猶他州出土的化石揭櫫了哺乳類近親曾經出走到全球主要陸塊

原文網址：www.sciencedaily.com/releases/2018/05/180523133203.htm

猶他州出土的化石揭櫫了哺乳類近親曾經出走到全球主要陸塊

這具小型化石證實了地球古代的超大陸之一――盤古大陸――分裂的時間比以往認為的還要晚了1500萬年左右。

這具在猶他州發現，年代將近1億3000萬年的顱骨化石在某方面來說是個驚天動地的發現。

這具小型化石證實了盤古大陸分裂的時間可能比科學家以往認為的還要晚發生，也意外證實了某一群介於哺乳類和爬蟲類之間的過渡生物，曾在數個大陸上經歷迅速的演化。

「這具幾近完整的顱骨化石是個令人難以置信的發現，據此我們辨識出一種新的原始哺乳類近親，牠們在世界各地皆有分布。」這項研究的主要作者Adam Huttenlocker表示，他是南加州大學凱克醫學院的臨床綜合解剖科學助理教授。

這項研究5月16日刊登在期刊《自然》(Nature)，它對於哺乳類在恐龍時代的演化歷程以及擴散至各大陸塊的過程提供了最新見解。作者提出盤古古陸的分裂持續時間比過往認為的還要再往後延長1500萬年；同時在白堊紀早期哺乳類仍然可以遷徙到各地，而牠們關係最近的親戚也還存活著。

「長久以來人們認為白堊紀時(1億4500萬年至6600萬年前)的早期哺乳類在解剖學上的構造皆很相似，且生態並不複雜。」Huttenlocker表示，「但我們和其他團隊的發現鞏固了以下理論：在現代哺乳類崛起許久以前，哺乳類的遠古親戚就已經發展出各種相當專一的生態棲位，像是食蟲動物、草食動物、肉食動物、水棲動物、飛行動物等。基本上，牠們佔有的各種生態區位跟我們現今看到哺乳類佔據的生態區位是相同的。」

本研究的資深作者，芝加哥大學的古生物學家Zhe-Xi Luo表示，此研究顯示了這些早期哺乳類的前身先從亞洲遷徙到歐洲，然後再進入北美洲，最後前往南半球的主要陸塊。

發現新物種的化石

Huttenlocker和猶他州地質調查具與芝加哥大學的同事將此新物種命名為Cifelliodon wahkarmoosuch。

這具化石在猶他州東部的白堊紀岩層發現，屬名的由來是紀念知名古生物學家Richard Cifelli；種名「wahkarmoosuch」在生活於發現區域的猶特族語言中的意思是「黃貓」。

科學家利用高解析度的電腦斷層掃描儀來分析這具頭骨。

「在西部內陸富含大量化石的地區中可以找到超過150種的哺乳類和類似爬蟲類的哺乳類前身，但牠們發現時大都只有幾顆牙齒或是僅有顎骨，因此Cifelliodon的頭骨確實是極其罕有的發現。」主要負責挖掘過程的研究共同作者，猶他州立大學的古生物學家James Kirkland表示。

他們估計這隻Cifelliodon的體重只有2.5磅(約1.5公斤)，跟現生的多數哺乳類相比小了許多，但牠和生活於白堊紀的其他同類相比已經是巨無霸了。一隻成年的Cifelliodon的體型大概跟一隻小型兔子或鼠兔差不多(鼠兔是一種小型哺乳類，其耳朵為圓形，四肢短小，尾巴很短)。

牠的牙齒類似於果蝠，可以咬下、撕裂並嚼碎食物。飲食當中可能包括了植物。

這個新命名的物種的腦容量並不大，負責處理嗅覺的「嗅球」相對而言卻很巨大。而顱骨上的眼窩相當小巧，因此Cifelliodon的視力或者辨色能力或許不太好。Huttenlocker表示牠可能是一種夜行型動物，依靠嗅覺來找到牠的食物。

超大陸的存在時間比以往認為的還要久

Huttenlocker和他的同事將Cifelliodon 分類在小賊獸目(Haramiyida)之下，這支已經滅絕的哺乳類祖先和真正的哺乳類之間有親緣關係。這具化石為其所屬的亞群Hahnodontidae第一次在北美發現的種類。

這具化石的發現強調出在侏儸紀和白堊紀交替之際，小賊獸目和其他某些種類的脊椎動物在全球皆有分布，代表可以讓這些動物遷徙至盤古大陸各處的廊道在白堊紀早期仍然完好無缺。

侏儸紀和白堊紀的小賊獸目化石大都發現在歐洲、格陵蘭和亞洲的三疊紀和侏儸紀岩層中(原文就是這樣)。先前已知的Hahnodontidae化石只出現在非洲北部的白堊紀岩層中。Huttenlocker主張Cifelliodon 屬於Hahnodontidae這件事提供了證據顯示當時依然存在可以讓動物遷徙的路徑，使牠們分布至現在分散於南北半球的陸塊。

「不只是小賊獸目之下的這個族群有此現象。」Huttenlocker表示，「今年以及之前的其它研究也根據在非洲和歐洲的白堊紀岩層發現類似的恐龍化石，得出盤古大陸在此時依然連結的結論。而我們的發現也反映出同一件事。」

Utah fossil reveals global exodus of mammals' near relatives to major continents

A small fossil is evidence that Earth's ancient supercontinent, Pangea, separated some 15 million years later than previously believed

A nearly 130-million-year-old fossilized skull found in Utah is an Earth-shattering discovery in one respect.

