Heterochrony and tooth evolution in hyperodapedontine rhynchosaurs (Reptilia, Diapsida)

The tooth arrangement of hyperodapedontine rhynchosaurs shows clear pattern s of morphological derivation, which can be summarized as three main apomor phic trends: the increase in the number of tooth rows lateral to the main m axillary groove, the loss of dental structures (medial groove and lingual t eeth) medial to the main maxillary groove, and the loss of dental structure s (medial crest and lingual teeth) medial to the main dentary crest. The an alysis of these trends from a heterochronic viewpoint reveals that accelera tion was the most probable process involved in the increase in number of th e lateral maxillary tooth rows, while the loss of the medial structures of the maxilla and dentary seem to be related respectively to neoteny and post -displacement. Both peramorphic and paedomorphic processes are, therefore, thought to have directed the main modifications seen in the tooth arrangeme nt of the hyperodapedontine rhynchosaurs. Heterochrony plays an important r ole in the evolution of the Late Triassic rhynchosaurs, which are different iated mainly on the basis of their dental morphology. square Heterochrony, Hyperodapedontinae, rhynchosaurs, tooth morphology, Triassic.