Michael I. Coates評述文章譯文

一條罕見的完整保存的古魚進一步揭示，對于現(xiàn)生脊椎動物起源的探索需要沖出泥盆紀（魚類時代），踏入更老的年代。

通常說來，現(xiàn)生生物類群的最早化石記錄往往是零碎的，這些破碎材料常常導致它們在分類上極具爭議。就“硬骨魚類”而言，硬骨魚類是脊椎動物的一個分支，包括從比目魚到人類的所有脊椎動物。完整的硬骨魚類化石記錄最早追溯到距今約４億年的下泥盆統(tǒng)。早期硬骨魚類的演化歷史因為化石材料的缺乏（僅有零散的牙齒與鱗片的報道）顯得混亂。在某些情況下，零碎的材料被組合在一起研究，它們中的一些具有出人意料的特征組合。在本期雜志的469頁，朱敏等為這段了解得并不多的脊椎動物演化歷史提供了一條活靈活現(xiàn)的古魚。至關緊要的是，這條魚代表著人類遙遠祖先的一個分支，它不但罕見地被完整保存下來，而且出人意料的古老。
那么它是一條什么樣的魚呢？對脊椎動物的多樣性做一下概括將有助于回答這個問題。在現(xiàn)生的5萬1千多種脊椎動物中，99.9%是有頜的：它們統(tǒng)稱有頜類。有頜類包括硬骨魚類以及軟骨魚類。軟骨魚類（鯊魚、鰩、銀鮫）僅占有頜類物種數(shù)的2%，其余98%的有頜類都屬于硬骨魚類。在硬骨魚類中，大約一半是輻鰭魚類，剩余的統(tǒng)稱肉鰭魚類。輻鰭魚類包括從斑馬魚到多鰭魚的大約2萬8千個種?，F(xiàn)生的肉鰭魚類中魚類僅有肺魚的3個屬以及空棘魚的1個屬。陸生的四足動物占了肉鰭魚類的絕大部分。

迄今為止，這些現(xiàn)生有頜類主要類群的起源可以追溯到泥盆紀，距今約3.59-4.16億年。這一時期又被稱為魚類時代。確切的軟骨魚類化石出現(xiàn)在大約4-4.05億年前的地層中，但是，我們并不知道它屬于軟骨魚類的現(xiàn)生類群（冠群），還是代表軟骨魚類祖先類群的一個旁支（干群）。對于硬骨魚類來說，盡管其最確切的化石證據(jù)來自早泥盆世，屬于硬骨魚類冠群，但是在晚志留世（距今約4.16-4.23億年）地層已經發(fā)現(xiàn)了不太肯定的硬骨魚類化石碎片。

這個故事并沒有結束，有頜類的起源問題還涉及另外兩個在地質歷史上出現(xiàn)得更早并業(yè)已滅絕的類群：盾皮魚類和棘魚類。重要的是，最近的系統(tǒng)發(fā)育分析開始揭示早期脊椎動物新的演化關系：棘魚類與盾皮魚類作為有頜類的單系類群受到質疑；特別是棘魚類，它們在軟骨魚類與硬骨魚類的干群中出現(xiàn)。顯而易見，現(xiàn)生有頜類類群的起源并非在泥盆紀時發(fā)生，原始硬骨魚類與軟骨魚類的分化應該出現(xiàn)得更早。

在此背景下，朱敏等命名并描述了早期硬骨魚類的一新屬種——夢幻鬼魚。夢幻鬼魚發(fā)現(xiàn)于中國南方距今約4.18億年前的石灰?guī)r中，長約1尺，展示了一個小型肉鰭魚的解剖學特征。鬼魚作為肉鰭魚這一事實充分證明了有頜類冠群一系列的分支事件在志留紀末之前便已發(fā)生。

