In Western Europe, the change from the Middle Miocene forest-adapted mammalian faunas to the more open woodland faunas of the later Neogene, took place through an abrupt critical period known as the Vallesian Crisis. The Vallesian Crisis involved the disappearance of most of the forest-adapted elements characterizing the middle Miocene, such as tapirs, rhinoceroses, wet-adapted artiodactyls, hominoids, rodents, and the large carnivores of the families Nimravidae and Amphicyonidae. Detailed analysis in the well-calibrated and mostly complete sequence of the type area of the Vallesian Mammal stage, the Vallès-Penedès Basin (NE Spain), reveals that entries were always above the level of exits during the early Vallesian, reaching a diversity maximum at the end of this time. At 9.75Ma, the Vallesian Crisis involved a sudden decrease of diversity, caused by high extinction levels, well above the number of entries. This event can be correlated with Mi7, a minor isotopic shift at about 9.6Ma. The correlation of the Vallesian Crisis with such a minor isotopic shift is in contrast with previous, more significant isotopic shifts which had little ecological effects on the terrestrial ecosystems. This inconsistency between the global climate change inferred from the oceanic record and its effect on the structure of the late Miocene terrestrial ecosystems calls for some caution when inferring direct, linear relationships between climate change and mammalian turnover. In contrast with the Plio-Pleistocene glacial-interglacial dynamics, the effects of this climate forcing seem to have been more complex during Miocene times. An alternative pattern of change can be envisaged by proposing a “House of Cards” effect for the Vallesian Crisis. Increasing diversity levels after a prolonged period of stability enhanced terrestrial ecosystems to evolve into a self-organized critical state, which suddenly dropped after a critical threshold was surpassed. In contrast to other critical periods, the final decline of the Vallesian chronofauna was more dependent on the critical state of the system than on the magnitude of the agent which induced the crisis.