Most groups of animals have many small species, somewhat fewer medium-size species, and even fewer large species. In contrast, the extinct dinosaurs—especially carnivores—had plenty of species no bigger than modern-day chickens and also many giant species, but few medium-size ones. Paleontologists have wondered whether juvenile dinosaurs crowded out medium-size adults by exploiting the habitats and food sources those species might have taken. To test this idea, researchers, combed a global collection of fossil data to determine the size distribution of more than 550 dinosaur species in 43 ancient ecosystems across 136 million years and seven continents. They then modeled the role that juvenile carnivores like Tyrannosaurus rex might have played in ecosystems, finding that the teenagers may have crowded out competing species.

大多数动物群都有许多小型物种，中型物种较少，大型物种甚至更少。相比之下，已灭绝的恐龙——尤其是食肉动物——有很多不比现代鸡大的物种，也有许多巨型物种，但中型的很少。古生物学家想知道幼年恐龙是否通过利用这些物种可能已经采取的栖息地和食物来源来排挤中等大小的成年人。为了验证这一想法，研究人员梳理了全球收集的化石数据，以确定 550 多种恐龙物种在 1.36 亿年和七大洲的 43 个古代生态系统中的大小分布。然后他们模拟了像霸王龙这样的幼年食肉动物的角色可能在生态系统中玩耍，发现青少年可能已经排挤了竞争物种。