Thoracic epaxial muscles in living archosaurs and ornithopod dinosaurs

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. Anat Rec Part A, 288A: 782-793, 2006.

The Tail of Tyrannosaurus: Reassessing the Size and Locomotive Importance of the M. caudofemoralis in Non‐Avian Theropods

“…The term ''caudal ribs'' is likely more accurate for all but the most posterior caudal vertebra]. Both the M. longissimus and M. spinalis are continues into the dorsal region (Organ, 2006).…”

Reconstruction of the thoracic epaxial musculature of diplodocid and dicraeosaurid sauropods

“…The anatomical nomenclature is adopted from Frey (1988a) and Tsuihiji (2005) for myology and from and Wilson (1999) for osteology. The hypothesized homology of m. transversospinalis in extant Sauria follows Tsuihiji (2005), and that of thoracic epaxial muscles of extant Archosauria follows Organ (2006) (Table 1). …”

“…1). M. tendinoarticularis is either regarded as an autonomous muscle (Hair, 1868;Frey, 1988a;Salisbury, 2001), or as part of the m. transversospinalis group (Gasc, 1981;Bornhauser and Ziswiler, 1983;Tsuihiji, 2005;Organ, 2006). The latter hypothesis (Table 1) is supported by the innervation pattern of the epaxial muscles in the trunk (Murakami et al, 1991).…”

“…interspinales and mm. interarticulares is present (Vallois, 1922;Gasc, 1981;Tsuihiji, 2005;Organ, 2006). M. longissimus dorsi is separated from m. transversospinalis by the septum intermusculare dorsi, and from the laterally positioned m. iliocostalis by the septum transversum (Fig.…”

“…Consequently, the variability in the epaxial trunk musculature and how it might relate to locomotor styles in different sauropod groups remains unknown. Whereas reconstructions of thoracic epaxial muscles in ornithischian dinosaurs are facilitated by the presence of ossified tendon systems (e.g., Norman, 1986;Organ, 2006), no such indications are available in dorsal vertebrae of sauropods. Additionally, the highly derived morphology of sauropod dorsal vertebrae makes comparisons with extant amniotes difficult.…”