我们对Chordata进化起源的理解,最不同和最具生态意义的动物门之一,受到缺乏明确的茎群亲属的阻碍。有问题的寒武纪化石被认为是候选的脊索动物包括vetulicolians,1Yunnanozoon,2和标志性的Pikaia。3然而,他们的系统发育位置仍然受到限制,阻碍了沿着脊索茎谱系的性格进化的重建。在这里,我们重新解释皮卡亚的形态,为消化道提供证据,至关重要的是,背侧神经索-强健的脊索突触。对这些结构的识别为Pikaia的新解剖模型奠定了基础,该模型表明该化石以前被颠倒解释。我们揭示了文昌鱼和脊椎动物之间的肌团构型,并建立了Yunnanozoon之间的形态联系,皮卡亚,和无争议的部落。在这种情况下,我们进行了新的系统发育分析,使用修订后的,全面的氘刀数据集,并建立了脊索茎谱系。我们将vetulicolians分解为一个共生组,包括最早的分叉茎脊索,对包括Yunnanozoon和Pikaia的更多衍生茎群脊索的等级。我们的系统发育结果表明,逐步获得了cho索状冠组的特征诊断。此外,他们绘制了早期脊索进化的一个阶段,该阶段由咽部区域与分段轴向肌肉组织的逐渐整合所定义。支持脊索起源的经典进化-发育假设4,并揭示了门历史上的“失落篇章”。
Our understanding of the evolutionary origin of
Chordata, one of the most disparate and ecologically significant animal phyla, is hindered by a lack of unambiguous stem-group relatives. Problematic Cambrian fossils that have been considered as candidate chordates include vetulicolians,1Yunnanozoon,2 and the iconic Pikaia.3 However, their phylogenetic placement has remained poorly constrained, impeding reconstructions of character evolution along the chordate stem lineage. Here we reinterpret the morphology of Pikaia, providing evidence for a gut canal and, crucially, a dorsal nerve cord-a robust chordate synapomorphy. The identification of these structures underpins a new anatomical model of Pikaia that shows that this fossil was previously interpreted upside down. We reveal a myomere configuration intermediate between amphioxus and vertebrates and establish morphological links between Yunnanozoon, Pikaia, and uncontroversial chordates. In this light, we perform a new phylogenetic analysis, using a revised, comprehensive deuterostome dataset, and establish a chordate stem lineage. We resolve vetulicolians as a paraphyletic group comprising the earliest diverging stem chordates, subtending a grade of more derived stem-group chordates comprising Yunnanozoon and Pikaia. Our phylogenetic results reveal the stepwise acquisition of characters diagnostic of the chordate crown group. In addition, they chart a phase in early chordate evolution defined by the gradual integration of the pharyngeal region with a segmented axial musculature, supporting classical evolutionary-developmental hypotheses of chordate origins4 and revealing a \"lost chapter\" in the history of the phylum.