科技 古生物学 八字足，而非平足

　　A new fossil shows that evolution does not always mean change

　　一个新的化石表明：进化并不总是意味着改变

　　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.

　　以前人们一度认为一种叫做腔棘鱼的鳍鱼是鱼类和两栖类之间缺少的环节，1938年当腔棘鱼在南非海岸沿海被发现时，古生物学家对它的出现大感震惊。在此之前，这种动物最近的遗迹出现在6500万年前恐龙时代后期的岩层中。而它这次的出现方式，就好像一个活生生的霸王龙属被发现藏身在蒙大拿州的模糊地带一样令人惊讶。现在，同样的经历再次让古生物学家碰上了，不过这次相反。他们不是找到了一个与古兽相同的"活化石"，而是找到了一个跟现代兽类相同的真正化石。

　　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.

　　所讨论的的化石采自巴西东北部，是一亿年前的标本，属于节肢动物类。这类节肢动物是大型类似板球的食肉昆虫，其足向不同方向伸展开来。现代节肢动物在沙地爬行寻找猎物时用它们雪鞋般的脚来帮助身体保持稳定。

　　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（其发现刚由伊利诺斯州自然史调查的负责人山姆和瑞士弗里堡大学的李?劳伊辛格发表在《动物图谱》上）仅是类似现代的八字足昆虫，那么这个发现不会特别令人惊讶：它只是证明了一种被称为进化停滞的现象，在这种现象中特定类型的体型存留很长一段时间了。令人惊讶的是节肢动物停止进化竟有这么久了。

　　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.

　　在林耐生物分类（类，属和门）系统的高等类别中进化停滞现象是相当普遍的。自然选择的图案都是最好的。然后，这种图案就被一个接一个的物种以略有不同的形式所采纳。例如，海龟的壳体是在2.5～2.0亿年前进化的，而蝎子的体型方案已超过4亿年的历史。然而这并不意味着一个动物学家会弄错2亿岁的海龟或是4亿岁的蝎子属于现在存活的何种物种。

　　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.

　　新发现引人注目的是它与现代动物如此相似，以致它可以被列入现有生物分类种类（林奈物种以上分类的最低等），而不是只被列入某一更高等的分类组。这的确罕见。经过仔细观察，甚至就连现代腔棘鱼都必须得归类于不同于任何已知化石的种类。

　　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."

　　显然，节肢动物的体型方案不仅是最好的，也是最佳的，至少对这种动物生活的环境而言是如此。唉，对节肢动物来说，它所喜爱的沙质沙漠已经从巴西东北部退却，它在那里的最佳体态已经消失。但它的发现恰好说明这部分的大千世界跟1亿年前的样子很象，也说明了进化论的一个重点，而这点在生物学家关注理解新颖性的发展时往往被遗忘了。自然选择的第一条规则是："如果没坏，就不要修理它。"