Зауролоф: гребнистый динозавр позднего мелового периода.

Hadrosaurid dinosaur genus from the Late Cretaceous period

Saurolophus (/s//ɔː/'/r//ɒ//l//ə//f//ə//s/; meaning "lizard crest") is a genus of large hadrosaurid dinosaur from the Late Cretaceous period of Asia and North America, that lived in what is now the Horseshoe Canyon and Nemegt formations about 70 million to 66 million years ago. It is one of the few genera of dinosaurs known from multiple continents. 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. Saurolophus is distinguished by a spike like crest which projects up and back from the skull. It was a herbivorous dinosaur which could move about either bipedally or quadrupedally.

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. 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. kryschtofovici (Riabinin, 1930) is not considered valid; either it is regarded as a dubious name, A 2013 study placed the two specimens in a new species, S. morrisi.

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 long, with its skull long. S. angustirostris, the Mongolian species, was larger; it got as large as in length, and larger remains are reported. It has been estimated to have potentially weighed up to .

The most distinctive feature of Saurolophus is its cranial crest, which is present in young individuals, but is smaller. It is long and spike like and projects upward and backward at about a 45° angle, starting from over the eyes. This crest is often described as solid, but appears to be solid only at the point, with internal chambers that may have had a respiratory and/or heat regulation function. The unique crest of Saurolophus is made up almost completely by the nasal bones, and in S. angustirostris it is solid. In adult specimens the crests are a rounded triangular shape in cross section. The crest protrudes past the edge of the skull backwards. Thin processes from the frontals and prefrontals extend along the underside of the crest, probably to strengthen it. At the end of the crest is a swelling of the nasal, which is often termed differently.

Barnum Brown, who described the first specimens, put it in its own subfamily in "Trachodontidae" (=Hadrosauridae), the Saurolophinae. At the time, this also included Corythosaurus and Hypacrosaurus, the only well known examples of what would become the Lambeosaurinae. Brown thought that Saurolophus had an expanded tip to the ischium bone in the hip, as dinosaurs now recognized as lambeosaurines had, but this appears to have been based on a mistakenly associated lambeosaurine ischium. Additionally, he misinterpreted the crests of Saurolophus and lambeosaurines as being made of the same bones. Most publications before 2010 classified Saurolophus as a member of Hadrosaurinae, often known colloquially as the "flat headed hadrosaurs". In 2010, the subfamily Saurolophinae was brought back into use because Hadrosaurus appears to have branched off prior to the "hadrosaurine"–lambeosaurine split. As a result, Hadrosaurinae by definition cannot include the traditional "hadrosaurines". Saurolophinae is the oldest available name for the former "hadrosaurine" clade. Saurolophus, as the name suggests, is a saurolophine, as it has a saurolophine pelvis and a (largely) solid crest. The following cladogram of hadrosaurid relationships was published in 2013 by Alberto Prieto Márquez et al. in Acta Palaeontologica Polonica:

As a hadrosaurid, Saurolophus would have been a bipedal/quadrupedal herbivore, eating a variety of plants. Its skull permitted a grinding motion analogous to chewing, and its teeth were continually replacing 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 held in the jaws by a cheek like organ. Its feeding range would have extended from the ground to about above. Maryańska and Osmólska, noting the hollow base, suggested that the crest increased the surface area of the respiratory cavity, and helped in thermoregulation. This idea has been picked up by authors of popular dinosaur works, such as David B. Norman, who discussed hadrosaurid display at length and included a life restoration of such an adaptation in action.

In 2015 Leonard Dewaele and colleagues described a small and partial nest containing several juveniles of S. angustirostris. The specimen (MPC D 100/764) was recovered from the notorious Dragon's Tomb assambleage of the Nemegt Formation. The team noted that among remains, three or even four juveniles can be recognized, and two fragmentary eggshells were found in association. Juveniles within this block were identified as perinates, as they had skull lengths less than five percent of the length of the skulls of the adults, indicating they were in the earliest developmental stage at the time of their deaths. Based on these juveniles, Dewaele and team indicated that during the ontogeny of S. angustirostris the distinct crest found in adults was poorly developed in infancy, the snout grew proportionally longer, the orbit became more oval shaped, the doming of the frontal became less prominent, and the coronoid process became higher.

