The functional anatomy of the hindlimb of bipedal dinosaurs has been intensively studied. Yet, surprisingly little work has been done concerning functional adaptation of digits for terrestrial locomotion. While complete and articulated pes skeletons are scarce, pes shape is abundantly recorded by fossil footprints. We elucidate the significance of footprint shape and size for locomotion using a large sample (n = 303) of tridactyl dinosaur footprints from a broad range of geographical localities and time slots. Size and shape variation are characterized separately for theropods and ornithischians, the two principal trackmaker taxa. At smaller sizes, theropod footprints are best discriminated from ornithischian footprints by their smaller interdigital angle and larger projection of digit III; at larger sizes digital widths are effective discriminants. Ornithischian footprints increase in size from the Early Jurassic to the Late Cretaceous, a trend not observed in theropod footprints. Size and function are argued to be important determinants of footprint shape, and an attempt made to infer function from shape. Digit III projection and length‐to‐width ratio of the footprints are negatively correlated with size in both groups; digit impression width is positively correlated with size only in ornithischians. Digit III projection appears to be positively correlated with cursorial ability. Increased interdigital angles are associated with a decrease in digital width, possibly an adaptation for stability. Weak digit III projection and increased digital width are interpreted as adaptations for graviportality. Footprints yield great potential for the understanding of the functional morphology of dinosaur feet.

中文翻译：

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三角恐龙足迹中的形状变异性：大小和功能的重要性

对两足恐龙后肢的功能解剖学进行了深入研究。然而，令人惊奇的是，很少有关于数字功能适应地面运动的工作。尽管缺少完整且清晰表达的pes骨骼，但化石足迹记录了pes的形状。我们使用大范围的地理位置和时间范围内的大型三足龙恐龙足迹样本（n = 303）阐明了足迹形状和大小对于运动的重要性。大小和形状变化分别针对兽脚亚目和鸟眼兽（这两个主要的追踪者类群）进行了表征。在较小的尺寸上，兽脚类脚印最好与鸟嘴兽脚印区分开，因为它们的叉指角度较小且数字III的投影较大。在较大的尺寸上，数字宽度是有效的判别方法。从侏罗纪早期到白垩纪晚期，鸟嘴兽脚印的大小增加，在兽脚类动物脚印中没有观察到这种趋势。大小和功能被认为是足迹形状的重要决定因素，并且尝试从形状推断功能。两组的数字III投影和足迹的长宽比与大小负相关；仅在鸟眼动物中，数字印象宽度与大小呈正相关。数字III投影似乎与光标能力正相关。叉指角度的增加与叉指宽度的减小有关，可能是为了适应稳定性。数字III的弱投影和数字宽度的增加被解释为对重力的适应。