Saurolophus

Saurolophus (/sɔːˈrɒləfəs/; meaning 'lizard crest') is a genus of large saurolophine hadrosaurid dinosaurs that lived about 70.0–66 million years ago, in the Late Cretaceous of North America and Asia; it is one of the few genera of dinosaurs known from multiple continents. It is distinguished by a spike-like crest which projects up and back from the skull. Saurolophus was a herbivorous dinosaur which could move about either bipedally or quadrupedally. The type species, S. osborni, was described by Barnum Brown in 1912 from Canadian fossils. A second valid species, S. angustirostris, is represented by numerous specimens from Mongolia, and was described by Anatoly Konstantinovich Rozhdestvensky. Barnum Brown recovered the first described remains of Saurolophus in 1911, including a nearly complete skeleton (AMNH 5220). Now on display in the American Museum of Natural History, this skeleton was the first nearly complete dinosaur skeleton from Canada. It was found in rocks of early Maastrichtian age, in the Upper Cretaceous Horseshoe Canyon Formation (then known as the Edmonton Formation) near Tolman Ferry on the Red Deer River in Alberta. Brown wasted little time in describing his material, giving it its own subfamily. Saurolophus was an important early reference for other hadrosaurs, as seen in the names of Prosaurolophus ('before Saurolophus') and Parasaurolophus ('near Saurolophus'). However, little additional material has been recovered and described. Instead, more abundant remains from Asia have provided more data. Initial remains were not promising; a partial fragmentary ischium from Heilongjiang, China, that Riabinin named S. kryschtofovici. Much better remains were soon recovered, though, but from Mongolia's early Maastrichtian-age Nemegt Formation. The 1946–1949 Russian-Mongolian paleontological expeditions recovered the large skeleton that became S. angustirostris as described by Anatoly Rozhdestvensky. Other skeletons from a variety of growth stages have also been discovered, and S. angustirostris is now the most abundant Asian hadrosaurid. Two species are regarded as valid today: the type species S. osborni, and S. angustirostris. S. osborni Brown, 1912 is known from a skull and skeleton, two other complete skulls, and skull fragments. S. angustirostris (Rozhdestvensky, 1952) is known from at least 15 specimens. It differs from S. osborni by some details of the skull, as well as in the pattern of scales found in skin impressions. The Mongolian species had a longer skull (by 20%) and the front of the snout (the premaxillary bones) were more upwardly directed. S. angustirostris also had a distinctive row of rectangular scales along the midline of the back and tail, known as 'midline feature-scales'; these are not currently preserved in S. osborni. In S. angustirostris, the scales on the tail flank were arranged in vertical patterns, which may have corresponded to striped coloration in life. This area was covered in radial scale patterns in S. osborni, possibly indicating a more mottled or spotted coloration. S. kryschtofovici Riabinin, 1930 is not considered valid; either it is regarded as a dubious name, or as a synonym of S. angustirostris (although the name antedates S. angustirostris). Until a 2011 reevaluation of the species by Phil R. Bell, S. angustoristris was not well-described. No autapomorphies, unique derived traits, had been established distinguishing it from S. osborni. Bell found in a publication earlier in the year that the two previous studies of S. angustirostris, by Rozhdestvensky in 1952, and Maryanska and Osmolska in 1981, do not provide a comprehensive enough description to compare the species with S. osborni. In 1939–40, two partial skeletons were found in the late Maastrichtian age Moreno Formation of California. These specimens were referred to cf. Saurolophus sp. In 2010, one of the skulls was instead assigned to Edmontosaurus. A 2013 placed the two specimens in a new species, S. morrisi. In 2014, the species was reassigned to a new genus, Augustynolophus. Saurolophus is known from material including nearly complete skeletons, giving researchers a clear picture of its bony anatomy. S. osborni, the rarer Albertan species, was around 8.2 m (27 ft) long, with its skull 1.0 m (3.3 ft) long. It has been estimated to have weighted around 3 tonnes (3.0 long tons; 3.3 short tons). S. angustirostris, the Mongolian species, was larger; it got as large as 13 m (43 ft) in length, and larger remains are reported. It has been estimated to have potentially weighed up to 11 tonnes (11 long tons; 12 short tons). The largest known skull of S.angustirostris measures 1.22 m (4.0 ft) in length. Aside from size, the two species are virtually identical, with differentiation hindered by lack of study.