Development and metaplasia of ossified tendons in two hadrosaurine dinosaurs (Maiasaura peeblesorum and Brachylophosaurus canadensis) are described by comparison with the known developmental processes of ossified tendons in turkeys (Meleagris gallopavo). Mineralized primary tendon tissue and replacement patterns suggest that ossified tendons in hadrosaurs grew initially in a manner similar to those in turkeys. That is, biomineralization begins with apatite deposition and is followed by resorption of mineralized primary tissue and subsequent bone formation. Earlier ontogenetic onset of tendon ossification and later offset among ossified tendons of hadrosaurs differentiates them from tendons of turkeys. In addition, an external fundamental system (EFS) in hadrosaurian tendons indicates that they grew radially beyond the original tendon boundary, a process unknown among avian tendons. Tendon histology indicates that hadrosaurian ossified tendons developed in step with the skeletal system and that ossification was not induced by biomechanical stresses.