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Background  Brachylophosaurini is a clade of hadrosaurine dinosaurs currently known from the Campanian (Late Cretaceous) of North America. Its members include:  Acristavus gagslarsoni , which lacks a nasal crest;  Brachylophosaurus canadensis , which possesses a flat paddle-shaped nasal crest projecting posteriorly over the dorsal skull roof; and  Maiasaura peeblesorum , which possesses a dorsally-projecting nasofrontal crest.  Acristavus , from the lower Two Medicine Formation of Montana (~81–80 Ma), is hypothesized to be the ancestral member of the clade.  Brachylophosaurus  specimens are from the middle Oldman Formation of Alberta and equivalent beds in the Judith River Formation of Montana; the upper Oldman Formation is dated 77.8 Ma.    Methodology/Principal Findings  A new brachylophosaurin hadrosaur,  Probrachylophosaurus bergei  (gen. et sp. nov.) is described and phylogenetically analyzed based on the skull and postcranium of a large individual from the Judith River Formation of northcentral Montana (79.8–79.5 Ma); the horizon is equivalent to the lower Oldman Formation of Alberta. Cranial morphology of  Probrachylophosaurus , most notably the nasal crest, is intermediate between  Acristavus  and  Brachylophosaurus . In  Brachylophosaurus , the nasal crest lengthens and flattens ontogenetically, covering the supratemporal fenestrae in large adults. The smaller nasal crest of  Probrachylophosaurus  is strongly triangular in cross section and only minimally overhangs the supratemporal fenestrae, similar to an ontogenetically earlier stage of  Brachylophosaurus . Sutural fusion and tibial osteohistology reveal that the holotype of  Probrachylophosaurus  was relatively more mature than a similarly large  Brachylophosaurus  specimen; thus,  Probrachylophosaurus  is not simply an immature  Brachylophosaurus .    Conclusions/Significance  The small triangular posteriorly oriented nasal crest of  Probrachylophosaurus  is proposed to represent a transitional nasal morphology between that of a non-crested ancestor such as  Acristavus  and the large flat posteriorly oriented nasal crest of adult  Brachylophosaurus . Because  Probrachylophosaurus  is stratigraphically and morphologically intermediate between these taxa,  Probrachylophosaurus  is hypothesized to be an intermediate member of the  Acristavus - Brachylophosaurus  evolutionary lineage.

plos one

A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana