Saurolophus angustirostris (PIN 551/8) was discovered during 1946-49 I. A. Efremov-led Polish-Mongolian expeditions to Mongolia's Gobi Desert (Nemegt Formation) and named by Anatoly Rozhdestvensky in 1952. It is the best represented species of Saurolophus, being known from at least 15 specimens of infants to adults, some with skin impressions, from a single bone bed that became known as “the Dragon's Tomb”. Its skull is around 20% longer than that of Saurolophus osborni and has a more up-turned tip of the snout. Remains now known as Barsboldia were once assigned here. Saurolophus kryschtofovici (Riabinin, 1930) is based on a fragmentary hip bone which was discovered in the Village of Jiayin, Tsagayan Group, Tsagayanskaya Svita, Heilongjiang, China. Most paleontologists consider it to be a junior synonym of Saurolophus angustirostris though, funnily enough, it was named first. Saurolophus morrisi is known from holotype "LACM/CIT 2852", a partial skeleton and skull from the Moreno Formation in the Panoche-Tumey Hills of Fresno County, California, and referred specimen LACM/CIT 2760, a partial skeleton from the Moreno Formation in San Benito County, California, which were excavated by Chester Stock in 1939 and 1940. William J. Morris referred them to Saurolophus sp. in 1973. However, in 2010 Bell and Evans argued that the holotype skull was no different to that of Edmontosaurus. In 2013, Prieto-Márquez and Wagner went to great lengths to prove that Morris was right in the first place and named Saurolophus morrisi in his honor. Then they renamed it Augustynolophus in 2014! In 2015, Leonard Dewaele et al. described perinatal specimens of Saurolophus angustirostris, associated with eggshell fragments, likely from a riverbank nest that was carried away by flood water during the wet summer season. The skull length of these babies is around 5% that of adult S. angustirostris specimens, making them the earliest known development stage of this giant hadrosaur.