The reactivation of dormant genes in fossil lineages after long time periods is considered a likely evolutionary mechanism. This would not violate Dollo's law on evolutionary irreversibility in a genetic sense, but the law might appear to be contradicted by certain intra- and inter-cladogenic phenotypic variations to at least the generic level. Practical effects would include potential complications for unravelling palaeontological phylogenies, and the recognition of the true status of some so-called Lazarus taxa.