The newly described Probrachylophosaurus bergei, a member of the Brachylophosaurini clade of dinosaurs, has a small flat triangular bony crest extending over the skull and may represent the transition between a non-crested ancestor, such as Acristavus, and the larger crests of adult Brachylophosaurus, according to a study published November 11, 2015 in the open-access journal PLOS ONE by Elizabeth Freedman Fowler and John Horner from Montana State University, USA.
Brachylophosaurini is a clade of hadrosaurine dinosaurs currently known from the Late Cretaceous of North America. Its members include Acristavus gagslarsoni (lived ca. ~81-80 million years ago), which lacks a nasal crest, and Brachylophosaurus canadensis (lived ca. ~77.8 million years ago), which possesses a flat paddle-shaped nasal crest projecting back over the top of its skull. The authors of this study describe a new brachylophosaurin hadrosaur, Probrachylophosaurus bergei, from the Judith River Formation in northcentral Montana, dated to ~79.8-79.5 million years ago, and compared its skull and body shape to related dinosaurs.
Elizabeth Freedman Fowler notes: "This part of the Judith River Formation represents a slice of time intermediate between areas where lots of dinosaur fossils have been collected in the past. The new species that we find here are 'missing links' between known dinosaur species, so it's a really exciting field area."
Probrachylophosaurus' head shape, most notably the bony triangular nasal crest, falls between Acristavus and Brachylophosaurus. The nasal crest of Probrachylophosaurus is proposed to represent a transitional nasal shape between the non-crested ancestor, such as Acristavus, and the large flat posteriorly oriented nasal crest of adult Brachylophosaurus. Because Probrachylophosaurus is estimated to have lived between the time of the Acristavus and Brachylophosaurus, and have a nose shape intermediate between them, Probrachylophosaurus is hypothesized to be an intermediate member of the Acristavus-Brachylophosaurus evolutionary lineage.
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