2021年12月29日 星期三

地球第一隻巨獸

 原文網址:https://nhm.org/stories/earths-first-giant

By: Tyler Hayden

新發現的巨型魚龍是地球上已知的第一種巨型動物,牠長達兩公尺的頭顱揭曉了已經消失的海洋爬蟲類和現代的鯨魚如何長得如此龐大。

這具新發現的物種是已知最早的巨型魚龍,牠長達兩公尺的頭顱闡明了這種海洋爬蟲類的體型如何快速增長而成為恐龍時代的海中巨獸,同時也有助於我們了解現代的鯨豚如何一路變成地球有史以來以來最大的動物。

在恐龍統治陸地的同時,魚龍以及其他水棲爬蟲類(特別強調牠們並非恐龍)則主宰了海洋。同樣地,牠們也發展出相當巨大的體型以及各式各樣的種類。魚龍演化出魚類和鯨豚擁有的流線形外觀與鰭,幾乎在整個恐龍時代的古代海洋都能看到牠們的蹤影。

這具保存良好的頭顱以及部分脊椎、肩膀和手臂是從內華達州奧古斯塔山的岩層――化石丘段(Fossil Hill Member)挖掘出來。牠的年代為三疊紀中期(2.472億年到2.37億年前),是體型達到史詩般大小的魚龍的最早例子。這個新物種被命名為Cymbospondylus youngorum,身長跟大型抹香鯨一樣超過17公尺,與迄今發現同時代的動物相比,不論是在海洋或是陸地都是體型最大的。

在內華達州的另一座山脈,古生物學家從1902年就開始從化石丘組的石灰岩、頁岩和粉砂岩裡挖出許多化石,開啟了通往三疊紀的窗口。這些山脈連結了我們所處的當下以及古代海洋,其中出產了許多種類的菊石(墨魚和章魚等現代頭足類帶有殼的祖先)以及海洋爬蟲類。這些樣本合稱為化石丘動物群,其中許多為C. youngorum的獵物或是競爭對手。

C. youngorum大概是在2.46億年前稱霸海洋,距離第一隻魚龍的鰭進入水裡不過隔了三百萬年,在如此短時間之內就變得如此巨大實在是令人驚訝。從C. youngorum長形的吻部以及錐形的牙齒可以推測牠的獵物為魷魚和魚類,但以體型來看牠或許也會獵食體型較小和未成年的海洋爬蟲類。

這種巨大的掠食者或許還是有其他動物與之抗衡。研究作者透過精密的電腦模擬,探討能量在化石丘動物群的食物網中可能的流動方式。他們根據數據重建出來的古代環境,發現海洋食物網還可以支持一些巨型的肉食性魚龍。當時的魚龍有許多不同的大小,生存策略也各有千秋,就像現代的鯨豚一樣――有較小的海豚、龐大的濾食性鬚鯨,還有捕獵魷魚的巨型抹香鯨。

鯨魚和魚龍的相似之處不僅是體型的範圍。牠們擁有類似的身體結構,而且最初都是在大滅絕事件之後崛起。這些雷同之處使得比對牠們的研究具有很重要的科學價值。作者結合了電腦模擬以及傳統古生物學,探討這些海洋動物如何各自發展出破紀錄的大小。

他們發現雖然鯨豚和魚龍都演化出非常巨大的體型,但邁向巨大化的演化軌跡卻各不相同。魚龍的體型一開始就快速增長,使牠們的演化史很早就出現巨獸;相較而言鯨豚則花了很長一段時間到達巨大化的上限。他們也發現體型變大和兇猛的掠食方式有關,像是抹香鯨會下潛到深海捕食巨型魷魚;另一個有關現象則是牙齒消失,像是龐大的濾食性鬚鯨就是地球有史以來最大的動物。

促使魚龍最初邁入巨大化的原因可能是在二疊紀末大滅絕過後,菊石和無頷、狀似鰻魚的牙形類為了填補空出來的生態棲位而快速增加。雖然鯨魚和魚龍有著不一樣的演化軌跡,但兩者皆是利用了自身在食物鏈中的地位而變得相當巨大。

「我們的發現與研究結果強調出不同類型的海洋四足動物,如何在類似情況下演化出壯觀的體型,但是速率卻有著驚人的差異,」自然歷史博物館哺乳動物學門(海洋哺乳類)的副研究員Jorge Velez-Juarbe博士表示。「有了我們彙整出來的資料庫以及測試過的分析方法之後,我們可以開始考慮加入其他次要的水生脊椎動物族群,來瞭解牠們在這方面的演化史。」

 

Earth's first giant

The two-meter skull of a humongous new ichthyosaur species, Earth’s first known giant creature, reveals how both the extinct marine reptiles and modern whales became giants.

