<p>Skeletochronology combined with growth curve reconstruction is routinely used to assess the age and growth dynamics of extinct and extant vertebrates. Here we performed in vivo labelling studies of the bone histology of four 2 years-old <i>Crocodylus niloticus</i> individuals. We found that all the crocodiles have more growth marks in their compacta than expected for their age, i.e., they deposited stochastic growth marks in their bones. Using the fluorochrome markers we determined that these stochastic growth marks were deposited during their favourable season of growth. The variable preservation of growth marks in the crocodile bones highlights developmental plasticity in their growth, which can be extrapolated to extinct archosaurs, and other reptiles. We caution the use of growth marks in fossil bones as a reliable estimator of age and discuss the far-reaching implications this has for growth curve reconstruction and life history assessments of extinct vertebrates, such as nonavian dinosaurs.</p>