Skeletochronology is a method used to determine the age of an individual from bone histology. The method is based on growth lines found in cortical bone, an area until now seldom examined in mosasaurs. For the first time, diaphysial thin sections of Tylosaurus, Platecarpus and Clidastes limb bones are studied using skeletochronological techniques. Results indicate that sexual maturity in mosasaurs was reached between ages five and seven, and that the mosasaurs studied exhibit a typical sauropsid growth pattern, although their growth rates are interpreted as faster than those of extant taxa because of fully aquatic adaptations.