The overarching trajectory of Palaeozoic vegetation history can be interpreted as the sequential replacement of the Eotracheophytic, Eophytic, Palaeophytic and Mesophytic evolutionary floras. Each evolutionary flora was characterised by a group of co-existing supra-generic plant taxa (families and orders) that formed relatively coherent communities in time and space. In most cases, the transition between floras was relatively brief and usually reflected the appearance of evolutionary adaptations (e.g. seeds, robust steles) that favoured the plants of the new flora. The main exception was the diachronous appearance of the Mesophytic Flora during the late Carboniferous and Permian, apparently the result of the invasion by upland or extra-basinal vegetation pre-adapted to the drier substrates that were developing then in the lowlands. The mass extinctions that had such a major effect on Sepkoski's evolutionary faunas had little effect on the dynamics of the evolutionary floras.