The Zambrana vertebrate fossil locality (Álava, Basque-Cantabrian Region) is located within Upper Eocene continental deposits of the Miranda-Treviño Basin. According to the sedimentological studies of this work, the fossiliferous beds are palustrine-lacustrine coal-bearing marls and limestones. They correspond to a shallow, low-gradient lake margin palaeoenvironment, with a peripheral swamp. The sedimentation was controlled by changes of water level. The fossil rich levels occur on top of swamp related organic matter rich facies, which were rapidly buried because of a rise of the water level. Therefore, the swamps are essential environment to prevent fast subaerial oxidation of the bones, and after buried by lacustrine facies, to form a reducing environment in water-saturated sediments and to promote the fossUization of the bone
