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2021年3月10日出版的《自然》杂志在线发表了美国芝加哥大学Tetsuto Miyashita研究小组的最新成果，他们发现古生代七鳃鳗分支存在非七鳃鳗幼虫。

研究人员揭示了来自古生代七鳃鳗四种分支的幼虫和幼虫形式（Hardistiella、Mayomyzon、Pipiscius和Priscomyzon），其中包括了泥盆纪晚期冈瓦纳Priscomyzon属成虫到成年的生进化过程。四个属的幼虫都缺乏七鳃鳗的特征。取而代之的是，它们具有现存七鳃鳗所独有的功能，包括醒目的眼睛、尖锐的进食器具和尾部联合鳃状篮。值得注意的是，系统发育分析表明这些七鳃鳗幼虫至少出现在现存七鳃鳗的三个独立谱系中。这种分布强烈地暗示了七鳃鳗是现代七鳃鳗的特异进化物种，而非脊椎动物祖先的遗物。

这些系统发育见解还表明，盲鳗和七鳃鳗最后的共同祖先是巨噬性捕食者，没有滤食性幼体期。因此，在圆口纲脊椎动物和颌口类分支中普遍存在的“介皮类”甲壳可能比现存圆口纲脊椎动物更能代表所有现存脊椎动物的最后共同祖先。

据了解，七鳃鳗是现存七鳃鳗的滤食性幼虫，长期以来对脊椎动物祖先假说产生影响。现存七鳃鳗的进化史是从形似文昌鱼的七鳃鳗进化为特异掠食性成年动物，这似乎概括了脊椎动物起源的广泛情况。但是，尚无直接证据验证七鳃鳗的进化历程也不确定它们作为原始脊椎动物解剖模型的地位。

附：英文原文

Title: Non-ammocoete larvae of Palaeozoic stem lampreys

Author: Tetsuto Miyashita, Robert W. Gess, Kristen Tietjen, Michael I. Coates

Issue&Volume: 2021-03-10

Abstract: Ammocoetes—the filter-feeding larvae of modern lampreys—have long influenced hypotheses of vertebrate ancestry1,2,3,4,5,6,7. The life history of modern lampreys, which develop from a superficially amphioxus-like ammocoete to a specialized predatory adult, appears to recapitulate widely accepted scenarios of vertebrate origin. However, no direct evidence has validated the evolutionary antiquity of ammocoetes, and their status as models of primitive vertebrate anatomy is uncertain. Here we report larval and juvenile forms of four stem lampreys from the Palaeozoic era (Hardistiella, Mayomyzon, Pipiscius, and Priscomyzon), including a hatchling-to-adult growth series of the genus Priscomyzon from Late Devonian Gondwana. Larvae of all four genera lack the defining traits of ammocoetes. They instead display features that are otherwise unique to adult modern lampreys, including prominent eyes, a cusped feeding apparatus, and posteriorly united branchial baskets. Notably, phylogenetic analyses find that these non-ammocoete larvae occur in at least three independent lineages of stem lamprey. This distribution strongly implies that ammocoetes are specializations of modern-lamprey life history rather than relics of vertebrate ancestry. These phylogenetic insights also suggest that the last common ancestor of hagfishes and lampreys was a macrophagous predator that did not have a filter-feeding larval phase. Thus, the armoured ‘ostracoderms’ that populate the cyclostome and gnathostome stems might serve as better proxies than living cyclostomes for the last common ancestor of all living vertebrates.

DOI: 10.1038/s41586-021-03305-9

Source: https://www.nature.com/articles/s41586-021-03305-9

Nature：《自然》，创刊于1869年。隶属于施普林格·自然出版集团，最新IF：69.504
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