Plesiosaurs are one of the longest-ranging tetrapod groups in the Mesozoic and underwent a major adaptive radiation in the Late Triassic/Early Jurassic, so they are an ideal clade to study the long-term implications and deep-time evolution of specific developmental patterns. We compiled a database of all published plesiosaur specimens and recorded their skeletal maturity status. We use statistical modelling to demonstrate that the abundance of allegedly ‘juvenile’ specimens increases through time, which contradicts the null hypothesis that the relative proportion of juvenile to adult specimens should remain constant throughout evolution. These results indicate that many ‘juvenile’ specimens are really adults exhibiting heterochronic traits, particularly paedomorphism. Heterochrony is a developmental pattern particularly widespread in secondarily adapted organisms such as plesiosaurs. However, heterochronic patterns are typically only studied in individual genera/species or restricted clades. We demonstrate that the pervasiveness of paedomorphism in plesiosaurs increased gradually throughout the evolution of the clade, rather than being a specialization of specific clades.