Mediumiugamiini is erected as new mordellid tribe based on Mediumiuga sinespinis gen. et sp. nov. It is a fossil beetle fromEarly Cretaceous amber from Spain and constitutes the first Spanish beetle described in amber. A new key for worldwide tribes of Mordellinae was necessary and it is provided, including new tribe Mediumiugamiini. A set of primitive characters were unchanged in fossil mordellids described from Jurassic and Cretaceous outcrops until the description of some specimens from Eocene Baltic amber, more resemble to modern mordellids. With the description of Mediumiuga sinespinis gen. et sp. nov. in amber, the change of some mordellid characters is discussed along Cretaceous, probably related to better ability to jump and keeping balance, getting more spores or pollen, and crawling and staying on leaves and petals.