Relationship between scapular position and structural strength of rib cage in quadruped animals

Abstract

Determining scapular position is a major issue in reconstructing the skeletal systems of extinct quadruped archosaurs and mammals, because the proximal portion of the scapulae has no direct skeletal joint with the vertebrae or ribs. When quadrupeds stand or walk, their trunk is suspended between the forelimbs by the serratus muscles, which arises from the lateral sides of the "thoracic" ribs and inserts into the proximal portion of the costal surface of the scapula. Therefore, the "thoracic" ribs are subjected to a static or dynamic vertical compression between the lifting force from the muscle and the gravitational force from the vertebral column. To investigate the body support function of the ribs, we analyzed the mechanical strength of the ribs of extant tetrapods by the two-dimensional finite element method, and compared the degree of strength through their craniocaudal scapular positions. The result of this simulation showed that the "thoracic" ribs of quadrupeds, to which the serratus muscles attach, have a relatively higher strength against compaction than the other ribs. In bipeds, however, we did not find a similar correlation between the strength of ribs and the serratus muscle. This implies that the location of robust ribs is associated with the arrangement of the serratus muscle, and provides a probable candidate for determination of the scapular position for extinct quadruped archosaurs and mammals.

Molecular Footprints of Aquatic Adaptation Including Bone Mass Changes in Cetaceans

“…In addition to the environmental pressures and respiratory function, biomechanical constraints linked to locomotion and body support would strongly affect the ribs and rib cage, particular in limbless tetrapods (e.g., Bramble and Carrier 1983; Carrier 1996; Houssaye et al. 2013, 2014; Fujiwara et al. 2009).…”

Elbow joint adductor moment arm as an indicator of forelimb posture in extinct quadrupedal tetrapods

“…However, this joint tends to have a wide range (or wide interpretations of range) of three-dimensional motion in most taxa, and bony articulations alone may be unreliable indicators of limb orientation and motion [24]. Furthermore, determination of the typical scapular position and orientation on the ribcage still remains problematic or even overlooked [25]. Differences in the shapes of articular cartilage caps and calcified epiphyses can also be a problem, especially in fossil archosaurs whose cartilages are often expected to be thick [26][27][28][29][30].…”

Fitting unanchored puzzle pieces in the skeleton: appropriate 3D scapular positions for the quadrupedal support in tetrapods

“…The maximum stress is inversely proportional to the strength of the rib against vertical compression (Fujiwara et al. ). Therefore, the muscle fascicle origin at which the maximum von Mises stress on the rib was minimized is the rib most strengthened against vertical compression.…”

“…For more mechanical approaches, the bending moments of the vertebral column (Christian & Preuschoft, ) and the bending strengths of the rib against vertical compression (Fujiwara et al. ) have been used as the indices of the cranio‐caudal scapular position in quadrupedal amniote tetrapods, although the dorso‐ventral and medio‐lateral scapular positions remain unresolved. In another study, stress analysis of simple structural models synthesized from polyhedral finite elements assuming the trunk is supported on a forelimb was conducted for crocodile and sauropod dinosaur models to estimate the appropriate scapular angle to the rib cage (Hohn, ), although the approach was not developed for deducing the scapular position to the trunk.…”