LAUSR

Significance Vertebrates and arthropods are two of the most successful and frequently fossilized animal groups, but direct evidence of their interaction in deep time – entailing the joint, intimate fossilization of remains from both groups – is extremely rare. Our discoveries in fossilized plant resin (amber) from the Early Cretaceous of Spain show that a symbiotic relationship, likely commensal/mutualistic, was established between beetle larvae feeding on detached feathers and feathered dinosaurs (theropods) more than a hundred million years ago. Only two previous records of arthropod–theropod symbiosis involving direct fossil evidence were known, both parasitic. Our findings demonstrate that beetles and feathered theropods have interacted since the Mesozoic, and shed light on the evolutionary importance of early symbiotic relationships between arthropods and vertebrates.