Probrachylophosaurus

Probrachylophosaurus is a genus of large herbivorous brachylophosaurin hadrosaurid dinosaur known from the Late Cretaceous (Campanian) Judith River Formation of Montana and the Foremost Formation of Alberta. The type (and only named) 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 are known, and fragmentary remains would later 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 highly distinct genus Maiasaura.

As a hadrosaur, Probrachylophosaurus would have been a large herbivore, 10 metres (33 ft) in length, that walked 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's tallest in the middle, forming a triangular shape as seen from behind. This distinguishes it from Brachylophosaurus, which evolved a longer crest with a flat shape formed by the nasals and the frontals. 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 only known 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 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 that, similar to other hadrosaurs, it had a strategy of fast growth. 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.