Abstract：
The difficulty of quantifying asymmetrical limb movements, compared with symmetrical gaits, has resulted in a dearth of information concerning the mechanics and adaptive benefits of these locomotor patterns. Further, no study has explored the evolutionary history of asymmetrical gaits using phylogenetic comparative techniques. Most foundational work suggests that symmetrical gaits are an ancestral feature and asymmetrical gaits are a more derived feature of mammals, some crocodilians, some turtles, anurans and some fish species. In this study, we searched the literature for evidence of the use of asymmetrical gaits across extant gnathostomes, and from this sample (n=308 species) modeled the evolution of asymmetrical gaits assuming four different scenarios. Our analysis shows strongest support for an evolutionary model where asymmetrical gaits are ancestral for gnathostomes during benthic walking and could be both lost and gained during subsequent gnathostome evolution. We were unable to reconstruct the presence/absence of asymmetrical gaits at the tetrapod, amniote, turtle and crocodilian nodes with certainty. The ability to adopt asymmetrical gaits was likely ancestral for Mammalia but was probably not ancestral for Amphibia and Lepidosauria. The absence of asymmetrical gaits in certain lineages may be attributable to neuromuscular and/or anatomical constraints and/or generally slow movement not associated with these gaits. This finding adds to the growing body of work showing the early gnathostomes and tetrapods may have used a diversity of gaits, including asymmetrical patterns of limb cycling.
摘要：
量化不对称肢体运动的难度，与对称步态相比，导致缺乏有关这些运动模式的力学和适应性优势的信息。Further,没有研究使用系统发育比较技术探索不对称gaits的进化史。大多数基础工作表明，对称步态是祖先的特征，而不对称步态是哺乳动物的更衍生特征，一些鳄鱼,一些海龟,无尾动物和一些鱼类。在这项研究中,我们搜索了文献，寻找在现存的颌骨上使用不对称步态的证据，并从该样本（n=308种）中模拟了假设四种不同情况的不对称步态的演化。我们的分析表明，对进化模型的支持最强，在该模型中，底栖行走过程中，不对称步态是下颌造口的祖先，并且在随后的下颌造口进化过程中可能会丢失和获得。我们无法重建四足动物不对称步态的存在/不存在，羊水,乌龟和鳄鱼的节点肯定。采用不对称步态的能力可能是哺乳动物的祖先，但可能不是两栖动物和Lepidosauria的祖先。某些谱系中不存在不对称步态可能归因于神经肌肉和/或解剖学约束和/或与这些步态无关的通常缓慢的运动。这一发现增加了越来越多的工作，表明早期的颌骨和四足动物可能使用了多种步态，包括肢体自行车的不对称模式。
