Data from: Walk before you jump: new insights on early frog locomotion from the oldest known salientian

Understanding the evolution of a Bauplan starts with discriminating phylogenetic signal from adaptation and the latter from exaptation in the observed biodiversity. Whether traits have predated, accompanied, or followed evolution of particular functions is the basic inference to establish the type of explanations required to determine morphological evolution. To accomplish this, we focus in a particular group of vertebrates, the anurans. Frogs and toads have a unique Bauplan among vertebrates, with a set of postcranial features that have been considered adaptations to jumping locomotion since their evolutionary origin. This interpretation is frequently stated but rarely tested in scientific literature. We test this assumption reconstructing the locomotor capabilities of the earliest known salientian, Triadobatrachus massinoti. This extinct taxon exhibits a mosaic of features that have traditionally been considered as representing an intermediate stage in the evolution of the anuran Bauplan, some of which were also linked to jumping skills. We considered T. massinoti in an explicit evolutionary framework by means of multivariate analyses and comparative phylogenetic methods. We used length measurements of major limb bones of 188 extant limbed amphibians (frogs and salamanders) and lizards as a morphological proxy of observed locomotor behavior. Our findings show that limb data correlate with locomotion, regardless of phylogenetic relatedness, and indicate that salamander-like lateral undulatory movements were the main mode of locomotion of T. massinoti. These results contrast with recent hypotheses and indicate that derived postcranial features that T. massinoti shared with anurans might have been later co-opted as exaptations in jumping frogs.

要理解躯体规划（Bauplan）的演化历程，首先需从观测到的生物多样性中区分出系统发育信号与适应性，再将适应性与外适应（exaptation）加以甄别。判断某类性状是先于、伴随还是紧随特定功能的演化，是确立解释形态演化所需理论框架的基础推论。为此，我们将研究聚焦于一类特殊的脊椎动物——无尾目（Anura）动物。蛙与蟾蜍在脊椎动物中拥有独特的躯体规划，其一系列躯干后特征自演化起源以来便被认为是适应跳跃运动的适应性特征。这一阐释虽常被提及，但在学术文献中鲜有实证检验。为验证这一假设，我们通过重建已知最古老的蛙型类（Salientia）物种马氏三叠蛙（Triadobatrachus massinoti）的运动能力来开展研究。这一已灭绝类群呈现出一系列镶嵌演化的特征，传统上被认为代表了无尾目躯体规划演化过程中的中间阶段，其中部分特征也曾被关联至跳跃能力。我们借助多元分析与比较系统发育方法，在明确的演化框架下对马氏三叠蛙展开研究。我们选取了188种现生四肢两栖动物（蛙类与蝾螈）以及蜥蜴的主要肢骨长度数据，将其作为反映观测到的运动行为的形态学代理指标。研究结果显示，无论系统发育亲缘关系如何，肢骨数据均与运动模式存在关联，且表明马氏三叠蛙的主要运动方式为类似蝾螈的侧摆运动。这一结果与近期提出的诸多假说相悖，并表明马氏三叠蛙与无尾目共有的衍生躯干后特征，后续可能在跳跃蛙类中被征募为外适应特征。