アメリカ大陸のクロコダイル、起源はアフリカ　研究

【パリＡＦＰ＝時事】南北アメリカ大陸に生息しているクロコダイルの複数の種は、すべてアフリカを起源とするという研究論文が23日、英科学誌ネイチャー系列のオンライン科学誌「サイエンティフィック・リポーツ」に掲載された。（写真はリビアの砂漠で化石が発掘されたクロコダイルの絶滅種のイメージ図。ペルージャ大学提供）
　それらの種の祖先は、５００万～７００万年前に大西洋の海流に乗ってアメリカ大陸へと渡った、卵を抱えた１頭の雌の可能性があると、研究者らは推測している。
　この説は１９３９年にリビアの砂漠で発掘されたクロコダイルの頭蓋骨の断片の最新技術による分析に基づいたもので、遺伝的証拠によっても補強されたと研究者らは主張している。
　研究死者らは、大学の博物館に何十年も保管されていた、絶滅種のクロコダイル「Ｃｒｏｃｏｄｙｌｕｓ　ｃｈｅｃｃｈｉａｉ」の７００万年前の頭部と上顎の化石を再分析した。
　ＣＴスキャンと３Ｄモデリングを使用して調べたところ、化石の口先の中央部に特徴的な突起があることが分かった。これは、現存するか絶滅したかにかかわらず、アフリカを起源とする他の種には見られないが、アメリカ大陸に現在生息する４種すべてにある。
　研究者らは、Ｃｒｏｃｏｄｙｌｕｓ　ｃｈｅｃｃｈｉａｉはアフリカ系とアメリカ系の間の進化の隙間を埋める「ミッシングリンク（失われた環）」だと指摘。「約７００万年前、少なくとも５００万年前に、１頭かそれ以上の個体、おそらく１頭の卵を持った雌がアフリカからアメリカに渡って広がったと考えられる」と述べた。
　そうした移動が可能であることは、オーストラリアのイリエワニによって証明されている。イリエワニは、海流に乗って約１か月で５００キロ移動できることが衛星追跡で分かっている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/07/27-13:39）

The several species of crocodiles plying rivers and brackish byways in the Americas -- from Florida to Peru -- all came from Africa, according to a study published Thursday.
They may have descended, researchers speculate, from a single pregnant specimen that bobbed along Atlantic Ocean currents to the New World at least five million years ago, probably longer.
Based on the high-tech analysis of a skull fragment unearthed from the Libyan desert in 1939, the findings are bolstered by genetic evidence pointing in the same direction, they reported in the journal Scientific Reports.
This is a really exciting discovery, lead authors Massimo Delfino from the University of Turin and Dawid Iurino, a palaeontologist at Sapienza University in Rome, told AFP by email.
It supports the results of molecular biologists that proposed the origin of American crocodiles had to be found in Africa.
The out-of-Africa narrative is based on the re-examination of the skull and upper jaw of a seven-million-year-old fossil that had been tucked away for decades in a university museum drawer.
It belonged to an extinct species called Crocodylus checchiai.
Using CT-scans and 3D-modelling, the scientists identified a tell-tale protrusion in the middle of the animal's snout not found in any other African crocodile, living or extinct, but present in all four species currently found in the Americas.
In the world of paleontology, this is pretty close to a smoking gun.
Our results are solid, the researchers said when asked if the evidence was conclusive.
The main problem for palaeobiologists is the rarity and fragmentary nature of fossil remains.
Four other fossils dug up in Libya at the same time -- including a complete skull and jaw -- were either destroyed during World War II or lost.
C. checchiai rewrites the story of how crocodiles spread across the planet in at least two ways.
- Missing link -
It lays to rest the already fading hypothesis that the giant, flesh-ripping reptiles -- which first emerged from Asia -- arrived in the Americas before moving on to Africa, and not the other way around.
The long-neglected fossil also supplants another contender from Africa -- Crocodylus niloticus, aka the Nile crocodile -- as the closest forebear of the American species.
According to our results, C. checchiai nests between the Nile crocodile and the American species, the authors told AFP.
It represents the missing link between the African and American lineages.
We can therefore assume one or more specimens -- perhaps a pregnant female -- dispersed from Africa to America about seven million years ago, at the very least five million.
That such a voyage is possible has been demonstrated by a present-day cousin, Australia's saltwater crocodile, which satellite tracking has shown can travel 500 kilometres (310 miles) in about a month while passively transported by ocean currents.
More closely related to birds than dinosaurs, egg-laying crocodiles have been around for about 55 million years.
There are 16 species spread across the tropics of Africa, Asia, Australia and, of course, the Americas.
They vary in size from less than two metres (six feet) for the dwarf crocodile, to more than seven metres and 1,000 kilos (2,200 pounds) for the saltwater species.
The carnivores are able to replace each of their 80 teeth up to 50 times during their lifespan, which can top 60 years.