Phylogenetic variation in hind-limb bone scaling of flightless theropods

The robusticity of the weight-bearing limbs of large terrestrial animals is expected to increase at a more rapid rate than in their smaller relatives. This scaling has been hypothesized to allow large species to maintain stresses in the limb bones that are similar to those seen in smaller ones. Curvilinear scaling has previously been found in mammals and nonavian theropods but has not been demonstrated in birds. In this study, polynomial regressions of leg-bone length and circumference in terrestrial flightless birds were carried out to test for a relationship similar to that seen in nonavian theropods. Flightless birds exhibit curvilinear scaling, with the femora of large taxa becoming thicker relative to length at a greater rate than in smaller taxa. Evidence was found for nonlinear scaling in the leg bones of nonavian theropods. However, unlike in avians, there is also phylogenetic variation between taxonomic groups, with tyrannosaur leg bones in particular scaling differently than other groups. Phylogenetically corrected quadratic regressions and separate analyses of taxonomic groupings found little phylogenetic variation in flightless birds. It is suggested here that the nonlinear scaling seen in avian femora is due to the need to maintain the position of the knee under a more anterior center of mass, thereby restricting femoral length. The femur of nonavian theropods is not so constrained, with greater variability of the linear scaling relationships between clades. Phylogenetic variation in limb-bone scaling may broaden the errors for mass-predictive scaling equations based on limb-bone measurements of nonavian theropods.

大型陆生动物的承重肢骨骼粗壮度，预计相较于其小型同类，增长速率更快。该异速生长模式被假说认为，可使大型物种维持与小型物种相近的肢骨应力水平。此前已有研究在哺乳类与非鸟类兽脚类恐龙中发现曲线异速生长现象，但鸟类中尚未得到证实。本研究针对陆生不会飞鸟类的肢骨长度与周径开展多项式回归分析，以检验是否存在与非鸟类兽脚类恐龙相似的生长关系。结果表明，不会飞鸟类呈现曲线异速生长模式：大型分类群的股骨相较于体长，其粗壮程度的增长速率高于小型分类群。研究发现非鸟类兽脚类恐龙的肢骨存在非线性异速生长现象，但与鸟类不同的是，非鸟类兽脚类的不同分类群间存在系统发育差异，其中暴龙类的肢骨生长模式与其他类群尤为迥异。经系统发育校正的二次回归分析，以及针对分类群的独立分析均显示，不会飞鸟类的系统发育差异极小。本研究提出，鸟类股骨所呈现的非线性异速生长，源于需将膝关节维持在更靠前的质心位置，进而限制了股骨长度；而非鸟类兽脚类的股骨则无此限制，其不同演化支间的线性异速生长关系存在较大变异性。肢骨异速生长的系统发育差异，可能会增大基于非鸟类兽脚类肢骨测量数据构建的体重预测异速生长方程的误差。