[N] 2006 Thoracic epaxial muscles in living archosaurs and ornithopod dinosaurs
Organ, C.L. (2006) Thoracic epaxial muscles in living archosaurs and ornithopod dinosaurs The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology Volume 288A, Issue 7, Date: July 2006, Pages: 782-793 Organ, C.L.
Abstract: 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.