Forelimb orientation and locomotion of sauropod dinosaurs: insights from the ?Middle Jurassic Tafaytour tracksites (Argana Basin, Morocco)

Forelimb posture in sauropod dinosaurs is still poorly understood. Although a laterally directed (semisupinated) manus is the plesiomorphic condition in sauropodomorphs, the sauropod track record prevailingly shows anterolateral to anterior manus orientations, suggesting a high degree of manus pronation. The ?Middle Jurassic Tafaytour tracksites described herein preserve at least 10 trackways, nine of which show laterally oriented, and in two examples even posterolaterally oriented, manual impressions. Located in the Argana Basin of the Western High Atlas, Morocco, the tracksite yields hundreds of footprints on a highly bioturbated surface, including examples with well-preserved digit and claw impressions. Footprint morphology and trackway configuration vary greatly between trackways. A literature review indicates that laterally directed manual impressions are restricted to small- and medium-sized trackmakers. Statistical analysis was performed on a larger sample (n = 79) of small sauropod trackways from around the world, indicating that lateral manual orientations are correlated with low locomotion speeds and narrow trackway gauges. Manus pronation in sauropods is hypothesized to occur when the forelimb is actively contributing to the propulsion of the animal, which would be the case at faster speeds or at wider trackway gauges where the center of mass is located more anteriorly. We present new approaches to the quantitative analysis of trackway data. Mean configuration plots allow for direct graphical comparisons of different trackways. Two types of trackway asymmetries are defined and quantified. The apparent glenoacetabular distance (GAD) represents a feasible proxy for body size, at least for the smaller sauropod trackmakers analyzed herein.

蜥脚类恐龙（sauropod dinosaurs）的前肢姿态研究至今仍有待深入。尽管蜥脚形亚目（sauropodomorphs）的祖征（plesiomorphic condition）为前足（manus）呈侧向指向、处于半旋前状态（semisupinated），但蜥脚类的足迹记录普遍显示前足朝向从前外侧延伸至正前方，提示前足存在高度旋前现象。本文记述的疑为中侏罗世（Middle Jurassic）的塔法图尔足迹点（Tafaytour tracksites）至少保存了10条行迹（trackways），其中9条的前足印痕呈侧向指向，另有2条甚至呈后外侧指向。该足迹点地处摩洛哥西高阿特拉斯山脉（Western High Atlas）的阿尔加纳盆地（Argana Basin），其产出的数百枚足迹分布于经强烈生物扰动（bioturbated）的岩层表面，部分足迹保存有清晰的趾骨与爪印痕。不同行迹间的足迹形态（footprint morphology）与行迹构型（trackway configuration）差异悬殊。文献调研显示，侧向指向的前足印痕仅见于中小型造迹者（trackmakers）。研究团队对全球范围内79条小型蜥脚类行迹样本（n=79）开展统计分析，结果表明前足侧向朝向与较低的运动速度（locomotion speeds）以及较窄的行迹轨距（trackway gauges）呈显著相关性。学界假说提出，蜥脚类前足旋前（manus pronation）现象发生于前足主动参与推进运动（propulsion）时，而当运动速度更快、行迹轨距更宽——此时动物的质心（center of mass）位置更靠前——便会出现此类情况。本文提出了针对行迹数据开展定量分析（quantitative analysis）的全新方法。平均构型图（mean configuration plots）可实现不同行迹间的直接图形化对比。研究定义并量化了两类行迹不对称性（trackway asymmetries）。表观肩髋臼间距（apparent glenoacetabular distance, GAD）可作为体型估算的可行替代指标（proxy），至少适用于本文所分析的小型蜥脚类造迹者。