Data from: Devonian tetrapod-like fish reveals substantial parallelism in stem tetrapod evolution

The fossils assigned to the tetrapod stem group document the evolution of terrestrial vertebrates from lobe-finned fishes. During the past 18 years the phylogenetic structure of this stem group has remained remarkably stable, even when accommodating new discoveries such as the earliest known stem tetrapod Tungsenia and the elpistostegid (fish–tetrapod intermediate) Tiktaalik. Here we present a large lobe-finned fish from the Late Devonian period of China that disrupts this stability. It combines characteristics of rhizodont fishes (supposedly a basal branch in the stem group, distant from tetrapods) with derived elpistostegid-like and tetrapod-like characters. This mélange of characters may reflect either detailed convergence between rhizodonts and elpistostegids plus tetrapods, under a phylogenetic scenario deduced from Bayesian inference analysis, or a previously unrecognized close relationship between these groups, as supported by maximum parsimony analysis. In either case, the overall result reveals a substantial increase in homoplasy in the tetrapod stem group. It also suggests that ecological diversity and biogeographical provinciality in the tetrapod stem group have been underestimated.

归入四足动物干群（tetrapod stem group）的化石，记录了肉鳍鱼类（lobe-finned fishes）向陆生脊椎动物的演化历程。在过去18年中，该干群的系统发育结构始终保持显著稳定，即便纳入新发现类群（如已知最早的四足动物干群成员通恩鱼（Tungsenia）、鱼-四足类过渡类群希望螈类（elpistostegid）的代表提克塔利克鱼（Tiktaalik））亦是如此。本文报道一件产自中国晚泥盆世的大型肉鳍鱼类化石，它打破了这一稳定格局。该化石兼具根齿鱼类（rhizodont fishes，通常被视为干群中远离四足类的基干分支）的特征，以及衍化的希望螈类与四足类性状。这种性状混杂现象，在贝叶斯推断分析（Bayesian inference analysis）的系统发育框架下，可能反映根齿鱼类与希望螈类、四足类之间存在精细的趋同演化；而在最大简约法分析（maximum parsimony analysis）的支持下，则可能意味着这些类群间存在此前未被认知的紧密亲缘关系。无论哪种情形，整体研究结果均表明四足动物干群的同塑现象（homoplasy）显著增多。该发现还提示，四足动物干群的生态多样性与生物地理分区性此前被严重低估。