Mass extinctions punctuate the palaeontological record. These overturns of species and groups of species are so monumental they help to define the framework of the geological timescale, with many smaller extinction events providing finer subdivisions. Perhaps the most famous mass extinction is that which occurred at the boundary between the Cretaceous Period (of the Mesozoic Era) and the Palaeogene Period (of the Cenozoic Era), 65.5 million years ago when, among countless other creatures, the dinosaurs finally went to the wall. The cause of these biological mega-crises has been sought in several directions but there are two main competing mechanisms – bolide (meteorite or comet) impacts versus volcanism on a massive scale. The extra-terrestrial theory argues that especially large bolides – perhaps 10 kilometres or more across – strike Earth from time to time and the collisions would generate sufficiently vast dust clouds to chill the surface, killing off organisms in the oceans and on land. For a long time, the Cretaceous–Palaeogene boundary and associated extinction have been linked to the impact that formed a 170-km-diameter crater at Chicxulub in México.