トリケラトプス:三角恐竜の生涯と時代についての事実

2020-07-01
角のある恐竜トリケラトプスは、白亜紀の終わり近くで、約6800万年前に最初に出現しました。トリケラトプスは、約6550万年前に大量絶滅が白亜紀を閉鎖したとき、まだ大規模でした。このモデルは、テキサス州シーダークリークの恐竜公園にあります。Michael / Flickr(CC BY-SA 2.0)
私が子供の頃、これはいつも私のお気に入りでした。今、私は彼女に会います。彼女は私が今まで見た中で最も美しいものです。
アラン・グラント、「ジュラシックパーク」(1993)

Even if your dinosaur phase came and went a few decades ago, you'll probably recognize Triceratops. The frilled, three-horned herbivore died out 65.5 million years ago, but you can find its likeness pretty much anywhere — from museums to toy stores to a National League baseball park.

Our fossil record has been kind to this beast. Hundreds of Triceratops bones have turned up in the American West and they're actually hard to avoid at the fossil-rich Hell Creek Formation of Montana, Wyoming and the Dakotas.

Treasure in the Badlands

In 1887, Othniel Charles Marsh — the U.S. Geological Survey's official vertebrate paleontologist — received a fossilized animal brow with two large horns that a colleague had found near Denver, Colorado.

Unaware of the specimen's age, Marsh figured it came from a prehistoric bison, which he named Bison alticornis. Later discoveries proved the creature was actually a dinosaur far larger than any cow relative. One incomplete skull bearing three horns and a partial beak found its way to Marsh after some Wyoming cowboys had lassoed the thing, snapping off a horn in the process.

Marsh dubbed the animal Triceratops, meaning "three-horned face," in an 1889 scientific paper. Note that this is a genus name; under the system we use to classify living things, a genus is one step above a species. So within the same genus, you can have multiple, closely-related species.

Today, two Triceratops species (both named by Marsh) are considered valid: Triceratops horridus and Triceratops prorsus.

Frills and Extras

Close attention is required to tell these animals apart. All Triceratops had three-horned skulls, with two massive horns above the eye sockets and a smaller one over the nose. However, adult Triceratops prorsus had slightly longer nasal horns than their cousins did. Another difference? The beaks of Triceratops prorsus looked proportionately shorter.

We know both species changed as they aged. And not just in terms of overall body size. A bony frill protruded from the backside of every Triceratops skull. When these animals matured, their frills grew longer.

多くの場合、中世の盾と比較して、これらのフリルは防御的な目的を果たした可能性があります。あるいは、コミュニケーションツールとして進化したのかもしれません。構造は鮮やかに着色されていて、個々の恐竜の健康とスタミナを宣伝している可能性があります。

トリケラトプスの孵化したばかりの子は、目の上にずんぐりした小さな眉の角がありました。これらの角は青年期に伸び、後方に曲がりました。しかし、時間が経つにつれて、劇的な逆転が起こりました。古い標本では、角がまっすぐになり、前に曲がっていました。

成熟は成長の急増を伴いました。非常に若い動物からの最小の既知のトリケラトプスの頭蓋骨は、長さがわずか11.8インチ(30センチメートル)です。長さが6.5フィート(2メートル)をはるかに超える他の回収された頭蓋骨は明らかに成人のものでした。

In an undated photo, Dr. Barnum Brown, left, Associate Curator of fossil reptiles at the University of Wisconsin, and Charles Lang (in charge of preparation and mounting) discuss a Triceratops skeleton from Hell Creek, Montana.

Weighing an estimated 6.5 to 13 tons (6 to 12 metric tons), the biggest Triceratops were 29.5 feet (9 meters) long from nose to tail. The tips of their shoulders would've stood 9.8 feet (3 meters) off the ground.

Unlike alligators and monitor lizards, Triceratops held their arms and legs in a (more or less) vertical position. Yet as Donald R. Prothero notes in his 2019 book "The Story of Life in 25 Fossils: Tales of Intrepid Fossil Hunters and the Wonders of Evolution," most Triceratops fingers didn't point dead ahead. Instead, these hand digits were angled away from the body. (Think "jazz hands.")

Horning In

On top of the U.S. states we've mentioned, Triceratops fossils also occur in west-central Canada. The genus first appeared around 68 million years ago, near the tail end of a geologic period called the Cretaceous. Triceratops was still at large when a mass extinction closed out the Cretaceous 65.5 million years ago.

Thus, Triceratops has the distinction of being one of the last non-avian dinosaurs to ever evolve.

It was also a late-arriving ceratopsian. Distributed across Eurasia and North America, the ceratopsians were a group of plant-eating dinosaurs that predominantly lived in the Cretaceous. Known for their beaks and long, flaring cheek bones, they ranged from bipedal, dog-sized animals to ... well, giant quadrupeds like Triceratops. Many had horns and/or frills, arranged in a spectacular array of different combinations.

Triceratops belonged to the chasmosaurinae, a major subgroup of these dinos characterized mainly by their elongated snouts (among other features).

Large eyebrow horns were another common trait among the chasmosaurines, Triceratops included. Lesions and fractures are regularly seen on Triceratops frill and cheek bones. These could indicate that the animals used their brow horns in one-on-one combat with each other.

Don't Lose Your Head

The jury's still out on whether or not Triceratops moved in herds. But we have a pretty good idea of what the creatures ate. Their teeth were arranged in so-called "dental batteries." Every individual tooth was stacked in a vertical column of three to five teeth. And these formed rows, with 36 to 40 tooth-loaded columns occurring side by side. Altogether, a single Triceratops could have 800 teeth at its disposal.

With its narrow beak, powerful jaws and replaceable (self-sharpening) teeth, Triceratops probably fed on cycads and other tough, fibrous plants.

Ah, but what ate Triceratops? Well, we know Tyrannosaurus rex lived in the same time and place — and it occasionally made a meal of the big herbivore. Bite marks matching T. rex's distinctive teeth have been found on several Triceratops skeletons, with the frills and faces getting more than their fair share of the injuries.

Pointing to these scars, some have argued that T. rex would methodically rip the heads off dead Triceratops. By grabbing ahold of the frills and tugging, the predators could decapitate the bodies, exposing choice neck muscles below. Nothing beats a hard-earned meal.

Now That's Interesting

Torosaurus was another chasmosaurine from the late Cretaceous of North America. Its skull could grow to 8.8 feet (2.7 meters) long, giving it one of the biggest heads of any land animal in Earth's history. A 2010 paper, published in the Journal of Vertebrate Paleontology, argued Torosaurus should be reclassified, claiming the beast didn't really belong in its own, separate genus. Instead, the authors felt the materials we have on record actually came from very old (and very large) Triceratops adults. However, most ceratopsian specialists now dispute that idea.

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