Bell and team in 2018 described the famous Dragon's Tomb assambleage of the Altan Uul II locality, Nemegt Formation, which contains a large sized bonebed of S. angustirostris. This bonebed is largely monodominant (one dominant species), with at least three size classes (juveniles, subadults, and adults) of S. angustirostris. Examinations made to Dragon's Tomb suggest that at least 21 Saurolophus individuals can be currently found. The team indicated that this bonebed has a minimum size of about 2000 m2, which suggest that over 100 Saurolophus carcasses may have contributed to the event. However, they discussed that even though evidence clearly reflects a catastrophic mass mortality of a social group of S. angustirostris and provide the first evidence of gregariousness in this taxon, the exact conditions and cause surrounding the group death can not be determined. Bell and team also noted that while Dragon's Tomb provides direct evidence for social behaviour in S. angustirostris, there is yet no evidence for it in S. osborni. Nevertheless, gregariousness is apparently widespread in hadrosaurines.

David W. E. Hone and Mahito Watabe in 2011 reported the left humerus of a nearly complete S. angustirostris skeleton (MPC D 100/764) from the Bügiin Tsav locality of the Nemegt Formation, which was heavily damaged from bite marks attributed to the sympatric Tarbosaurus. As suggested by the lack of damage to the rest of the skeleton (such as large wounds in skeletal remains indicative of predation), this tyrannosaurid was likely scavenging an already dead S. angustirostris. It is unlikely that a large bodied predator such as Tarbosaurus would have left sparse feeding traces on a single humerus having an entire carcass to feed on. The humerus shows three distinctive feeding methods, interpreted as punctures, drag marks, and bite−and−drag marks. Hone and Watabe noted that bite marks were mostly located at the deltopectoral crest, suggesting that this Tarbosaurus was actively selecting which biting style employ to scavenge the bone.

Comparisons between the scleral rings of Saurolophus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day at short intervals.

S. osborni is known only from the upper part (unit 4) of the Horseshoe Canyon Formation. The formation is interpreted as having a significant marine influence, due to an encroaching Western Interior Seaway, the shallow sea that covered the midsection of North America through much of the Cretaceous. The dinosaurs from this formation form part of the Edmontonian land vertebrate age. A 2001 study suggested that Saurolophus osborni was part of a distinct inland fauna characterized by an association between Anchiceratops ornatus and it, while the contemporary coastal fauna was characterized by the association of Pachyrhinosaurus canadensis and Edmontosaurus regalis. However, the association between S. osborni and Anchiceratops was later noted to be in error, Anchiceratops only occurs lower in the Horseshoe Canyon Formation, before the major transgression of the Western Interior Seaway represented by the Drumheller Marine Tongue.

S. angustirostris was one of the largest herbivores of the Nemegt Formation, which lacked large ceratopsians, but had sauropods and a more diverse theropod fauna. Unlike other Mongolian formations like the well known Djadochta Formation that includes Velociraptor and Protoceratops, the Nemegt is interpreted as being a well watered region, like the Dinosaur Park Formation in Alberta. It coexisted with the rare hadrosaurid Barsboldia, flat headed pachycephalosaurian Homalocephale and domed Prenocephale, the large ankylosaurid Saichania, rare titanosaurs sauropods Nemegtosaurus and Opisthocoelicaudia, the alvarezsaurid Mononykus, three types of troodontids including Zanabazar, several oviraptorosaurians including Rinchenia and Nemegtomaia, the ornithomimosaurs Anserimimus and Gallimimus, and the giant theropods Deinocheirus and Therizinosaurus, including the tyrannosaurid Tarbosaurus.