The two-meter skull of a newly discovered species of giant ichthyosaur, the earliest known, is shedding new light on the marine reptiles’ rapid growth into behemoths of the Dinosaurian oceans, and helping us better understand the journey of modern cetaceans (whales and dolphins) to becoming the largest animals to ever inhabit the Earth.

While dinosaurs ruled the land, ichthyosaurs and other aquatic reptiles (that were emphatically not dinosaurs) ruled the waves, reaching similarly gargantuan sizes and species diversity. Evolving fins and hydrodynamic body-shapes seen in both fish and whales, ichthyosaurs swam the ancient oceans for nearly the entirety of the Age of Dinosaurs.

Excavated from a rock unit called the Fossil Hill Member in the Augusta Mountains of Nevada, the well-preserved skull, along with part of the backbone, shoulder, and arm, date back to the Middle Triassic (247.2-237 million years ago), representing the earliest case of an ichthyosaur reaching epic proportions. As big as a large sperm whale at more than 17 meters (55.78 feet) long, the newly named Cymbospondylus youngorum is the largest animal yet discovered from that time period, on land or in the sea.

In other mountain ranges of Nevada, paleontologists have been recovering fossils from the Fossil Hill Member’s limestone, shale, and siltstone since 1902 opening a window into the Triassic. The mountains connect our present to ancient oceans and have produced many species of ammonites, shelled ancestors of modern cephalopods like cuttlefish and octopuses, as well as marine reptiles, specimens collectively known as the Fossil Hill Fauna, representing many of C. youngorum’s prey and competitors.

C. youngorum stalked the oceans some 246 million years ago, or only about three million years after the first ichthyosaurs got their fins wet, an amazingly short time to get this big. The elongated snout and conical teeth suggest that C. youngorum preyed on squid and fish, but its size meant that it could have hunted smaller and juvenile marine reptiles as well.

The giant predator likely had some hefty competition. Through sophisticated computational modeling, the authors examined the likely energy running through the Fossil Hill Fauna’s food web, recreating the ancient environment through data, finding that marine food webs were able to support a few more colossal meat-eating ichthyosaurs. Ichthyosaurs of different sizes and survival strategies proliferated, comparable to modern cetaceans’— from relatively small dolphins to massive filter-feeding baleen whales, and giant squid-hunting sperm whales.

Whales and ichthyosaurs share more than a size range. They have similar body plans, and both initially arose after mass extinctions. These similarities make them scientifically valuable for comparative study. The authors combined computer modeling and traditional paleontology to study how these marine animals reached record-setting sizes independently. 

They found that while both cetaceans and ichthyosaurs evolved very large body sizes, their respective evolutionary trajectories toward gigantism were different. Ichthyosaurs had an initial boom in size, becoming giants early on in their evolutionary history, while whales took much longer to reach the outer limits of huge. They found a connection between large size and raptorial hunting —think of a sperm whale diving down to hunt giant squid—and a connection between large size and a loss of teeth—think of the giant filter-feeding whales that are the largest animals ever to live on Earth.

Ichthyosaurs' initial foray into gigantism was likely thanks to the boom in ammonites and jawless eel-like conodonts filling the ecological void following the end-Permian mass extinction. While their evolutionary routes were different, both whales and ichthyosaurs relied on exploiting niches in the food chain to make it really big.

“This discovery and the results of our study highlight how different groups of marine tetrapods evolved body sizes of epic proportions under somewhat similar circumstances, but at surprisingly different rates,” says NHM’s Associate Curator of Mammalogy (Marine Mammals), Dr. Jorge Velez-Juarbe. “Moving forward, with the dataset we’ve compiled and analytical methods we've tested, we can start thinking about including other groups of secondarily aquatic vertebrates to understand this aspect of their evolutionary history.”

原始論文:P. Martin Sander, Eva Maria Griebeler, Nicole Klein, Jorge Velez Juarbe, Tanja Wintrich, Liam J. Revell, Lars Schmitz. Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceansScience, 2021; 374 (6575) DOI: 10.1126/science.abf5787

引用自:Natural History Museum of Los Angeles County. "Earth's first giant." 

 

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