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世界最大の「猛禽類」恐竜は、考えられていたよりも 1000 万年早く生きていた


Csotonyi のイラスト: ユタラプトル尾根の Stikes Quarry 骨床の流砂堆積物に閉じ込められたユタラプトルと獲物の描写。 最近の地質学的研究により、世界最大の「ラプトル」恐竜であるユタラプトルは、これまで考えられていたよりも 1000 万年古く、現在では少なくとも 1 億 3500 万歳と推定されていることが判明しました。 クレジット: Julius Csotonyi、自然史イラストレーター

世界最大の「ラプトル」恐竜であるユタラプトルは現在、1 億 3500 万歳と推定されており、これまで考えられていたよりも 1000 万年古い。 この発見は、恐竜の進化の歴史に重要な意味を持ち、[{” attribute=””>Jurassic and Cretaceous Periods in Utah.

Utahraptor is going to need 10 million more candles on its next birthday cake.

A geological study of the rock formation that encased a fossilized example of the world’s biggest “raptor” shows it’s 10 million years older than previously understood. The report, co-written by a researcher with the University of Kansas, was recently published in the journal Geosciences.

“We determined the age of the dinosaur Utahraptor and found that it was much older than previously supposed,” said Gregory Ludvigson, emeritus senior scientist with the Kansas Geological Survey at KU, who collaborated on the investigation. “That finding has important implications for the evolutionary history of dinosaurs.”

The fieldwork took place in Utah at the well-known Utahraptor Ridge site, named for larger cousins of the ferocious velociraptor dinosaur (known to fans of “Jurassic Park”).

The ridge is home to Stikes Quarry, a fossil quicksand deposit packed with dinosaur fossils that are largely intact and preserved — in much the same positions as when they died. Stikes Quarry is part of the Cedar Mountain Formation, a rock unit containing fossils of more kinds of dinosaurs than any formation in the world.

Greg Ludvigson

Greg Ludvigson on outcrop of colorful buried soils of the Yellow Cat Member in May 2009. Credit: Photo courtesy of Matt Joeckel

“We also learned to our complete surprise that the rock strata from the Stikes Dinosaur Quarry were deposited during a global change episode known as the Weissert Event,” Ludvigson said. “This is an agenda-setting discovery that will reverberate for decades.”

More than a decade ago, Ludvigson, along with Jim Kirkland, state paleontologist with the Utah Geological Survey, and Matt Joeckel, state geologist and director of the Conservation and Survey Division at the University of Nebraska-Lincoln, joined together to tackle that question.

The scientists and colleagues they recruited took two research approaches: One path — uranium/lead dating of zircon crystals — involved analyzing samples of these minerals collected at different depths in the rock layers. The second looked at changes in the relative abundance of two types of stable carbon isotopes found in buried organic matter.

Andreas Möller and Noah McLean

Andreas Möller and Noah McLean at the Isotope Geochemistry Laboratory in the KU Department of Geology. Credit: University of Kansas

By comparing results to periods in Earth’s history when global changes in the carbon cycle were known to have occurred, the team showed rocks in the Yellow Cat Member of the Cedar Mountain Formation — and the Utahraptor fossils found within — are 10 million years older than previously known.

Earlier estimates put the age of the rocks and fossils at 125 million years old.

“That’s a lot of evolutionary time,” Ludvigson said. “It’s kind of a vindication of something Jim’s argued for some time, but arguing doesn’t put an absolute age on it, and that’s important to him.”

The revised age indicates the rocks at the Stikes Quarry are at least 135 million years old. The lower part of the Yellow Cat Member encompasses even older strata. The findings narrow the gap in the rock record at the boundary between the Jurassic and Cretaceous Periods in Utah.

“Before, we had a 25-million-year gap between them,” said Kirkland, who first named and described the dinosaur Utahraptor ostrommaysi in 1993. “That’s a third of the age of mammals, more than double the length of time for hominid evolution. It’s a big chunk of time. Anything can happen over 25 million years if you have no record of what’s going on. We’ve plugged that record, for the most part.”

Reference: “Berriasian–Valanginian Geochronology and Carbon-Isotope Stratigraphy of the Yellow Cat Member, Cedar Mountain Formation, Eastern Utah, USA” by Robert M. Joeckel, Celina A. Suarez, Noah M. McLean, Andreas Möller, Gregory A. Ludvigson, Marina B. Suarez, James I. Kirkland, Joseph Andrew, Spencer Kiessling and Garrett A. Hatzell, 26 January 2023, Geosciences.
DOI: 10.3390/geosciences13020032

Additional co-authors are Celina Suarez, University of Arkansas; Noah McLean, Andreas Möller, Marina Suarez, Joseph Andrew and Spencer Kiessling, KU; and Garrett Hatzell, University of Arkansas.

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