Prosaurolophus

Prosaurolophus(meaning "before Saurolophus", in comparison to the later dinosaur with a similar head crest) is a genus of hadrosaurid (or duck-billed) dinosaur from the Late Cretaceous of North America. It is known from the remains of at least 25 individuals belonging to two species, including skulls and skeletons, but it remains obscure. Around 9 meters long (29.5 ft), its fossils have been found in the late Campanian-age Upper Cretaceous Dinosaur Park Formation in Alberta, and the roughly contemporaneous Two Medicine Formation in Montana, dating to around 76-75 million years ago.[1] Its most recognizable feature is a small solid crest formed by the nasal bones, sticking up in front of the eyes.

The type species is P. maximus, described by American paleontologist Barnum Brown of the American Museum of Natural History in 1916. A second species, P. blackfeetensis, was described by Jack Horner of the Museum of the Rockies in 1992. The two species are differentiated mainly by crest size and skull proportions.

Description
Prosaurolophus was a large-headed duckbill; the most complete described specimen has a skull around 0.9 meters long (2.9 ft) on a ~8.5 meter long skeleton (~28 ft).[2] It had a small, stout, triangular crest in front of the eyes; the sides of this crest were concave, forming depressions. The upper arm was relatively short.[3]

Otherwise, its anatomy was unremarkable for a hadrosaurine hadrosaurid. Like nearly all hadrosaurids, the whole front of the skull was flat and broadened out to form a beak, ideal for clipping leaves and twigs from the forests of North America. The back of the mouth contained thousands of teeth suitable for grinding food before it was swallowed.

The two species are differentiated by details of the crest, and in profile, P. blackfeetensis is restored with a steeper, taller face than P. maximus. In P. blackfeetensis, at least, the crest migrated backward toward the eyes during growth.

Classification
Because of its name, Prosaurolophus is often associated with Saurolophus. However, this is contentious; some authors have found the animals to be closely related,[5][6] whereas others have not, instead finding it closer to Brachylophosaurus, Edmontosaurus, Gryposaurus, and Maiasaura.[7] It was a hadrosaurine hadrosaur, meaning it lacked a hollow crest.

The cladogram to the right is based on the 2004 review by Jack Horner, David B. Weishampel, and Catherine Forster, in the second edition of The Dinosauria.[7] In this review, Prosaurolophus is closest to Gryposaurus. This is only one of many proposed cladograms for hadrosaurids.