Corethron Castracane is a marine planktonic diatom genus that is widespread in cold-water assemblages at the present day. Its fossil record was known previously to range from the middle Eocene to the Holocene. Here, we report the earliest occurrence of Corethron in Late Cretaceous (Cenomanian to Santonian) amber from Vend ee, northwestern France, thus extending the first appearance datum by about 40e55 million years and supporting the molecular phylogenetic estimates proposed for the origin of this lineage. The presence of Corethron in amber is a special case of biostratinomy where a marine taxon is incorporated into the liquid resin produced by trees, implying that the Vendean amber forest grew in a nearshore environment where wind, sea-spray, or high tide introduced the marine diatom frustules into the terrestrial realm.