Characterizing the Evolution of Wide-Gauge Features in Stylopodial Limb Elements of Titanosauriform Sauropods via Geometric Morphometrics.

Wide-gauge posture of titanosauriform sauropods remains an enigmatic peculiarity among terrestrial vertebrates. Here, two-dimensional geometric morphometrics and thin plate splines analyses were used to quantitatively analyze shape differences among sauropodomorph humeri and femora to identify how these elements may differ according to body gauge. Results demonstrate that titanosauriforms generally possess proportionately gracile humeri in comparison to other sauropods, with relatively more medially oriented humeral heads and proximally located deltopectoral crests. Myological repercussions of these features demonstrate a relative sacrificing of muscular torque for forelimb abduction/adduction in exchange for minimization of necessary muscle contraction to generate the same degree of limb excursion. Regarding femora, titanosauriforms possess significantly broader femora mediolaterally than other sauropods, with comparatively proximomedially placed fourth trochanters. Canonical variates results also identify a trend for titanosauriform femora to present distal condyles that are more frequently perpendicular to the long axis of the shaft or beveled medially. All of these femoral shape characteristics are expressed to the greatest degree by titanosaurians. Myologically, mediolateral femoral broadening increases relative mechanical advantages for hind limb abductor and adductor musculature. This supports previous hypotheses that suggested titanosauriforms were capable of a greater degree of hind limb abduction and adduction. This capability may have been necessary to maintain dynamic stability during wide-gauge locomotion over uneven terrain. Overall, our results corroborate previous qualitative assessments of wide-gauge attributes, afford new insights into statistically significant but obscure shape patterns, and add new clarity to aspects of the functional morphology of wide-gauge posture. Anat Rec, 300:1618-1635, 2017.