新たに発見された恐竜イアニは、変化する惑星における種の「最後のあえぎ」となる可能性がある

アーティストの再構築 イアニ・スミシー。 最近発見されたこの初期の鳥脚類恐竜は、約9,900万年前の白亜紀中期の地球温暖化の時期にこの種が最後に抵抗し、恐竜の個体数に劇的な変化をもたらした時代を代表するものである可能性がある。 クレジット: ホルヘ・ゴンザレス

新たに発見された恐竜、 イアニ・スミシー彼は中世の気候変動の時代に生きた[{” attribute=””>Cretaceous era and might be the last of its lineage, replaced by duckbill dinosaurs. This dinosaur, found in Utah, represents a critical transitional period in dinosaur history, affected by rising CO2 levels, global warming, and shifting dinosaur populations.

A newly discovered plant-eating dinosaur may have been a species’ “last gasp” during a period when Earth’s warming climate forced massive changes to global dinosaur populations.

The specimen, named Iani smithi after Janus, the two-faced Roman god of change, was an early ornithopod, a group of dinosaurs that ultimately gave rise to the more commonly known duckbill dinosaurs such as Parasaurolophus and Edmontosaurus. Researchers recovered most of the juvenile dinosaur’s skeleton – including skull, vertebrae and limbs – from Utah’s Cedar Mountain Formation.

Iani smithi lived in what is now Utah during the mid-Cretaceous, approximately 99 million years ago. The dinosaur’s most striking feature is its powerful jaw, with teeth designed for chewing through tough plant material.

白亜紀中期は大きな変化の時代であり、恐竜の個体数に大きな影響を与えました。 この時期の大気中の二酸化炭素の増加により、地球の温度が上昇し、海面が上昇し、恐竜が占める陸地面積はますます小さくなっていきました。 非常に暖かかったので、極地では熱帯雨林が生い茂りました。 開花植物が沿岸地域を占領し、草食動物の自然の食料源に取って代わりました。

北米では、かつては大地の巨人だった植物を食べる巨大な竜脚類が、捕食者であるアロサウルスとともに姿を消しつつある。 同時に、古代のアヒルの嘴恐竜や角のある恐竜などの小型の草食動物や、ティラノサウルスや巨大なオヴィラプトロサウルスなどの羽毛のある獣脚類もアジアから到来していました。

入る イアニ・スミシーは、新しく発見されたというだけでなく、北米の化石記録の中での希少性と恐竜の歴史におけるその位置によってもユニークです。

「発見 自分 それは幸運の連続だった。 孤立した歯があちこちで収集されていたため、この生態系に似たものが生息していることは知られていましたが、特に地球の歴史のこの時期に、これほど美しい骨格に遭遇するとは予想していませんでした。 ほぼ完成に近い頭蓋骨があったことは、物語をつなぎ合わせる上で非常に貴重でした」とリンジー・ザノ准研究教授は語る。[{” attribute=””>North Carolina State University, head of paleontology at the North Carolina Museum of Natural Sciences, and corresponding author of the work.

The lower jaw and teeth of new dinosaur Iani smithi. Credit: National Geographic, Mark Thiessen and Becky Hale

Zanno and her team used the well-preserved skeleton to analyze the evolutionary relationships of Iani and were surprised – and a bit skeptical – of the results.

“We recovered Iani as an early rhabdodontomorph, a lineage of ornithopods known almost exclusively from Europe,” Zanno says. “Recently, paleontologists proposed that another North American dinosaur, Tenontosaurus – which was as common as cattle in the Early Cretaceous – belongs to this group, as well as some Australian critters. If Iani holds up as a rhabdodontomorph, it raises a lot of cool questions.”

Key among these is, could Iani be a last gasp, a witness to the end of a once successful lineage? Zanno thinks that studying this fossil in the context of environmental and biodiversity changes during the mid-Cretaceous will give us more insight into the history of our planet.

Iani smithi is named for Janus, the two-faced god who symbolized transitions – an apt name, given its position in history.

“Iani may be the last surviving member of a lineage of dinosaurs that once thrived here in North America but were eventually supplanted by duckbill dinosaurs,” Zanno says. “Iani was alive during this transition – so this dinosaur really does symbolize a changing planet.

“This dinosaur stood on the precipice,” she says, “able to look back at the way North American ecosystems were in the past, but close enough to see the future coming like a bullet train. I think we can all relate to that.”

Reference: “An early-diverging iguanodontian (Dinosauria: Rhabdodontomorpha) from the Late Cretaceous of North America” by Lindsay E. Zanno, Terry A. Gates, Haviv M. Avrahami, Ryan T. Tucker and Peter J. Makovicky, 7 June 2023, PLOS ONE.
DOI: 10.1371/journal.pone.0286042

The work appears in PLOS ONE and was supported by the National Science Foundation. Zanno is lead author as well as corresponding. Terry Gates and Haviv Avrahami, both of NC State and the North Carolina Museum of Natural Sciences, along with Ryan Tucker of Stellenbosch University and Peter Makovicky of the University of Minnesota, also contributed to the work.