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Saurolophus

Saurolophus (meaning "reptile crest") is a genus of large hadrosaurine duckbill that lived about 70 million years ago, in the Late Cretaceous of North America (Canada) and Asia (Mongolia); 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. The other valid species, S. angustirostris, lived in Asia, and was described by Anatoly Konstantinovich Rozhdestvensky. A third species is considered dubious.

Description

Saurolophus is known from material including nearly complete skeletons, giving us a clear picture of its bony anatomy. S. osborni, the rarer Albertan species, was around 9.8 meters long (32 feet), with its skull a meter long (3.3 feet). It weight is estimated at 1.9 tonnes (2.1 tons). S. angustirostris, the Mongolian species, was larger; the type skeleton is roughly 12 meters long (39.4 ft), and larger remains are reported. Aside from size, the two species are virtually identical, with differentiation hindered by lack of study.

Its most distinctive feature 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 degree 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.

Classification

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 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. The subfamily Saurolophinae is no longer accepted, although a clade within Hadrosaurinae including Saurolophus and a few other genera (including Kritosaurus and Prosaurolophus) is sometimes found; this clade is informally known as Saurolophini. Saurolophus is accepted as a good hadrosaurine, as it has a hadrosaurine pelvis and a (largely) solid crest.

Discovery and history

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 1947-49 Polish-Mongolian Paleontological Expedition recovered the large skeleton that became S. angustirostris as described by A.K. Rozhdestvensky. Other skeletons from a variety of growth stages have also been discovered, and S. angustirostris is now the most abundant Asian hadrosaurid.

Species

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 fifteen specimens. S, kryschtofovici (Riabinin, 1930) is not considered valid; either it is regarded as a dubious name, or as a synonym of S. angustirostris (although it predates S. angustirostris).

Paleoecology

S. osborni shared the Horseshoe Canyon Formation with fellow hadrosaurids Edmontosaurus and hollow-crested Hypacrosaurus, hypsilophodont Parksosaurus, ankylosaurid Euoplocephalus, nodosaurid Edmontonia, horned dinosaurs Montanoceratops, Anchiceratops, Arrhinoceratops, and Pachyrhinosaurus, pachycephalosaurid Stegoceras, ostrich-mimics Ornithomimus and Struthiomimus, a variety of poorly-known small theropods including troodontids and dromaeosaurids, and the tyrannosaurs Albertosaurus and Daspletosaurus. The dinosaurs from this formation are sometimes known as Edmontonian, after a land mammal age, and are distinct from those in the formations above and below. The Horseshoe Canyon 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. S. osborni may have preferred to stay more landward.

S. angustirostris was one of the largest herbivores of the Nemegt Formation, which lacked large horned dinosaurs but had sauropods and a more diverse theropod fauna. It coexisted with the rare crested hadrosaurid Barsboldia, flat-headed pachycephalosaurian Homalocephale and domed Prenocephale, the large ankylosaurid Tarchia, rare saltasaurid sauropods Nemegtosaurus and Opisthocoelicaudia, the alvarezsaurid Mononykus, three types of troodontids including Saurornithoides, several oviraptorosaurians including Rinchenia and Nomingia, the ostrich-mimics Gallimimus and Deinocheirus, therizinosaurid Therizinosaurus, tyrannosaurid relative Bagaraatan, and the tyrannosaurid Tarbosaurus. Unlike other Mongolian formations like the well-known Djadochta Formation that includes Velociraptor and Protoceratops, the Nemegt is interpreted as being well-watered, like the Dinosaur Park Formation in Alberta.

Paleobiology

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 ~4 meter (13 ft) above. Common S. angustirostris would have been an important large herbivore in the Nemegt Formation, but S. osborni was rare in the Horseshoe Canyon Formation and faced competition from other duckbills (Edmontosaurus and Hypacrosaurus).

Crest

The distinctive spike-like crest of Saurolophus has been interpreted in multiple ways, and could have had multiple functions. Brown compared it to the crest of a chameleon, and suggested it could provide an area for muscle attachment and a connection point for a nonbody back frill like that seen in the basilisk lizard. Peter Dodson interpreted similar features in other duckbills as having use in sexual identification. Maryańska and Osmólska, noting the hollow base, suggested that the crest increased the surface area of the respiratory cavity, and helped in thermoregulation. James Hopson supported a function as a visual signal, and further mentioned the possibility that there were inflatable skin flaps over the nostrils that could have acted as resonators and additional visual signals. 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.

References

External links

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