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doi: 10.1371/journal.pone.0025186. Epub 2011 Sep 28.

Cranial growth and variation in edmontosaurs (Dinosauria: Hadrosauridae): implications for latest Cretaceous megaherbivore diversity in North America

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Cranial growth and variation in edmontosaurs (Dinosauria: Hadrosauridae): implications for latest Cretaceous megaherbivore diversity in North America

Nicolás E Campione et al. PLoS One.2011.

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.

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Conflict of interest statement

Competing Interests:The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Type skulls and current biostratigraphic distributions of North American edmontosaurs.
(A) Skeleton reconstruction ofEdmontosaurus regalis. Scale bar, 100 cm. (B) Type skulls of the five named edmontosaur taxa:Thespesius edmontoni (CMN 8399),Edmontosaurus regalis (CMN 2288),E. saskatchewanensis (CMN 8509),E. annectens (YPM 2182, from [15]),Anatotitan copei (AMNH 5730). Scale bar, 20 cm. (C) Biostratigraphic distributions of edmontosaur species (in black) based on published synonymies. Grey bars indicate ghost ranges inferred by proposed taxonomic schemes. Abbreviations: Alta, Alberta; BS, Brett-Surman ; Camp, Campanian; Estd, Eastend Formation; Fr man, Frenchman Formation; HC, Hell Creek Formation; HWF, Horner et al. ; L Sco, Lower Scollard Formation; Lar, Laramie Formation; Lnc, Lance Formation; LW, Lull and Wright ; PM, Prieto-Márquez ; Sask, Saskatchewan; WH, Weishampel and Horner .
Figure 2
Figure 2. Compilation of virtually all known complete edmontosaur skulls from North America.
All skulls are in lateral view (sometimes reversed). Labels below each skull include the symbol used in the morphometric plots, whether the specimen represents a holotype (type), the formation where it was uncovered (HCF, Horseshoe Canyon Formation; HF, Hell Creek Formation; FF, Frenchman Formation; LF, Lance Formation), and the species name based on traditional edmontosaur taxonomy . Scale bar, 20 cm.
Figure 3
Figure 3. Measurements and landmarks used in this study.
(A) The suite of 13 linear measurements taken and used in the principal component analysis and bivariate allometric analyses. Numbers correspond to those indicated in the table of measurements (Table S1). (B) The set of 13 landmarks used in the geometric morphometric analysis.
Figure 4
Figure 4. Multivariate and geometric morphometric results.
(A) Plot of the first and second principal component axes (87.3% of the total variation) from the PCA of linear measurements. Arrows show the direction of increase along a PC axis of particular variables. (B) Plot of the first and second relative warps from the geometric morphometric analysis (65% of the total variation). (C) RMA analysis of relative warp 1 (RW1) against skull size. Solid and dashed lines represent the Maastrichtian and Campanian subsamples, respectively (Table 1). The orange triangle with black outline represents the holotype ofA. copei (AMNH 5730), and the orange triangle represents the largest skull in the dataset (MOR 003), here assigned toA. copei. Abbreviations: diasl, length of edentulous portion of the dentary; pol, postorbital length; prnl, prenarial length; rmw, width of reflected margin of premaxilla; snh, snout height.
Figure 5
Figure 5. Bivariate allometric results.
Bivariate plots and RMA lines for a variety of skull measurements against skull length (proxy for size) comparing late Campanian (circles/dashed lines) and late Maastrichtian (triangles/solid lines). (A) Width of the reflected margin of the premaxilla, (B) snout height, (C) length of prenarial region, (D) maxilla height, (E) Quadrate height, and (F) postorbital length. The orange triangle with black outline represents the holotype ofA. copei (AMNH 5730), and the orange triangle represents the largest skull in the dataset (MOR 003). RMA statistics are presented in table 1.
Figure 6
Figure 6.Edmontosaurus growth series.
Hypothesized growth series for the two recognizedEdmontosaurus species. (A) Specimens from left to right: CMN 8399 (holotype ofT. edmontoni), USNM 12711, ROM 801. (B) Specimens from left to right CMN 8509 (holotype ofE. saskatchewanensis), ROM 57100, MOR 003. Scale bar, 20 cm.
Figure 7
Figure 7. Biostratigraphy and evolution of edmontosaurs and hadrosaurid disparity during latest Cretaceous.
(A) Revised biostratigraphic ranges of edmontosaur species during the latest Cretaceous. Based on our results either a cladogenetic (left) or anagenetic mode (right) of evolution is possible for this genus. (B) Results from the morphometric analysis including virtually all hadrosaurid skulls known from the latest Cretaceous (Figures 2 and S3A). The minimum convex polygons represent specimens known from the three time intervals described in the text. The centroid for each cluster and 95% confidence intervals is marked by an ‘X’ and the dotted lines. (C) Pattern of hadrosaurid morphological disparity, as measured by Foote's Disparity Metric, from 73 to 65 Ma, which shows a significant drop from the early to late Maastrichtian.
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