和其它化石一樣，鬼魚兼具原始和進步的性狀。它提供了完整的解剖學信息，證實了之前復原的另一早期硬骨魚——斑鱗魚擁有出人意料的特征組合是正確的。由于斑鱗魚僅有一些零散的材料，研究者對這種奇異的特征組合心存疑慮，但是鬼魚的出現(xiàn)證實這些特征組合是完全可信的。和斑鱗魚及許多其它的肉鰭魚類（包括拉蒂邁魚，現(xiàn)生空棘魚）一樣，鬼魚的腦顱也分為前后兩個部分。與斑鱗魚一樣，鬼魚的頰部骨骼與早期的輻鰭魚類相似。鬼魚的肩帶在胸鰭之前具有一個棘刺，這與斑鱗魚和許多其它的早期有頜類一致（包括至少一種軟骨魚）。此外，鬼魚的背鰭棘刺和其前方具有棘刺的骨板很可能也是原始特征。上述特征在早期有頜類中廣泛分布，而它們卻同時出現(xiàn)鬼魚身上，使得我們首次觀察到了肉鰭魚類一系列重要鑒別特征的演化順序。

朱敏等構建的進化樹推進了對早期硬骨魚類和有頜類的系統(tǒng)發(fā)育關系的研究。目前對于非硬骨魚類的分支格局仍然存在很多不確定性，鬼魚的加入沒有改變早期硬骨魚類的基本系統(tǒng)發(fā)育關系。不過，它支持了近年來有關肉鰭魚類演化格局的假說。

最后，鬼魚作為毫無爭議的肉鰭魚這一結論有什么意義呢？總體上看，早期化石由于不完整或者缺少充足的分類依據(jù)，其作為進化分歧事件的最近時間校正點被認為是不可靠的。鬼魚則是一個例外，因為它為脊椎動物進化的一個重大分歧事件（輻鰭魚類與肉鰭魚類的分化）提供了一個新的確鑿無疑的最早化石參考點。鬼魚的發(fā)現(xiàn)將有頜類進化的一系列重大分歧事件從泥盆紀推回到志留紀，同時指示早期脊椎動物進化的這段歷史仍疑云密布。

新構建的有頜類演化樹表明早期肉鰭魚類、輻鰭魚類和軟骨魚類應該在志留紀就已經出現(xiàn)。但是它們都在哪里呢？現(xiàn)生有頜魚類的根在志留紀，然而，這些魚類在目前所繪出的志留紀生態(tài)復原圖中是缺失的。鬼魚的發(fā)現(xiàn)，將會掀起新一輪的野外考察熱潮，并讓我們以新的視角重新觀察已有的泥盆紀之前的化石材料。

Beyond the Age of Fishes

Michael I. Coates

Discovery of an unusually intact and ancient fossil fish provides further evidence that the search for modern vertebrate origins requires breaking out of the Devonian and into the preceding period.

As a rule, the earliest fossils of living groups tend to be scrappy, and such fragments lend themselves to contentious interpretations. For ‘bony fishes’, Osteichthyes — the division of vertebrates that includes everything from humans to halibut — the record of articulated fossils peters out within the Lower Devonian1, some 400 million years ago. Earlier stretches of osteichthyan history are littered with fossil detritus, such as isolated teeth and scales. In certain instances, bits and pieces have been reassembled into conjectural species2–4, some of which have surprising combinations of anatomical features2. On page 469 of this issue, Zhu et al.5 introduce a fresh — albeit long-dead — fish into this poorly resolved patch of vertebrate evolution. Crucially, this piscine offshoot of our own distant past is both unusually intact and exceptionally old.

So what kind of fish is it? A summary of vertebrate diversity helps to make sense of the answer. Of the 51,000 or more living species of vertebrates, 99.9% have jaws: these are the gnathostomes. Gnathostomes include the bony Osteichthyes and the cartilaginous Chondrichthyes. Chondrichthyes (sharks, rays and chimaeras) account for only 2% of gnathostome species, the Osteichthyes accounting for the other 98%. Around half of the Osteichthyes are Actinopterygii, or ‘ray-finned fishes’, and half are Sarcopterygii, or ‘lobe-finned fishes’. Actinopterygians include some 28,000 species, from zebrafish to bichirs, and living sarcopterygian fishes are limited to three genera of lungfishes and one coelacanth. Land-dwelling tetrapods constitute the remaining majority of sarcopterygians.

