经济学人下载:牙齿的痕迹

Science and technology
科学技术

Palaeontology
古生物学

Remember the tooth
牙齿的痕迹

A link is made in an ancient ecosystem
古代生态系统中的线索

WHO ate whom in the food chains of the past is rarely clear.
在古代谁以吃谁为生是不清楚的。

Though it is obvious which species were predators and which prey,
尽管谁是捕食者和谁是猎物是很明显的,

the subtle specialisations of feeding habit that allow many types of carnivore to co-exist are rarely preserved in the fossil record.
但是专门的微妙的捕食习惯,能让多种食肉动物能够共存的资料很少有保存完好的化石记录。

Rarely, but not never.
很少有记录不表示没有。

That is why the recent discovery of a Jurassic ammonite with a shark's tooth embedded in its shell has excited palaeontologists.
这就是为什么最近发现侏罗纪菊石，连同壳上面的鲨鱼牙齿让古生物学家非常兴奋。

The ammonite in question, Orthaspidoceras, a species that thrived 155m years ago,
发现的鹦鹉螺是一种叫Orthaspidoceras的生活在一亿五千五百万年前的生物，

was part of the collection of an amateur who did not realise the significance of what he had found.
化石是由业余爱好者收藏的，他没有意识到他发现的化石的意义。

Romain Vullo of the University of Rennes, however,
法国雷恩大学的Romain Vullo却明白，

did-and he has brought the discovery to the attention of the scientific world in the pages of Naturwissenschaften.
而他在《自然科学》杂志上的几页论文吸引了科学界的注意。

Ammonites were both predators and prey.
鹦鹉螺既是捕食者也是猎物。

They occupied a position in the Mesozoic oceans similar to that of modern squid.
他们生活在中生代的海洋中，跟现代乌贼相似。

Like squid, they were swimming tentacled molluscs-a group called the cephalopods.
跟乌贼一样，鹦鹉螺是游动的有触须的软体动物，属于头足纲动物。

Unlike squid, though, they had protective shells.
不像乌贼,鹦鹉螺有保护壳。

These shells were divided internally into gas-filled chambers.
这些保护壳的里面有充满气体的分隔小室。

The result had neutral buoyancy, allowing the animal to move by jet propulsion.
结果就是鹦鹉螺有浮力,让其像喷气发动机推进一样的运动，这点跟乌贼一样。

Modern pearly nautiluses, whose relationship to ammonites is much debated, have a similar arrangement.
现代珍珠一样的鹦鹉螺跟远古鹦鹉螺的亲缘关系尽管还有待验证，但是现代鹦鹉螺也有类似的充气推进机制。

What ammonites-or, at least, some of them-ate became clear earlier this year when an X-ray showed a small crustacean in the jaws of a species called Baculites.
鹦鹉螺吃什么的问题今年有了答案，x光显示杆鹦鹉螺的咽喉中有小甲壳类动物的痕迹。

But what ate ammonites has never been shown in such an unambiguous manner.
但是什么吃鹦鹉螺从来没有像现在一样模棱两可。

Some ammonite fossils have tooth marks that look as though they were made by huge reptilian predators called mosasaurs.
一些鹦鹉螺化石上有牙印，看上去仿佛是巨大爬虫类捕食者比如沧龙留下的。

Some appear to have been attacked by the beaks of other cephalopods.
有些牙印似乎是被其他头足纲攻击的痕迹。

And some seem to have been bitten by sharks.
有些牙印似乎是鲨鱼留下的。

On top of that, coprolites, as palaeontologists politely describe fossil faeces, have turned up with ammonite shells in them.
最重要的是,对古生物学家礼貌地描述中的粪化石的研究表明，有些粪便中有鹦鹉螺的壳。

Based on comparisons with modern evacua, these are probably from sharks.
随着现代技术的发展，研究表明这些粪便来自于鲨鱼。

But which sharks?
但是哪种鲨鱼吃鹦鹉螺呢?

Dr Vullo's ammonite nails one culprit.
Vullo博士的化石标本确定了其中的一个罪魁祸首。

The tooth belongs to a species called Planohybodus.
化石中鹦鹉螺外壳上的牙齿属于一种叫做平弓鲨的物种。

And that is a surprise. Those modern sharks that eat shelled animals have robust teeth for crunching through hard exteriors.
这是一个惊喜的发现。觅食有壳动物的现代鲨鱼通常长着强壮的牙齿，可以咬穿坚硬的外壳。

The teeth of Planohybodus, in contrast, were slender and pointed-the sort usually associated with grasping and tearing at flesh.
相反，平弓鲨的牙齿,是那种纤细而突出的，一般让人联想到抓住和撕裂肉。

In a modern predator, that would indicate the habit of eating fish.
在现代掠食者中,拥有这种牙齿意味着习惯吃鱼。

That a shark with teeth like this would try to make a meal of an ammonite is, at first sight, odd.
长着这样牙齿的鲨鱼,会试图吃掉鹦鹉螺，美餐一顿的理论,乍一看有点古怪。

But second thoughts provide a possible explanation.
但想一想就有了合理的解释。

Ammonites' manoeuvrability would have depended crucially on their buoyancy control.
鹦鹉螺的灵活性主要建立在浮力控制上面的。

Even a small puncture to the shell, which a pointed tooth would be well able to deliver,
即使是壳上的一个小洞,可能就是这样尖牙造成的,就会让水进入壳内,

would let the water in and cause that control to vanish.
鹦鹉螺从而无法控制浮力。

Since ammonites could not withdraw entirely into their shells for self-defence,
鹦鹉螺不能完全缩回外壳内自卫,

it would then just be a question of dragging the creature out of its chamber in order to eat it.
于是就很可能就会被从外壳中拽出，成了其他生物的美食。

And for that, sharp, pointed teeth are ideal.
这样说来，锋利的尖牙是完美的武器。