Data from: A computational analysis of locomotor anatomy and body mass evolution in Allosauroidea (Dinosauria Theropoda)

We investigate whether musculoskeletal anatomy and three-dimensional (3-D) body proportions were modified during the evolution of large (>6000 kg) body size in Allosauroidea (Dinosauria Theropoda). Three adaptations for maintaining locomotor performance at large body size, related to muscle leverage, mass, and body proportions, are tested and all are unsupported in this analysis. Predictions from 3-D musculoskeletal models of medium-sized (Allosaurus) and large-bodied (Acrocanthosaurus) allosauroids suggest that muscle leverage scaled close to isometry, well below the positive allometry required to compensate for declining muscle cross-sectional area with increasing body size. Regression analyses on a larger allosauroid data set finds slight positive allometry in the moment arms of major hip extensors, but isometry is included within confidence limits. Contrary to other recent studies of large-bodied theropod clades, we found no compelling evidence for significant positive allometry in muscle mass between exemplar medium- and large-bodied allosauroids. Indeed, despite the uncertainty in quantitative soft tissue reconstruction, we find strong evidence for negative allometry in the caudofemoralis longus muscle, the single largest hip extensor in non-avian theropods. Finally, we found significant inter-study variability in center-of-mass predictions for allosauroids, but overall observe that consistently proportioned soft tissue reconstructions produced similar predictions across the group, providing no support for a caudal shift in the center of mass in larger taxa that might otherwise reduce demands on hip extensor muscles during stance. Our data set provides further quantitative support to studies that argue for a significant decline in locomotor performance with increasing body size in non-avian theropods. However, although key pelvic limb synapomorphies of derived allosauroids (e.g., dorsomedially inclined femoral head) evolved at intermediate body sizes, they may nonetheless have improved mass support.

本研究旨在探讨异特龙超科（Allosauroidea，恐龙总目兽脚亚目（Dinosauria Theropoda））在演化至体型大于6000千克的巨型阶段时，其肌肉骨骼解剖结构与三维（3-D）身体比例是否发生了适应性改变。本研究检验了三类与维持巨型体型下运动性能相关的适应性机制，分别涉及肌肉杠杆效率（muscle leverage）、肌肉质量与身体比例，但本次分析未支持任何一类假说。基于对中型异特龙（Allosaurus）与大型高棘龙（Acrocanthosaurus）这类异特龙类的三维肌肉骨骼模型所做出的预测显示，肌肉杠杆效率的缩放接近于等距缩放，远低于补偿体型增大时肌肉横截面积（cross-sectional area）下降所需的正变率缩放水平。针对更大规模异特龙类数据集的回归分析（regression analyses）显示，主要髋部伸肌（hip extensors）的力臂（moment arms）存在轻微正变率，但等距缩放仍处于置信区间（confidence limits）范围内。与近期针对其他大型兽脚类支系的研究结果相悖，本研究未发现有力证据表明中型与大型异特龙类样本的肌肉质量存在显著正变率。事实上，尽管软组织重建的量化结果存在不确定性，我们仍发现非鸟类兽脚类中最大的单块髋部伸肌——尾股肌长肌（caudofemoralis longus）的质量呈现显著负变率。最后，本研究发现不同研究对异特龙类的质心（center-of-mass）预测结果存在显著差异，但整体而言，采用统一比例的软组织重建方案可在该类群中得到相似的质心预测结果，并未支持大型类群的质心向尾部偏移这一假说——而该偏移本可降低站立阶段髋部伸肌的负荷需求。本数据集为相关研究提供了进一步的量化支持，这些研究认为非鸟类兽脚类的运动性能会随体型增大而显著下降。不过，尽管衍生型异特龙类的关键盆腔肢骨共有衍征（synapomorphies，例如股骨头（femoral head）背内侧倾斜）在中等体型阶段即已演化出现，但这些特征仍可能提升了身体支撑能力。