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

Abstract

Deducing the scapular positions of extinct tetrapod skeletons remains difficult, because the scapulae and rib cage are connected with each other not directly by skeletal joint, but by thoracic muscles. In extant non-testudine quadrupedal tetrapods, the top positions of the scapulae/suprascapulae occur at the anterior portion of the rib cage, above the vertebral column and near the median plane. The adequacy of this position was tested using three-dimensional mechanical models of Felis, Rattus and Chamaeleo that assumed stances on a forelimb on a single side and the hindlimbs. The net moment about the acetabulum generated by the gravity force and the contractive forces of the anti-gravity thoracic muscles, and the resistance of the rib to vertical compression between the downward gravity and upward lifting force from the anti-gravity thoracic muscle depend on the scapular position. The scapular position common among quadrupeds corresponds to the place at which the roll and yaw moments of the uplifted portion of the body are negligible, where the pitch moment is large enough to lift the body, and above the ribs having high strength against vertical compression. These relationships between scapular position and rib cage morphology should allow reliable reconstruction of limb postures of extinct taxa.

Keywords: limb posture; moment analysis; reconstruction; stress analysis.

Figure 1

Positional relationships among body skeletons in vivo of Felis shown in the dorsal view. Note that the hindlimb skeletons are directly connected to the vertebral column via the pelvic girdle, whereas the forelimb skeletons have no direct skeletal connection to the rib cage.

Figure 2

Diagrams showing the difference of the scapular positions to the trunk in right lateral and cranial views. The scapulae are on (a) cranio‐dorso‐medial, (b) cranio‐ventro‐lateral and (c) caudo‐lateral positions, and (d) inside the ribcage. The dorsal portion of the scapular blade (double‐lined circle) and the glenoid (gl) were marked for each scapular position.

Figure 3

3D rotation model of Felis constructed in this study shown in right lateral and cranial views. The model assumes a stance on the right forelimb, and the body is supported via thoracic muscles (RHc, m. rhomboideus cervicis; RHt, m. rhomboideus thoracis; SVc, m. serratus ventralis cervicis; SVt, m. serratus ventralis thoracis). The roll, yaw and pitch axes through the acetabulum (point P) are perpendicular with each other. A plane parallel to the roll and yaw axes is defined as the ‘reference plane’. The uplifted body elements are allowed to rotate about the point P. The uplifted body elements are subjected to the downward force (F_COM) at the centre of mass (COM) generated by acceleration, and to the contractile forces of the thoracic muscles (RHc, RHt, SVc and SVt). Moment analyses were conducted under different orientations of the COM acceleration: (a) vertical; (b) 15° anterior; (c) 15° posterior; (d) 15° rightward; and (e) 15° leftward accelerations (see Table S3).

Figure 4

The appropriate scapular positions estimated for (a, b) Felis, (c, d) Rattus and (e, f) Chamaeleo in rotation models with the centre of mass (COM) accelerated vertically. (a, c, e) The scapular positions at which the net roll (SP_R) and yaw (SP_Y) moments generated by the contractive forces of the thoracic muscle fascicles (F _max) and the downward gravity forces (n × F_COM: n = 1, 2, 3, 4) applied to the fascicle origins and the centre of mass (COM), respectively, become zero; (b, d, f) The scapular positions at which the net pitch moment generated by the F _max and n × F_COM (SP _P−n: n = 1, 2, 3, 4) become zero. The results are shown in the oblique views in orthographic projection. COM, centre of mass of the uplifted body element (UB); P, a common pivot of the UB and TS about roll, yaw and pitch axes; TS, thoracic skeleton.

Figure 5

The appropriate scapular position against the vertical compression (SP_V): (a) Felis, (b) Rattus and (c) Chamaeleo. Graphs at the top show the relationship between the cranio‐caudal positions of the origins of m. serratus ventralis and the maximum von Mises stress distributed on the rib when each origin was subjected to upward lifting force (1 N) whereas the vertebral column was fixed to x‐, y‐, z‐axes. The uplifted body element (UB) and the thoracic skeleton (TS) were shown in the middle and the bottom, respectively, in the lateral and dorsal views in orthographic projections. The arrows on the images in the middle and the bottom indicate the positions on the rib cage where the vertical force was applied in the stress analyses, which correspond to the origins of the m. serratus ventralis. C4, the proximal (C4p), middle (C4m) and distal (C4d) origins of the m. serratus ventralis on the fourth cervical rib; C5, fifth cervical rib; Tn, nth thoracic rib.