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.要理解生物基本体型（Bauplan）的演化历程，首先需要从观测到的生物多样性中区分出系统发育信号（phylogenetic signal）与适应（adaptation），并进一步将适应与外适应（exaptation）相区分。若要明确阐释形态演化所需的解释框架，核心推论在于判定某一性状是先于、伴随还是追随着特定功能的演化进程出现。为完成这一研究目标，我们将聚焦对象锁定为一类特殊的脊椎动物——无尾类（anurans）。蛙与蟾蜍在脊椎动物中拥有独特的基本体型，其一系列躯干后骨骼特征自演化起源以来便被认为是适应跳跃运动的结果。该观点虽在学界被反复援引，却罕有实证检验。我们通过重建已知最古老的蛙形类（salientian）成员马氏三叠蟾（Triadobatrachus massinoti）的运动能力，对这一假设进行验证。这一已灭绝类群拥有一套镶嵌演化特征组合，传统观点认为其代表了无尾类基本体型演化历程中的过渡阶段，其中部分特征还被认为与跳跃能力相关。本研究采用多变量分析与比较系统发育方法，在清晰的演化框架下对马氏三叠蟾展开分析。我们选取188种现生四肢两栖类（蛙类与蝾螈）及蜥蜴的主要肢骨长度数据，作为观测到的运动行为的形态学替代指标。本研究结果显示，肢骨数据与运动行为显著相关，且不受系统发育亲缘关系的影响；同时表明马氏三叠蟾的主要运动模式为类似蝾螈的侧向摆动运动。这一结果与近期提出的相关假说相悖，并暗示马氏三叠蟾与无尾类共有的衍生躯干后骨骼特征，后续可能在跳跃蛙类中被招募为外适应结构。