Probrachylophosaurus is a genus of hadrosaurid dinosaur that lived during the Late Cretaceous in North America. Fossils have been found in rocks of the Judith River Formation in Montana and the Foremost Formation in Alberta, and date to the Campanian age, c. 80.45–79 million years ago. The only recognized species is Probrachylophosaurus bergei. First discovered in Montana in 1981 and not fully excavated until 2008, it was initially suspected to be a new species of Brachylophosaurus. However, in 2015 it was named as a distinct genus, with its name reflecting a suspected ancestral relationship. Both an adult and subadult specimen were originally recognized, and in 2021 fragmentary remains would extend its known range into the Foremost Formation. Thought to be a close relative of the later Brachylophosaurus and Ornatops, and a possible descendant of the older Acristavus, it represents a possible transitional state in brachylophosaurin evolution. Nonetheless, whether this represents a single evolutionary lineage or a series of diverging or convergent branches remains unclear, as does its relation to the genus Maiasaura. As a hadrosaur, Probrachylophosaurus would have been a large herbivore, 10 metres (33 ft) in length, that walked primarily on four legs and possessed a large beak. Like many other hadrosaurs, it was distinguished by its head crest, likely used for display. In Probrachylophosaurus this crest is a short, paddle-shaped extension of the nasal bone projecting over the back of the head. It is tallest in the middle and triangular when seen from behind. This distinguishes it from Brachylophosaurus, which had a longer crest with a flat shape formed by the nasals and the frontal bones. In both genera, the frontals bear long striation marks termed the "nasofrontal sturue", expanded for the sake of supporting the large crest. In Probrachylophosaurus these cover only half of the frontal surface, unlike the later Ornatops and Brachylophosaurus where they expand further and in the latter cover the entire frontal. Aside from the crest, its skull shows an intermediate anatomy between the robust Acristavus and the more lightly built Brachylophosaurus. The most complete specimen possesses several injuries, including a series of fractured tail vertebrae that may have been damaged by interactions with other hadrosaurs. During the Campanian when Probrachylophosaurus lived, Alberta and Montana were part of a wet coastal plain along the Western Interior Seaway, fed by sediments expelled by the growing Rocky Mountains to the west. Being amongst the first group of hadrosaurs to colonize the area upon its exposure by the sea receding, it would have initially lived in the coastal lagoons and estuaries of the Foremost Formation before its ecosystem later changed to be a more terrestrial floodplain with abundant rivers as seen in the Judith River Formation. It would have lived with a variety of other dinosaurs, including a diverse array of centrosaurine ceratopsids and large predatory tyrannosaurids that would have preyed upon it. After the time of Probrachylophosaurus, the area in which it lived would again be covered by the seaway. Histologic analysis indicates fast growth, similar to other hadrosaurs. However, comparisons with the related Maiasaura indicate that Probrachylophosaurus took around twice as long to reach sexual maturity. The oldest known specimen had likely reached over 95% of its adult size upon death at 14 years of age, but did not yet show signs of complete skeletal maturity.
Discovery and naming
In 1981 and 1994, Mark Goodwin of the University of California Museum of Paleontology excavated limb bones and a vertebra near Rudyard in the north of Montana, at a site originally discovered by Kyoko Kishi. After a school class found some more bones, in 2007 and 2008 a team of the Museum of the Rockies secured the remainder of a hadrosaur skeleton, among which the skull. The fossil was donated to the Museum of the Rockies by land owners Nolan and Cheryl Fladstol; and John and Claire Wendland. In 2009, 2010 and 2011, the find was reported in the scientific literature as a possible new species of Brachylophosaurus. In 2015, the type species Probrachylophosaurus bergei was named and described by Elizabeth A. Freedman Fowler and Jack Horner. The generic name is a combination of Latin pro, "before", and Brachylophosaurus and refers to the genus being situated in a lower position in the stratification than its relative Brachylophosaurus. The specific name honours Sam Berge, one of the landowners, and his friends who have supported the research. Probrachylophosaurus was one of eighteen dinosaur taxa from 2015 to be described in open access or free-to-read journals.
Known material The holotype, MOR 2919, was found in a layer of the Judith River Formation dating from the Campanian and being between 79.8 and 79.5 million years old, plus or minus two hundred thousand years. It consists of a partial skeleton with skull, of an adult individual. It contains the right premaxilla, both maxillae, the left jugal, a part of the right lacrimal, the rear of a left nasal bone, the middle part of a right nasal bone, the skull roof from the frontal bones to the exoccipitals, both squamosals, both quadrate bones, the predentary of the lower jaws, both dentaries, a right surangular, eleven neck vertebrae, eleven back vertebrae, twenty-nine tail vertebrae, nineteen chevron bones, nineteen ribs, the entire pelvis, both lower legs, a right second metatarsal and a right fourth metatarsal. The bones have not been found in articulation. Specimen MOR 1097, a fragmentary skull of a subadult individual, was also referred to the species in the 2015 study. It had been found at a kilometre distance from the holotype.
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