Imagine a warm spring day about 66 million years ago. Fish would have been swimming in the rivers in an area that’s now North Dakota in the US. Seconds later， the serenity was ended by destruction.

These could have been the very last moments of the dinosaur era when a city-sized asteroid struck Earth， killing off three quarters of all species on the planet. According to a study published in Nature on Feb 23， 2022 the asteroid hit in springtime.

Scientists have long been confused over the time of year the asteroid hit， and how some animals managed to survive while dinosaurs didn’t.

Researchers in 2019 discovered fossilized fishes in North Dakota that died shortly after the asteroid hit Earth. They examined the fossils with a particle accelerator and found out there was seasonal growth on the bones. All fish bone cell densities and volumes can indicate the season.

想象一下大約6 600萬年前一個溫暖的春天，在位于現(xiàn)今美國北達(dá)科他州區(qū)域的河流中，魚兒暢游著，幾秒鐘后，毀滅終結(jié)了這份寧靜。

這也許是恐龍時代的最后時刻，因為一顆城市大小的小行星撞擊地球，殺死了地球上四分之三的物種。根據(jù)2022年2月23日《自然》雜志上發(fā)表的一項研究，小行星撞擊地球是在春季。

長期以來，科學(xué)家們一直對小行星撞擊地球的時間困惑不清，也不清楚為何一些動物能存活下來，而恐龍卻未能活命。

2019年，研究人員在北達(dá)科他州發(fā)現(xiàn)了魚類化石，這些魚類在小行星撞擊地球后不久死亡。研究人員用粒子加速器檢查了化石，發(fā)現(xiàn)魚骨上顯示出季節(jié)性生長。所有的魚骨細(xì)胞密度和體積都可以表明季節(jié)。

The timing of the collision， at least for the Northern Hemisphere， came at a particularly sensitive stage. “I think spring puts a large group of the late Cretaceous biota in a very vulnerable spot because they were out and about looking for food， tending to offspring and trying to build up resources after the harsh winter，” Melanie During， the main author of the study， said at a news briefing.

In the Southern Hemisphere， it was autumn， a season when many creatures prepare for winter. Therefore， life in the Southern Hemisphere was a lot more prepared for this event.

It is unclear whether small animals in the Northern Hemisphere actually did worse than those in the south. There is evidence that Northern Hemisphere turtles were wiped out in the asteroid strike， after which their habitats were later repopulated by turtles from the south， Dennis Voeten， a researcher at Uppsala University in Sweden told The Guardian.

Nothing much larger than a house cat survived the asteroid impact and many species would have been doomed whenever it hit， Daniel Field， an assistant professor at the University of Cambridge in the UK， told The Guardian.

小行星撞擊地球的時間，至少對于北半球來說，是一個特別敏感的階段。這項研究的第一作者梅勒妮·杜林在一次新聞發(fā)布會上說：“我認(rèn)為春天令一大群白堊紀(jì)晚期生物處在了一個非常脆弱的境地，因為它們忙著外出尋找食物，照顧后代，以及在嚴(yán)冬后累積物資?！?/p>

南半球正值秋天，這是許多生物準(zhǔn)備過冬的季節(jié)。因此，南半球的生物對這一事件有了更充分的準(zhǔn)備。

目前尚不清楚北半球的生物是否真的比南半球的生物表現(xiàn)得更糟。瑞典烏普拉大學(xué)研究員丹尼斯·沃滕在接受英國《衛(wèi)報》采訪時表示，有證據(jù)表明，北半球的海龜在小行星撞擊地球時滅絕了，之后它們的棲息地被來自南半球的海龜所占據(jù)。

英國劍橋大學(xué)助理教授丹尼爾·菲爾德在接受《衛(wèi)報》采訪時表示，體型比家貓大的動物沒能在小行星撞擊地球時幸存下來，無論何時，撞擊一旦發(fā)生，許多物種就難逃厄運。

Word Bank

serenity /s?'ren?ti/ n. 平靜；寧靜

fossilized /'f?s?la?zd/ adj. 石化的

sensitive /'sens?t?v/ adj. 敏感的

My teeth are very sensitive to cold food.

offspring /'?fspr??/ n. 后代；（動物的）幼崽

evidence /'ev?d?ns/ n. 證據(jù)；證明