Middle ear ossicles (malleus, incus, stapes) are known only for few multituberculate taxa, and three different stapedial morphotypes have been suggested: (1) slender, columelliform and microperforate, (2) robust and rod-like, and (3) bicrural. Reinvestigation of Upper Jurassic (Kimmeridgian) mammalian petrosals from the Guimarota coal mine in central Portugal (Western Europe) revealed an asymmetric bicrural stapes (ABS) in the paulchoffatiid Pseudobolodon oreas. The middle ear bones displaced inside the osseous vestibule were detected by a microCT analysis. The Kimmeridgian age of the Guimarota stapes exceeds the stapes from the Early Cretaceous (Barremian) of Asia (about 122-124 ma) by ~30 ma, and is only slightly younger than the stapes of the recently described Oxfordian euharamiyid Arboroharamiya allinhopsoni. The Guimarota stapes indicates that the stapes of Lambdopsalis, described as columelliform and microperforate (small stapedial foramen), does not represent a general condition for multituberculates. The stapes of Pseudobolodon is bicrural, the anterior crus sits centrally on the oval footplate, and the stapedial head is simple and smaller than the footplate. We hypothesize that the ABS evolved from the symmetric bicrural stapes (SBS) of non-mammaliaform cynodonts. The ABS appears to be the ancestral morphotype of the mammalian SBS, and the mammalian columelliform imperforate stapes (CIS).