Cranial Growth and Variation in Edmontosaurs (Dinosauria: Hadrosauridae): Implications for Latest Cretaceous Megaherbivore Diversity in North America

The well-sampled Late Cretaceous fossil record of North America remains the only high-resolution dataset for evaluating patterns of dinosaur diversity leading up to the terminal Cretaceous extinction event. Hadrosaurine hadrosaurids (Dinosauria: Ornithopoda) closely related to Edmontosaurus are among the most common megaherbivores in latest Campanian and Maastrichtian deposits of western North America. However, interpretations of edmontosaur species richness and biostratigraphy have been in constant flux for almost three decades, although the clade is generally thought to have undergone a radiation in the late Maastrichtian. We address the issue of edmontosaur diversity for the first time using rigorous morphometric analyses of virtually all known complete edmontosaur skulls. Results suggest only two valid species, Edmontosaurus regalis from the late Campanian, and E. annectens from the late Maastrichtian, with previously named taxa, including the controversial Anatotitan copei, erected on hypothesized transitional morphologies associated with ontogenetic size increase and allometric growth. A revision of North American hadrosaurid taxa suggests a decrease in both hadrosaurid diversity and disparity from the early to late Maastrichtian, a pattern likely also present in ceratopsid dinosaurs. A decline in the disparity of dominant megaherbivores in the latest Maastrichtian interval supports the hypothesis that dinosaur diversity decreased immediately preceding the end Cretaceous extinction event.

北美采样完备的晚白垩世化石记录，仍是评估白垩纪末灭绝事件前夕恐龙多样性格局的唯一高分辨率数据集。与埃德蒙顿龙（Edmontosaurus）亲缘关系密切的鸭嘴龙亚科（Hadrosaurinae）鸭嘴龙类（恐龙总目：鸟脚亚目），是北美西部坎潘期晚期与马斯特里赫特期地层中最常见的巨型植食性恐龙类群之一。然而近三十年来，学界对埃德蒙顿龙物种丰富度与生物地层学的解读始终处于动态变化之中，尽管该演化支通常被认为在马斯特里赫特期晚期发生了演化辐射。本研究首次通过对几乎所有已知完整埃德蒙顿龙头骨开展严格的形态测量分析，探讨了埃德蒙顿龙的多样性问题。研究结果表明仅存在两个有效物种：产自坎潘期晚期的君王埃德蒙顿龙（Edmontosaurus regalis），以及产自马斯特里赫特期晚期的安氏埃德蒙顿龙（Edmontosaurus annectens）；此前命名的类群（包括存在争议的科佩异栉龙（Anatotitan copei））均被认为是基于个体发育体型增长与异速生长相关的过渡形态特征建立的。对北美鸭嘴龙类类群的修订显示，从马斯特里赫特期早期至晚期，鸭嘴龙类的多样性与形态差异度均出现下降，这一格局在角龙科（Ceratopsidae）恐龙中大概率同样存在。马斯特里赫特期最晚期优势巨型植食类群的形态差异度下降，支持了恐龙多样性在白垩纪末灭绝事件前夕即刻下降的假说。