he recognized three groups within the subfamily. Like Hopson, one group was called the Edmontosaurus lineage, the second the Kritosaurus group, and the third uniting Prosaurolophus and Saurolophus. Over a decade later in 1989, Brett-Surman scientifically named the groups of hadrosaurines, the first becoming Edmontosaurini, the second Kritosaurini, and the third Saurolophini. Phylogeny The first cladistic analysis to encompass the interrelationships of Hadrosauridae was conducted by Weishampel and Horner (1990). They found Saurolophinae synonymous with Hadrosaurinae, but only separated the subfamily into two groups. The first group included Gryposaurus, Aralosaurus, Maiasaura, and Brachylophosaurus. The other contained Edmontosaurus, Saurolophus, Prosaurolophus, Lophorhothon, and Shantungosaurus. A detailed cladgram of hadrosaurid relationships was published in 2013 by Acta Palaeontologica Polonica. The study was led by Alberto Prieto-Márquez, and recovered Prosaurolophus in a similar position as suggested by Brown in 1916. The below cladogram was the one recovered by their analysis: In 2001, Prosaurolophus was studied with other hadrosaurids by Wagner. The genus, along with Corythosaurus and Maiasaura, were considered by Wagner to be synonymous with Saurolophus, Hypacrosaurus and Brachylophosaurus respectively. Prosaurolophus maximus was reassigned to Saurolophus as S. maximus. The same year however, Prosaurolophus was found to be distinct from Saurolophus, in an analysis of Hu et al.. Their analysis was unique from any of the time, and they recovered Prosaurolophus in Saurolophinae, with Saurolophus, Lophorhothon, Tsintaosaurus, Jaxartosaurus, and Kritosaurus. No other analysis has recovered this group of dinosaurs. Horner et al. (2004) also recovered a different phylogeny of Saurolophinae. Prosaurolophus was, for the first time, recovered separate from Saurolophus, in fact not even closely related. Prosaurolophus was found in a group with Brachylophosaurus, Maiasaura, Grpyosaurus, and Edmontosaurus, while Saurolophus was grouped with Naashoibitosaurus (=Kritosaurus) and "Kritosaurus" australis. The Prosaurolophus-Saurolophus clade has been a problematic grouping when trying to place among hadrosaurines. Many skull features are similar to Edmontosaurus, while other are closer to Gryposaurus, so the group has been classified as close to both. However, the clade might be closer to Edmontosaurus, as the features are more numerous uniting them. Paleobiology As a hadrosaurid, Prosaurolophus would have been a large herbivore, eating plants with a sophisticated skull that permitted a grinding motion analogous to chewing. Its teeth were continually replaced and packed into dental batteries that contained hundreds of teeth, only a relative handful of which were in use at any time. Plant material would have been cropped by its broad beak, and