Supplementary cranial description of the types of Edmontosaurus regalis (Ornithischia: Hadrosauridae), with comments on the phylogenetics and biogeography of Hadrosaurinae.

Hai Xing,Margaret L Currie,Jordan C Mallon,Anthony Fiorillo

doi:10.1371/journal.pone.0175253

Abstract

The cranial anatomy of the flat-skulled hadrosaurine Edmontosaurus regalis (Ornithischia: Hadrosauridae) is extensively described here, based on the holotype and paratype collected from the middle part of the Horseshoe Canyon Formation in southern Alberta. Focus is given to previously undocumented features of ontogenetic and phylogenetic importance. This description facilitates overall osteological comparisons between E. regalis and other hadrosaurids (especially E. annectens), and revises the diagnosis of E. regalis, to which a new autapomorphy (the dorsal half of the jugal anterior process bearing a sharp posterolateral projection into the orbit) is added. We consider the recently named Ugrunaaluk kuukpikensis from the upper Campanian/lower Maastrichtian of Alaska a nomen dubium, and conservatively regard the Alaskan material as belonging to Edmontosaurus sp.. A phylogenetic analysis of Hadrosauroidea using maximum parsimony further corroborates the sister-taxon relationship between E. regalis and E. annectens. In the strict consensus tree, Hadrosaurus foulkii occurs firmly within the clade comprising all non-lambeosaurine hadrosaurids, supporting the taxonomic scheme that divides Hadrosauridae into Hadrosaurinae and Lambeosaurinae. Within Edmontosaurini, Kerberosaurus is posited as the sister taxon to the clade of Shantungosaurus + Edmontosaurus. The biogeographic reconstruction of Hadrosaurinae in light of the time-calibrated cladogram and probability calculation of ancestral areas for all internal nodes reveals a significantly high probability for the North American origin of the clade. However, the Laramidia–Appalachia dispersals around the Santonian–Campanian boundary, inferred from the biogeographic scenario for the North American origin of Hadrosaurinae, are in conflict with currently accepted paleogeographic models. By contrast, the Asian origin of Hadrosaurinae with its relatively low probability resulting from the biogeographic analysis is worth seriously considering, despite the lack of fossil material from the Santonian and lower Campanian of Asia. Extra fossil collecting in appropriate geographic locations and stratigraphic intervals of Asia and Europe will help to clarify the biogeographic dynamics of hadrosaurine dinosaurs in the near future.

Highlights

Hadrosauridae is a derived group of ornithopod dinosaurs, currently known from the Upper Cretaceous (Santonian–Maastrichtian) of Eurasia and America [1,2,3]
100 years after the initial description of the species, we provide an important supplement to the cranial morphology of the types of E. regalis, in order to systematically revise this taxon and make a more detailed assessment of the phylogenetics and biogeography of Hadrosaurinae
All other known Edmontosaurus specimens recovered from the middle portion of the Horseshoe Canyon Formation and Unit 4 of the Wapiti Formation

Summary

Introduction

Hadrosauridae is a derived group of ornithopod dinosaurs, currently known from the Upper Cretaceous (Santonian–Maastrichtian) of Eurasia and America [1,2,3]. Edmontosaurus regalis (the type species of the genus Edmontosaurus) has been recognized as a well-sampled flat-skulled hadrosaurine from North America [7, 8] This species has proved integral to understanding patterns of ornithischian diversity [8], morphological variation of specific structures (such as the jaw muscles and circumnarial depression) in dinosaurs [9, 10], and histological growth dynamics of hadrosaurids [11]. Since the beginning of the 20th century, a large number of E. regalis specimens including the holotype and paratype, together with multiple bonebeds, were documented in the uppermost Campanian Horsethief Member of the Horseshoe Canyon Formation, along the central region of the Red Deer River valley in southern Alberta [13,14,15] (Fig 1). The Liscomb quarry has proved difficult to precisely date because some studies provide inconsistent age estimates (late Campanian versus early Maastrichtian) for the bounding strata [20]

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Results

Conclusion