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Science and Technology Palaeontology Splay-footed, not flat-footed
科技 古生物學(xué) 八字足，而非平足
A new fossil shows that evolution does not always mean change
一個新的化石表明：進化并不總是意味著改變
WHEN a coelacanth, a type of lobe-finned fish once considered the missing link between fish and amphibians, was found off the coast of South Africa in 1938, it came as a shock to palaeontologists. Until then, the most recent traces of such a creature had been in rocks dating from the last days of the dinosaurs, 65m years ago. It was, in its way, as surprising as if a live Tyrannosaurus had been found hiding in an obscure part of Montana. Now the same experience is hitting palaeontologists again—but this time in reverse. Instead of finding a "living fossil" identical to an ancient beast, they have found a real fossil identical to a modern one.
以前人們一度認(rèn)為一種叫做腔棘魚的鰭魚是魚類和兩棲類之間缺少的環(huán)節(jié)，1938年當(dāng)腔棘魚在南非海岸沿海被發(fā)現(xiàn)時，古生物學(xué)家對它的出現(xiàn)大感震驚。在此之前，這種動物最近的遺跡出現(xiàn)在6500萬年前恐龍時代后期的巖層中。而它這次的出現(xiàn)方式，就好像一個活生生的霸王龍屬被發(fā)現(xiàn)藏身在蒙大拿州的模糊地帶一樣令人驚訝。現(xiàn)在，同樣的經(jīng)歷再次讓古生物學(xué)家碰上了，不過這次相反。他們不是找到了一個與古獸相同的"活化石"，而是找到了一個跟現(xiàn)代獸類相同的真正化石。
The fossil in question, a 100m-year-old specimen from north-east Brazil, belongs to the genus Schizodactylus. These are large, carnivorous, cricket-like insects whose feet splay out wildly in different directions. Modern Schizodactylus use their feet like snowshoes, to help them remain stable as they travel over sandy terrain in search of prey.
所討論的的化石采自巴西東北部，是一億年前的標(biāo)本，屬于節(jié)肢動物類。這類節(jié)肢動物是大型類似板球的食肉昆蟲，其足向不同方向伸展開來。現(xiàn)代節(jié)肢動物在沙地爬行尋找獵物時用它們雪鞋般的腳來幫助身體保持穩(wěn)定。
If the new fossil—whose discovery has just been published in ZooKeys by Sam Heads of the Illinois Natural History Survey and Léa Leuzinger of the University of Fribourg, in Switzerland—were merely similar to modern splay-footed insects, the find would not be particularly surprising: it simply demonstrates a phenomenon called evolutionary stasis, in which a specific type of body form hangs around for a long time. What is surprising is just how static Schizodactylus has been.
如果新化石（其發(fā)現(xiàn)剛由伊利諾斯州自然史調(diào)查的負(fù)責(zé)人山姆和瑞士弗里堡大學(xué)的李?勞伊辛格發(fā)表在《動物圖譜》上）僅是類似現(xiàn)代的八字足昆蟲，那么這個發(fā)現(xiàn)不會特別令人驚訝：它只是證明了一種被稱為進化停滯的現(xiàn)象，在這種現(xiàn)象中特定類型的體型存留很長一段時間了。令人驚訝的是節(jié)肢動物停止進化竟有這么久了。
Evolutionary stasis is fairly common at the higher levels of the Linnaean system of biological classification (class, order and family). Natural selection hits on a good design. That design is then adopted in slightly different forms by species after species. The shelled bodies of turtles, for example, evolved between 250m and 200m years ago, while the body plans of scorpions have been around for more than 400m years. That does not mean, however, that a zoologist would mistake a 200m-year-old turtle or a 400m-year-old scorpion for any species now alive.
在林耐生物分類（類，屬和門）系統(tǒng)的高等類別中進化停滯現(xiàn)象是相當(dāng)普遍的。自然選擇的圖案都是最好的。然后，這種圖案就被一個接一個的物種以略有不同的形式所采納。例如，海龜?shù)臍んw是在2.5～2.0億年前進化的，而蝎子的體型方案已超過4億年的歷史。然而這并不意味著一個動物學(xué)家會弄錯2億歲的海龜或是4億歲的蝎子屬于現(xiàn)在存活的何種物種。
What is remarkable about the new find is that it is so similar to modern animals that it can be assigned to an existing genus—the lowest level of Linnaean classification above a species—rather than just to some higher taxonomic group. That is rare indeed. Even the modern coelacanth, on closer examination, had to be put in a different genus from any known fossil.
新發(fā)現(xiàn)引人注目的是它與現(xiàn)代動物如此相似，以致它可以被列入現(xiàn)有生物分類種類（林奈物種以上分類的最低等），而不是只被列入某一更高等的分類組。這的確罕見。經(jīng)過仔細(xì)觀察，甚至就連現(xiàn)代腔棘魚都必須得歸類于不同于任何已知化石的種類。
Clearly the body plan of Schizodactylus is not merely good, but optimal, at least for the environment the animal lives in. Alas for Schizodactylus, the sandy deserts it prefers have retreated from north-eastern Brazil and its optimality there has vanished. But its discovery shows better what this part of the world was like 100m years ago—and also illustrates an important point about evolution that is often forgotten in biologists' understandable focus on the development of novelty. The first rule of natural selection is: "If it ain't broke, don't fix it."
顯然，節(jié)肢動物的體型方案不僅是最好的，也是最佳的，至少對這種動物生活的環(huán)境而言是如此。唉，對節(jié)肢動物來說，它所喜愛的沙質(zhì)沙漠已經(jīng)從巴西東北部退卻，它在那里的最佳體態(tài)已經(jīng)消失。但它的發(fā)現(xiàn)恰好說明這部分的大千世界跟1億年前的樣子很象，也說明了進化論的一個重點，而這點在生物學(xué)家關(guān)注理解新穎性的發(fā)展時往往被遺忘了。自然選擇的第一條規(guī)則是："如果沒壞，就不要修理它。"
