远古巨鱼

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Scientists have long struggled to explain why bony fishes are so small: The heaviest—the ocean sunfish—is just 2.3 metric tons, but cartilaginous fishes like whale sharks can weigh up to 34 metric tons. 

科学家们一直在试图解释为什么硬骨鱼会这么小：最重的—海洋翻车鱼—也只有2.3公吨重，而软骨鱼，比如鲸鲨则可以长到34公吨重。

Now, a new study of an ancient giant suggests this modern difference is merely an evolutionary accident.

如今，一项对古代巨物的研究表明这种现代的不同仅仅是进化史上的意外事件。

Bony fish, which make up some 95% of all fish species, might be constrained by their metabolism according to one argument. 

硬骨鱼，占所有鱼类品种中的95%，有理论说（其大小）可能受到新陈代谢的限制。

Larger animals generally have to make do with less oxygen per gram of tissue; because bony fish seem to have higher metabolic requirements than sharks, it might simply be impossible for them to grow much larger than the ocean sunfish.

更大的动物平均下来一般每克组织所分配到的氧气更少；因为硬骨鱼比鲨鱼这样的软骨鱼的新陈代谢要求似乎更高，所以它们想长得比海洋翻车鱼还大恐怕不大可能。

Enter Leedsichthys problematicus. 

再来看看利兹鱼。

The extinct fish—thought to be the largest on record—lived about 165 million years ago in Europe and South America. 

这种灭绝了的鱼—据说是有记录的最大的鱼—生活在一亿六千五百万年前的欧洲和南美洲地区。

It grew to at least 16.5 meters in length and might have weighed 45 metric tons, which means it was larger even than today’s whale shark.

它可以长到至少16.5米长，超过45公吨重，也就是说比今天的鲸鲨还要大。

Realizing modern biologists had left ancient fish out of their equation, scientists decided to calculate L. problematicus’s metabolic requirements.

意识到现代生物学家们在方程中忽视了古代鱼之后，科学家们决定来计算一下利兹鱼的新陈代谢需求。

 They used data from living bony fish as a guide, and they found that it would have not only survived, but thrived: In theory, the giant fish could have cruised along at speeds of 17.8 kilometers per hour while still keeping its tissues adequately oxygenated, they report this month in Palaeontology. 

利用现代的硬骨鱼做参照，他们发现利兹鱼不仅存在过，而且十分兴盛过：从理论上说，这种大鱼的游速可达到17.8公里每小时，同时组织不会缺氧，他们在本月的《古生物学》杂志上进行了发表。

For comparison, the speediest living fish probably swim no faster than 30 kilometers per hour.

与之相比，现存的游得最快的鱼的游速也不超过30公里每小时。

It’s still a mystery why there are no gigantic bony fish today, but metabolically speaking there’s no reason why they shouldn’t exist, the researchers conclude.

如今为什么没有了这种巨型的硬骨鱼仍然是个谜，不过从新陈代谢的角度来说，它们是可以存在的，研究者们总结道。

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