Phenotypic plasticity, the ability of an individual organism to express different phenotypes in response to varying environmental conditions, is ubiquitous across life’s diversity. Evolutionary hypotheses that could in principle be tested in the fossil record include the origin of adaptive plasticity in heterogeneous environments. Many paleontological papers, acknowledging that observed variation might be due either to plasticity or to genetic differentiation, explicitly decline to choose between them. Since developmental plasticity can modify phenotype within a generation and can revert to the original form with equal speed if the environment reverses, very rapid and/or reversible change has been seen as a signal of plasticity, whereas genetic change is expected to take longer. Over longer time-periods, fossil morphology constantly fluctuating around a mean has been interpreted as environmentally-induced plasticity.