Epaxial muscles and ossified tendons in dinosaurs: Anatomy, development, histology, and biomechanics

Intratendinous ossification in dinosaurs (including birds) is a wide spread phenomenon that has implications for physiology, posture, biomechanics, behavior, and systematics. Four separate studies were undertaken to elucidate ossified tendon biology in extinct dinosaurs and birds. The first investigation reconstructs dorsal epaxial musculature in extinct dinosaurs. Crocodilian and avian dorsal epaxial muscles are homologized. Using the extant phylogenetic bracketing approach, the three-layered trellis of Hadrosauriformes is homologized to the M. transversospinalis slips in crocodilians and the M. longus colli dorsalis thoracica in birds. These tendons are ossified in Hadrosauriformes and some birds. The parallel ossified tendons in other ornithischians are homologized to the M. longissimus dorsi in Alligator. The second investigation determines how ossified tendons in non-avian dinosaurs developed. Atrophied muscle was previously thought to be the origin of ossified tendons in non-avian dinosaurs. Using different age classes of hadrosaurs (Brachylophosaurus and Maiasaura) and turkeys (Meleagris), the developmental process of ossified tendons in hadrosaurs is shown to be homologous with intratendinous ossification in birds. But, the degree of ossification is greater in hadrosaurs, whose tendons also possess primary osteons, an external fundamental system and lines of arrested growth. The third investigation determines the histological diversity of ossified tendons in Dinosauria. Despite various anatomical locations and large differences in size, ossified tendons have uniform microstructure even in specimens that do not normally experience intratendinous ossification (such as Ceratosaurus and Camarosaurus). The greater degree of ossification noted above occurs in all non-avian dinosaurs. Also, variation in periosteal bone development occurs along the length of individual tendons. Ossified tendons from marginocephalians are unique in that they have larger portions of fibrolamellar bone and radial vascularity. In the fourth investigation, computer finite element models are constructed for two ornithopods to assess ossified tendon biomechanics. An Alligator is used to empirically determine joint properties. Ossified tendons probably reduced tail deflection and played a role in locomotion in all ornithopods. The ossified tendon trellis of Hadrosauriformes may have increased the skeleton's ability to bear large body mass. Neural spine height and bone material properties have the greatest impact on spinal rigidity.