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

BackgroundBrachylophosaurini 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 FindingsA 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/SignificanceThe 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.背景 短冠龙族（Brachylophosaurini）是鸭嘴龙亚科（hadrosaurine）的一个演化支，目前仅发现于北美晚白垩世坎潘期（Campanian）地层。该类群的成员包括：无冠龙（Acristavus gagslarsoni），不具有鼻脊（nasal crest）；短冠龙（Brachylophosaurus canadensis），拥有向后延伸至颅顶上方的扁平桨状鼻脊；以及慈母龙（Maiasaura peeblesorum），具有向背侧突出的鼻额脊。产自蒙大拿州下双药组（Two Medicine Formation，约81 Ma至80 Ma）的无冠龙，被推测为该演化支的祖先类群。短冠龙标本发现于阿尔伯塔省中部长老曼组（Oldman Formation）以及蒙大拿州朱迪思河组（Judith River Formation）的同期地层；上部长老曼组的年代为77.8 Ma。 材料与方法/主要发现 本文描述并进行系统发育分析了一种新的短冠龙族鸭嘴龙类——伯杰前短冠龙（Probrachylophosaurus bergei，gen. et sp. nov.，新属新种），其研究材料为产自蒙大拿州中北部朱迪思河组（79.8 Ma至79.5 Ma）的一具大型个体的头骨与颅后骨骼（postcranium）；该地层相当于阿尔伯塔省的下老曼组。前短冠龙的颅骨形态，尤其是鼻脊，处于无冠龙与短冠龙之间。在短冠龙中，鼻脊会随个体发育（ontogenetically）逐渐变长、变扁平，大型成年个体的鼻脊可覆盖颞上窝（supratemporal fenestrae）。前短冠龙的鼻脊体积更小，横截面呈显著三角形，仅轻微覆盖颞上窝，与短冠龙个体发育早期的鼻脊形态相似。通过骨缝愈合（sutural fusion）情况与胫骨组织学（tibial osteohistology）分析可知，前短冠龙的正模标本（holotype）的成熟度高于一具体型相近的短冠龙标本；因此，前短冠龙并非仅仅是未成熟的短冠龙。 结论与意义 伯杰前短冠龙小型的三角形向后延伸鼻脊，被认为是介于无顶饰祖先类群（如无冠龙）与成年短冠龙大型扁平向后延伸鼻脊之间的过渡形态。由于前短冠龙在地层分布与形态特征上均处于这两个类群之间，因此被推测为无冠龙-短冠龙演化谱系中的过渡类群。