Abstract
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.
Highlights
The pattern of dinosaur diversity leading to the terminal Cretaceous extinction event continues to be hotly debated, with the well-sampled fossil record of North America forming the basis for differing hypotheses [1,2,3,4,5,6]
The Principal Component Analysis (PCA) plot (Figure 4A) reveals that all specimens associated with the positive side of the second axis are from late Maastrichtian time, whereas all of the specimens from the latest Campanian Horseshoe Canyon Formation are negatively associated with the second axis
Thespesius edmontoni is known from rare specimens and is generally smaller the contemporaneous E. regalis
Summary
The pattern of dinosaur diversity leading to the terminal Cretaceous extinction event continues to be hotly debated, with the well-sampled fossil record of North America forming the basis for differing hypotheses [1,2,3,4,5,6]. The number of species recognized varies, all schemes infer an increase in diversity leading up to the end-Cretaceous extinction event. These species occurrences and synonymies are often proposed without reference to particular specimens and lack supporting character data to justify assignments. This confusing taxonomic history has led to uncertainty about the diversity of edmontosaurs, with as many as four morphologically, and presumably ecologically, similar species present in the late Maastrichtian [14]
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