Thoracic epaxial muscles in living archosaurs and ornithopod dinosaurs.

Crocodylians possess the same thoracic epaxial muscles as most other saurians, but M. transversospinalis is modified by overlying osteoderms. Compared with crocodylians, the thoracic epaxial muscles of birds are reduced in size, disrupted by the synsacrum, and often modified by intratendinous ossification and the notarium. A phylogenetic perspective is used to determine muscle homologies in living archosaurs (birds and crocodylians), evaluate how the apparent disparity evolved, and reconstruct the thoracic epaxial muscles in ornithopod dinosaurs. The avian modifications of the epaxial musculoskeletal system appear to have coevolved with the synsacrum and notarium. The lattice of ossified tendons in iguanodontoidean dinosaurs (Hadrosauridae and Iguanodontidae) is homologized to M. transversospinalis in crocodylians and M. longus colli dorsalis, pars thoracica in birds. Birds have an arrangement of tendons within M. longus colli dorsalis, pars thoracica identical to that observed in the epaxial ossified tendons of iguanodontoid dinosaurs. Moreover, many birds (such as grebes and turkeys) ossify these tendons, resulting in a two- or three-layered lattice of ossified tendons, a morphology also seen in iguanodontoid dinosaurs. Although the structure of M. transversospinalis appears indistinguishable between birds and iguanodontoid dinosaurs, intratendinous ossification within this epaxial muscle evolved convergently.