Recognising and quantifying the evolution of skeletal paedomorphosis in Plesiosauria

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.