Ｔ・レックスに似た短い腕　新種の大型肉食恐竜を発見

【ワシントンＡＦＰ＝時事】ティラノサウルス・レックス（Ｔ・レックス）のような大きな頭部と短い腕を持つ大型肉食恐竜の新種を発見したとする論文が７日、米科学誌カレント・バイオロジーに発表された。（写真は新種肉食恐竜「メラクセス・ギガス」の想像図。米ミネソタ大学提供）
　新種恐竜の化石は、２０１２年にアルゼンチンのパタゴニア地方北部で頭蓋骨が見つかり、その後４年にわたる発掘でほかの部位も発見された。米人気ドラマ「ゲーム・オブ・スローンズ」の原作小説に登場するドラゴンにちなみ、「メラクセス・ギガス」と命名された。
　カルカロドントサウルス科に属し、９０００万年～１億年前の白亜紀に生息。化石の保存状態は非常に良好で、頭蓋骨の長さは１２７センチ、全長は約１１メートル、体重４トンと推測されている。腕の長さは約６０センチと頭蓋骨の半分ほどで、口まで届かなかったとみられる。
　Ｍ・ギガスはＴ・レックスが出現する２０００万年前に絶滅しており、両種は近縁でないため、Ｔ・レックスの短い腕はＭ・ギガスから受け継いだものではない。論文を執筆した米国とアルゼンチンの共同研究チームはこのことから、短い腕には進化上の利点があり、交尾の際に雄が雌を抱きかかえたり、休息や転倒時に体を支えて立ち上がったりするために使ったのではないかとみている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/07/08-13:44）

Paleontologists said Thursday they had discovered a new giant carnivorous dinosaur species that had a massive head and tiny arms, just like Tyrannosaurus rex.
The researchers' findings, published in the journal Current Biology, suggest that small forelimbs were no evolutionary accident, but rather gave apex predators of the time certain survival advantages.
Meraxes gigas -- named after a fictional dragon in the Game of Thrones book series -- was dug up over the course of four years during field expeditions in the northern Patagonia region of Argentina, starting with the skull which was found in 2012.
We won the lottery and found it literally on the first morning, senior author Peter Makovicky from the University of Minnesota told AFP.
The fossilized remains were remarkably well preserved. The skull is just over four feet long (127 centimeters), while the entire animal would have been some 36 feet long, and weighed four metric tons.
Its arms were two feet long, so it's literally half the length of the skull and the animal would not have been able to reach its mouth, said Makovicky.
T. rex didn't get its tiny arms from M. gigas. The latter went extinct 20 million years before the former arose, and the two species were far apart on the evolutionary tree.
Instead, the authors believe the fact that tyrannosaurids, carcharodontosaurids -- the group Meraxes belonged to -- and a third giant predator species called abelisaurids all evolved tiny arms points to certain benefits.
Makovicky believes that as their heads grew larger, it became the dominant tool of their predatory arsenal, taking on the function that forelimbs would have had in smaller species.
His co-author Juan Canale, the project lead at Ernesto Bachmann Paleontological Museum in Neuquen, Argentina, went further in suggesting other advantages.
- Mating and movement support -
I'm convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles, he said in a statement.
They may have used the arms for reproductive behavior such as holding the female during mating or support themselves to stand back up after a break or a fall.
Meraxes roamed the Earth between 90 to 100 million years ago during the Cretaceous, at a time when the region was wetter, more forested, and much closer to the sea, said Makovicky.
They would have preyed on a menagerie of contemporary sauropods -- some of whom were discovered at the same site.
The individual lived to around 40 years -- a ripe old age for dinosaurs -- and its skull was replete with crests, furrows, bumps and small hornlets.
It certainly would have looked very imposing and gargoyle like, said Makovicky.
Those are the kinds of features that in living animals are often under sexual selection, speculating the species used their massive skulls as billboards for advertising to would-be mates.