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.

归入四足动物茎干类群（tetrapod stem group）的化石记录了肉鳍鱼类向陆生脊椎动物的演化历程。在过去18年间，该茎干类群的系统发育结构始终保持显著稳定，即便纳入诸如已知最早的茎干四足动物腾氏鱼（Tungsenia）、鱼-四足动物过渡类群希望螈类（elpistostegid）以及提塔利克鱼（Tiktaalik）这类新发现时亦是如此。本文报道了一件产自中国晚泥盆世的大型肉鳍鱼类化石，它打破了这一稳定性。该化石兼具根齿鱼（rhizodont fishes）的特征——这类群曾被视为茎干类群中远离四足动物的基部分支——以及衍生的希望螈类样和四足动物样性状。这种性状组合现象，在基于贝叶斯推断分析推导的系统发育框架下，可能反映出根齿鱼类与希望螈类、四足动物间存在精细的趋同演化；或是支持最大简约法分析结果的、此前未被认知的这些类群间的紧密亲缘关系。无论哪种情形，本研究结果均揭示出四足动物茎干类群中的同塑现象（homoplasy）显著增多，同时也表明四足动物茎干类群的生态多样性与生物地理分区性此前被严重低估。