恐龍退下，魚類上場

原文網址：www.sciencedaily.com/releases/2015/06/150630100607.htm

Exit dinosaurs, enter fishes

Mass extinction event that killed the dinosaurs gave rise to modern 'age of fishes'

恐龍退下，魚類上場

毀滅恐龍的大滅絕事件造就了現今的魚類紀元

A pair of paleobiologists from Scripps Institution of Oceanography, UC San Diego have determined that the world's most numerous and diverse vertebrates ¬- ray-finned fishes -- began their ecological dominance of the oceans 66 million years ago, aided by the mass extinction event that killed off dinosaurs.

兩位來自加州大學聖地牙哥分校(UC San Diego)的斯克里普斯海洋學研究所(Scripps Institution of Oceanography)的古生物學家確認了世上數目最多且最多樣的脊椎動物—輻鰭魚(ray-finned fish)—於6600萬年前受惠於消滅恐龍的大滅絕事件而開始成為海洋生態系的主要角色。

Scripps graduate student Elizabeth Sibert and Professor Richard Norris analyzed the microscopic teeth of fishes found in sediment cores around the world and found that the abundance of ray-finned fish teeth began to explode in the aftermath of the mass die-off of species, which was triggered by an asteroid strike in the Yucatan Peninsula. Scientists refer to this episode as the Cretaceous-Paleogene (K/Pg) extinction event.

斯克里普斯的研究生Elizabeth Sibert與Richard Norris教授分析了世界各地的沉積物岩心中微小的魚類牙齒。他們發現輻鰭魚的牙齒數目在生物大滅絕後開始爆炸性的增長。此滅絕事件的成因為小行星撞擊猶加敦半島，科學家稱之為白堊紀-古第三紀(K/Pg)滅絕事件(Cretaceous-Paleogene (K/Pg) extinction event)。

Ninety-nine percent of all fish species in the world -- from goldfish to tuna and salmon -- are classified as ray-finned fishes. They are defined as species with bony skeletal structures and have teeth that are well preserved in deep ocean mud. Sharks, in contrast, have cartilaginous skeletons and are represented by both teeth and mineralized scales, also known as denticles, in marine sediments.

現今世上99%的魚類—從金魚至鮪魚及鮭魚—皆可歸類成輻鰭魚。牠們的定義為擁有硬骨質構造，而在深海底泥中可以發現牠們保存良好的牙齒。相反的，鯊魚則是擁有軟骨質構造，在海洋沉積物中可以發現牠們的牙齒與礦化的鱗片，稱作鱗突(denticle)。

"We find that the extinction event marked an ecological turning point for the pelagic marine vertebrates," write the authors in the study. "The K/Pg extinction appears to have been a major driver in the rise of ray-finned fishes and the reason that they are dominant in the open oceans today."

「我們發現這起滅絕事件標記了遠洋脊椎動物生態結構的轉捩點。」作者在此篇論文中寫道。「K/Pg滅絕事件顯然是輻鰭魚崛起的主要推手，以及牠們為何能主宰今日的開放海洋的原因。」

The breakthrough for the researchers in reaching their conclusion came through their focus on fossilized teeth and shark scales. In cores from numerous ocean basins, they found that while the numbers of sharks remained steady before and after the extinction event, the ratio of ray-finned fish teeth to shark teeth and scales gradually rose, first doubling then becoming eight times more abundant 24 million years after the extinction event. Now there are 30,000 ray-finned fish species in the ocean, making this class the most numerically diverse and ecologically dominant among all vertebrates on land or in the ocean.

研究人員之所以能夠得到此突破性的結論在於他們將焦點放在牙齒化石與鯊魚鱗片。從許多海盆鑽探出的岩心中，他們發現雖然在此次滅絕事件前後鯊魚的牙齒數目大致不變，輻鰭魚牙齒對鯊魚牙齒的數目比例卻逐漸上升。在滅絕事件後起初是兩倍，2400萬年後卻變成有八倍之多。現今的海洋中有多達30,000種的輻鰭魚，使得牠們成為陸地與海洋脊椎動物中多樣性最高，也最具有生態上的優勢的一類。

Scientists had known that the main diversification of ray-finned fishes had happened generally between 100 million and 50 million years ago.

科學家已經知道輻鰭魚的多樣化主要發生在1億至5000萬年前。

"The diversification of fish had never been tied to any particular event. What we found is that the mass extinction is actually where fish really took off in abundance and variety," said Sibert, who is the recipient of an NSF Graduate Research Fellowship. "What's neat about what we found is that when the asteroid hit, it completely flipped how the oceans worked. The extinction changed who the major players were."

「過往從未將魚類的多樣化跟任何特定事件牽上關連。而我們發現魚類實際上是從這次大滅絕事件後才開始變的多樣化且數目激增，」 Sibert說，其為美國國家科學基金會研究生獎學金的受獎者。「我們的研究精妙之處在於我們發現當小行星撞擊地球時，它同時也徹底顛覆了海洋的運作規則。這次的滅絕事件改變了誰才是擁有主導權的一方。」

Sibert and Norris believe that some key changes in the oceans might have helped ray-finned fishes along. Large marine reptiles disappeared during the mass extinction, as did the ammonites, an ancient cephalopod group similar to the chambered nautilus. Those species, the researchers believe, had been either predators of ray-finned fishes or competitors with them for resources.

Sibert與Norris認為發生在海洋的某些關鍵變化可能獨厚輻鰭魚。大型海洋爬蟲類在此次大滅絕中消失殆盡，同樣步上此命運的還有菊石，為一種類似鸚鵡螺的古代頭足鋼動物。研究人員認為這些動物在當時可能是輻鰭魚的天敵，或者會與牠們競爭資源。

"What's amazing," said Norris, "is how quickly fish double, then triple in relative abundance to sharks after the extinction, suggesting that fish were released from predation or competition by the extinction of other groups of marine life."

「迷人的是，」Norris說，「輻鰭魚的數量在此次大滅絕後迅速地增加為鯊魚的兩倍，然後是三倍之多，這顯示輻鰭魚從其他海洋生物的掠食或競爭壓力中解脫了。」

Sibert noted that before the extinction event, ray-finned fishes existed in a state of relative ecological insignificance, just like mammals on land.

Sibert發現在此次大滅絕事件之前，輻鰭魚在生態上的重要性相對而言並不高，就跟當時陸上的哺乳動物一樣。

"Mammals evolved 250 million years ago but didn't become really important until after the mass extinction. Ray-finned fishes have the same kind of story," said Sibert. "The lineage has been around for hundreds of millions of years, but without the mass extinction event 66 million years ago, it is very likely that the oceans wouldn't be dominated by the fish we see today."

「哺乳動物雖然在2500萬年前就已經演化出來，然而直到此次大滅絕發生之前牠們都不是那麼重要。而輻鰭魚的故事也有異曲同工之處。」Sibert說。「雖然牠們的血脈已流傳了數億年，但如果沒有那次發生在6600萬年前的大滅絕事件，海洋很有可能不會被我們今日所見的魚群主宰。」

引用自：University of California - San Diego. "Exit dinosaurs, enter fishes: Mass extinction event that killed the dinosaurs gave rise to modern 'age of fishes'." ScienceDaily. ScienceDaily, 30 June 2015.