Thus far, the origins of these major divisions of today’s gnathostomes can be traced back to the Devonian, between 416 million and 359 million years ago, the Age of Fishes. Fossils that are clearly chondrichthyan are known from around 400 million to 405 million years ago6, but we have little idea as to whether these belong within the living radiation, the ‘crown group’, or represent side branches of their common ancestry, the ‘stem group’. As for osteichthyans, although it is agreed that fossils from the earliest Devonian2,7 belong within the crown, osteichthyan fragments of less-certain affinity are also known from the Late Silurian3, 423 million to 416 million years ago.

But there’s more to this story, because the question of gnathostome origins also involves a pair of extinct groups of gnathostomes known to appear earlier in the geological record, the placoderms and acanthodians1. Importantly, recent analyses8 have begun to reveal new relationships between early vertebrates, in which acanthodians and placoderms are scattered among the early divisions of gnathostome evolution; acanthodians, in particular, are cropping up on chondrichthyan and osteichthyan stem groups. The straightforward message is that the origin of modern gnathostomes is not a Devonian phenomenon, after all. The basal divergence between osteichthyans and chondrichthyans occurred somewhat earlier.

This, then, is the context within which to place Guiyu oneiros, the new species of early osteichthyan named and described by Zhu et al.5. Preserved in 418-million-year-old limestone in what is now southern China, the fossils of Guiyu show the skeletal anatomy of a small sarcopterygian, around 33 centimetres long. The very fact that Guiyu can be identified as a sarcopterygian provides further and arguably clinching evidence that a whole series of major branching events within the gnathostome crown group must have taken place well before the end of the Silurian.

Like any other fossil, Guiyu is a mixture of primitive and advanced features. With regard to its anatomical completeness, Guiyu provides exceptional corroboration for the decidedly odd reconstruction of the early osteichthyan genus, Psarolepis2. Cobbled together from a disparate set of fossils, the incongruent suite of features9 displayed by Psarolepis has been viewed with caution. Now, it turns out to be thoroughly plausible. Like Psarolepis and other sarcopterygian fishes (including Latimeria, the living coelacanth), the braincase of Guiyu is divided into separate front and rear units. Like Psarolepis, the cheek bones resemble those of early actinopterygians. Like Psarolepis and many other early gnathostomes1, including at least one chondrichthyan6, the shoulder girdle bears a spine in front of the pectoral fin. Similarly, the dorsal-fin spine and anterior spinebearing plate of Guiyu are probably primitive. These are all widespread features of early gnathostomes, and seeing such characteristics in Guiyu provides a first glimpse of the sequential order of anatomical changes that resulted in the standard set of sarcopterygian traits.

The evolutionary tree proposed by Zhu et al.5 (see Fig. 5 on page 473) adds to a growing set of analyses of early osteichthyan and gnathostome interrelationships8,10. Uncertainties still surround the branching pattern of non-osteichthyans, but the addition of Guiyu to the cast of early fishes does not change the basic pattern of interrelationships among early osteichthyans. Instead, it adds support to notable consistencies in the emerging pattern of sarcopterygian evolution, including the clustering of some of the earliest-known examples to form an as-yet unnamed group.

Finally, what does the conclusion that Guiyu is unequivocally sarcopterygian imply? On the whole, early fossils are thought to be unreliable as minimum-date markers of evolutionary branching events11, because they are less complete and/or lack the full anatomical signature of the group to which they are assigned. Guiyu might be an exception that proves the rule, for it provides a new and exceptionally reliable earliest fossil marker for a major split in vertebrate evolution. By pushing a whole series of branching points in gnathostome evolution out of the Devonian and into the Silurian, the discovery of Guiyu also signals that a significant part of early vertebrate evolution is unknown (Fig. 1).

The new shape of the gnathostome tree shows that early sarcopterygians, as well as Actinopterygians and chondrichthyans, ought to be turning up in Silurian sediments. But where are they? Modern fish groups have Silurian roots, but these fishes are consistently absent from existing scenarios of Silurian life. The discovery of Guiyu should provoke a rash of new fieldwork and a fresh look at existing collections of pre-Devonian fossils.

Michael I. Coates is in the Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637, USA.

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