ABSTRACT

Fossil tracks should theoretically capture differences in pedal anatomy between growth stages of the same taxon, particularly those related to the soft tissue of the foot, providing a more realistic view of pedal ontogeny than skeletal material alone. However, recognizing these ontogenetic trajectories is complicated by the influence of preservation and kinematics on track morphology, as well as the inherent difficulty of referring different tracks to a single taxon. Here, we explore differences in track morphology from a collection of tracks attributed to tyrannosaurids from Unit 4 of the Wapiti Formation (upper Campanian) in western Canada. Along with morphology, close geographic and stratigraphic associations suggest that the tracks pertain to similar tyrannosaurid trackmakers. A geometric morphometric analysis of the track outlines reveals size-dependent increase in relative track robusticity, driven primarily by an increase in ‘heel’ breadth and surface area. This relationship is lost when the dataset is expanded to include tyrannosaurid tracks globally, which we attribute to increased stratigraphic and taxonomic ‘noise’ within the global dataset that masks the tightly constrained patterns obtained from the Wapiti Formation tracks. Although there is some substrate and kinematic influence on certain aspects of track morphology, we hypothesize that the observed size-dependent relationship reflects genuine expansion in the breadth of the heel soft tissues and probably their overall surface area associated with growth. Increased pedal robusticity likely assisted with weight bearing and locomotor stability as body mass increased over ontogeny, supporting previous hypotheses that some tyrannosaurids underwent a growth-related reduction in relative agility and/or cursorial performance.

摘要

从理论上讲，化石迹线应该能够捕捉同一分类群生长阶段之间的踏板解剖结构差异，尤其是与脚的软组织相关的那些，从而提供比单独的骨骼材料更为真实的踏板形态学视图。但是，由于保存和运动学对轨迹形态的影响以及将不同轨迹引用到单个分类单元的固有困难，识别这些本体发生的轨迹变得很复杂。在这里，我们从加拿大西部Wapiti组（Campanian上层）第4单元归因于霸王龙的轨道集合中，探索轨道形态的差异。除形态学外，紧密的地理和地层学联系还表明这些磁道属于类似的霸王龙的磁道制造者。轨道轮廓的几何形态分析表明，相对轨道鲁棒性的大小依赖于增加，这主要是由“后跟”宽度和表面积的增加引起的。当数据集扩展到全球包括霸王龙轨迹时，这种关系就消失了，这归因于全球数据集内地层学和分类学“噪声”的增加，掩盖了从Wapiti编队轨迹获得的严格约束的模式。尽管在磁道形态的某些方面存在一定的底物和运动学影响，但我们假设观察到的尺寸依赖性关系反映了脚跟软组织的宽度的真实扩展，并可能反映了与生长相关的整个表面积。