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Probrachylophosaurus, new hadrosaur from Judith River Formation of Montana
New in PLoS ONE:
Elizabeth A. Freedman Fowler & John R. Horner(2015)
A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an
Intermediate Nasal Crest from the Campanian Judith River Formation of
PLoS ONE 10(11): e0141304.
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.
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.
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.