School of Earth Sciences, University of Bristol, Bristol BS8 1QU, UK.
Département de sciences biologiques, Université de Montréal C.P. 6128, Succursale Centre-ville, Montréal, QC, Canada H3C 3J7.
Proc Biol Sci. 2022 May 11;289(1974):20220258. doi: 10.1098/rspb.2022.0258.
Deuterostomes comprise three phyla with radically different body plans. Phylogenetic bracketing of the living deuterostome clades suggests the latest common ancestor of echinoderms, hemichordates and chordates was a bilaterally symmetrical worm with pharyngeal openings, with these characters lost in echinoderms. Early fossil echinoderms with pharyngeal openings have been described, but their interpretation is highly controversial. Here, we critically evaluate the evidence for pharyngeal structures (gill bars) in the extinct stylophoran echinoderms and using virtual models based on high-resolution X-ray tomography scans of three-dimensionally preserved fossil specimens. Multivariate analyses of the size, spacing and arrangement of the internal bars in these fossils indicate they are substantially more similar to gill bars in modern enteropneust hemichordates and cephalochordates than to other internal bar-like structures in fossil blastozoan echinoderms. The close similarity between the internal bars of the stylophorans and and the gill bars of extant chordates and hemichordates is strong evidence for their homology. Differences between these internal bars and bar-like elements of the respiratory systems in blastozoans suggest these structures might have arisen through parallel evolution across deuterostomes, perhaps underpinned by a common developmental genetic mechanism.
后口动物包括三个具有截然不同身体结构的门。现存后口动物类群的系统发育框定表明，棘皮动物、半索动物和脊索动物的最近共同祖先为具两侧对称、咽部有口的蠕虫，这些特征在棘皮动物中消失了。具有咽部开口的早期化石棘皮动物已有描述，但它们的解释极具争议性。在这里，我们使用基于三维保存化石标本的高分辨率 X 射线断层扫描的虚拟模型，对已灭绝的海百合类棘皮动物和腕足动物的咽部结构（鳃条）进行了批判性评估。对这些化石中内部条的大小、间距和排列的多元分析表明，它们与现代肠腔半索动物和头索动物的鳃条相比，与化石星虫动物棘皮动物的其他内部条状结构更相似。海百合类棘皮动物 和 的内部条与现存脊索动物和半索动物的鳃条之间的密切相似性为它们的同源性提供了强有力的证据。这些内部条与星虫动物呼吸系统的条状元素之间的差异表明，这些结构可能是通过后口动物的平行进化而产生的，也许是由共同的发育遗传机制支撑的。
