トリケラトプスやトロサウルスの祖先にあたると考えられる、角竜の新種が発表されました。

Newly Discovered Dinosaur Likely Father of Triceratops Triceratops and Torosaurus have long been considered the kings of the horned dinosaurs. But a new discovery traces the giants' family tree further back in time, when a newly discovered species appears to have reigned long before its more well-known descendants, making it the earliest known member of its family. The new species, called Titanoceratops after the Greek myth of the Titans, rivaled Triceratops in size, with an estimated weight of nearly 15,000 pounds and a massive eight-foot-long skull.

ティタノケラトプスと名づけられたこの新種ですが、実は長い間ペンタケラトプスと勘違いされており、再調査によって新種と判断されました。時代としては7400万年前で、白亜紀後期のうちでもカンパニア期と呼ばれる時期のものです。トリケラトプスやトロサウルスが活躍するのは、その後に続く白亜紀最末期のマーストリヒト期なのですが、ティタノケラトプスのサイズはトリケラトプスに匹敵するそうで、この系統の大型化が早くから進んでいたことを示します。 ちなみに角竜の仲間ですが、大きくは二足歩行性のプッシタコサウルス科と四足歩行性のネオケラトプス類に分かれます。1億1000年前（白亜紀前期）の篠山層群から発見された角竜類は、このうちネオケラトプス類の初期のものです。ネオケラトプス類からその後プロトケラトプス科やケラトプス科が分かれ、それらの子孫が北米大陸での繁栄につながります。今回のティタノケラトプスは、このケラトプス科が分岐するセントロサウルス亜科とカスモサウルス亜科のうち、カスモサウルス亜科の初期の段階ということになります。 記載論文は下記。

Titanoceratops ouranous, a giant horned dinosaur from the Late Campanian of New Mexico At the end of the Cretaceous, 65.5 million years ago, the giant ceratopsids Triceratops and Torosaurus dominated North America's dinosaur fauna. The origins of these giant ceratopsids, the Triceratopsini, are poorly understood. This paper describes Titanoceratops ouranos, a giant ceratopsid from the late Campanian (73-74 Ma) of New Mexico, and the earliest known triceratopsin. The holotype was previously interpreted as an aberrant and exceptionally large specimen of Pentaceratops sternbergi, but the animal does not show the diagnostic features of Pentaceratops. Instead, cladistic analysis shows that Titanoceratops is the sister taxon of a clade formed by Eotriceratops, Triceratops, and Torosaurus. With an estimated mass of 6.5 tons, Titanoceratops is the largest dinosaur known from the Campanian of North America, and rivaled Triceratops in size. The recognition of Titanoceratops suggests that giant chasmosaurines evolved once, among the Triceratopsini, and that the group evolved large size five million years earlier than previously thought. The giant horned dinosaurs probably originated in the southern part of the North American continent during the Campanian but only became widespread during the Maastrichtian.