The small fossil is evidence that the super-continental split likely occurred more recently than scientists previously thought and that a group of reptile-like mammals that bridge the reptile and mammal transition experienced an unsuspected burst of evolution across several continents.

"Based on the unlikely discovery of this near-complete fossil cranium, we now recognize a new, cosmopolitan group of early mammal relatives," said Adam Huttenlocker, lead author of the study and assistant professor of clinical integrative anatomical sciences at the Keck School of Medicine of USC.

The study, published in the journal Nature on May 16, updates the understanding of how mammals evolved and dispersed across major continents during the age of dinosaurs. It suggests that the divide of the ancient landmass Pangea continued for about 15 million years later than previously thought and that mammal migration and that of their close relatives continued during the Early Cretaceous (145 to 101 million years ago).

"For a long time, we thought early mammals from the Cretaceous (145 to 66 million years ago) were anatomically similar and not ecologically diverse," Huttenlocker said. "This finding by our team and others reinforce that, even before the rise of modern mammals, ancient relatives of mammals were exploring specialty niches: insectivores, herbivores, carnivores, swimmers, gliders. Basically, they were occupying a variety of niches that we see them occupy today."

The study reveals that the early mammal precursors migrated from Asia to Europe, into North America and further onto major Southern continents, said Zhe-Xi Luo, senior author of the study and a paleontologist at the University of Chicago.

Fossil find: a new species

Huttenlocker and his collaborators at the Utah Geological Survey and The University of Chicago named the new species Cifelliodon wahkarmoosuch.

Found in the Cretaceous beds in eastern Utah, the fossil is named in honor of famed paleontologist Richard Cifelli. The species name, "wahkarmoosuch" means "yellow cat" in the Ute tribe's language in respect of the area where it was found.

Scientists used high-resolution computed tomography (CT) scanners to analyze the skull.

"The skull of Cifelliodon is an extremely rare find in a vast fossil-bearing region of the Western Interior, where the more than 150 species of mammals and reptile-like mammal precursors are represented mostly by isolated teeth and jaws," said James Kirkland, study co-author in charge of the excavation and a Utah State paleontologist.

With an estimated body weight of up to 2.5 pounds, Cifelliodon would seem small compared to many living mammals, but it was a giant among its Cretaceous contemporaries. A full-grown Cifelliodon was probably about the size of a small hare or pika (small mammal with rounded ears, short limbs and a very small tail).

It had teeth similar to fruit-eating bats and could nip, shear and crush. It might have incorporated plants into its diet.

The newly named species had a relatively small brain and giant "olfactory bulbs" to process sense of smell. The skull had tiny eye sockets, so the animal probably did not have good eyesight or color vision. It possibly was nocturnal and depended on sense of smell to root out food, Huttenlocker said.

Supercontinent existed longer than previously thought

Huttenlocker and his colleagues placed Cifelliodon within a group called Haramiyida, an extinct branch of mammal ancestors related to true mammals. The fossil was the first of its particular subgroup -- Hahnodontidae -- found in North America.

The fossil discovery emphasizes that haramiyidans and some other vertebrate groups existed globally during the Jurassic-Cretaceous transition, meaning the corridors for migration via Pangean landmasses remained intact into the Early Cretaceous.

Most of the Jurassic and Cretaceous fossils of haramiyidans are from the Triassic and Jurassic of Europe, Greenland and Asia. Hahnodontidae was previously known only from the Cretaceous of Northern Africa. It is to this group that Huttenlocker argues Cifelliodon belongs, providing evidence of migration routes between the continents that are now separated in northern and southern hemispheres.

"But it's not just this group of haramiyidans," Huttenlocker said. "The connection we discovered mirrors others recognized as recently as this year based on similar Cretaceous dinosaur fossils found in Africa and Europe."

原始論文：Adam K. Huttenlocker, David M. Grossnickle, James I. Kirkland, Julia A. Schultz, Zhe-Xi Luo. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature, 2018; DOI: 10.1038/s41586-018-0126-y

引用自：University of Southern California. "Utah fossil reveals global exodus of mammals' near relatives to major continents: A small fossil is evidence that Earth's ancient supercontinent, Pangea, separated some 15 million years later than previously believed." ScienceDaily. ScienceDaily, 23 May 2018.