Exceptional fossil preservation and evolution of the ray-finned fish brain

Abstract

SUMMARYBrain anatomy provides key evidence for ray-finned fish relationships1, but two key limitations obscure our understanding of neuroanatomical evolution in this major vertebrate group. First, the deepest branching living lineages are separated from the group’s common ancestor by hundreds of millions of years, with indications that aspects of their brain morphology–like other aspects of their anatomy2,3–are specialised relative to primitive conditions. Second, there are no direct constraints on brain morphology in the earliest ray-finned fishes beyond the coarse picture provided by cranial endocasts: natural or virtual infillings of void spaces within the skull4–8. Here we report brain and cranial nerve soft-tissue preservation in †Coccocephalichthys wildi, a ∼319-million-year-old (Myr) ray-finned fish. This oldest example of a well-preserved vertebrate brain provides a unique window into neural anatomy deep within ray-finned fish phylogeny. †Coccocephalichthys indicates a more complicated pattern of brain evolution than suggested by living species alone, highlighting cladistian apomorphies9 and providing temporal constraints on the origin of traits uniting all extant ray-finned fishes9–11. Our findings, along with a growing set of studies in other animal groups12–16, point to the significance of ancient soft tissue preservation in understanding the deep evolutionary assembly of major anatomical systems outside of the narrow subset of skeletal tissues17–20.