This is the first fossil evidence indicating that indirect development is the plesiomorphy of metazoan development.