Edmontosaurus

***Edmontosaurus*** (***ed·mon·to·saur·us***) sometimes known as the disputed ***Anatosaurus***or ***Anatotitan*** (referring to the secondary species *E. annectens*), was a large hadrosaur from the late Cretaceous Period. It lived in North America alongside dinosaurs such as Tyrannosaurus rex, Ornithomimus, Pachycephalosaurus, Struthiomimus, Ankylosaurus, and Triceratops.

*Edmontosaurus* were incredibly large hadrosaurs, one of largest in North America. Although not as large as their Asian counterparts such as *Shantungosaurus* reached an incredible length of 13 metres (42 feet) and was about 3.5 metres high at the hip. Rearing up on their powerful hind legs they stood higher than 4 meters (13-14 feet) tall.

Multiple specimens of *Edmontosaurus annectens* have been found with preserved skin impressions. Several have been well-publicized, such as the "Trachodon mummy" of the early 20th century, and the specimen nicknamed "Dakota", the latter apparently including remnant organic compounds from the skin. Because of these finds, the scalation of *Edmontosaurus annectens* is known for most areas of the body. Skin impressions are less well known for *E. regalis*, but some well-preserved examples have been studied, including one which preserves a soft tissue crest or wattle on the head. It is unknown whether such a crest was present on *E. annectens*, and whether it was an indicator of sexual dimorphism.

Like other hadrosaurids, *Edmontosaurus* is thought to have been a facultative biped, meaning that it mostly moved on four legs, but could adopt a bipedal stance when needed. It probably went on all fours when standing still or moving slowly, and switched to using the hind legs alone when moving more rapidly. Research conducted by computer modeling in 2007 suggests that *Edmontosaurus* could run at high speeds, perhaps up to 45 kilometres per hour (28 mph). Further simulations using a subadult specimen estimated as weighing 715 kilograms (1,576 lbs) when alive produced a model that could run or hop bipedally, use a trot, pace, or single foot symmetric quadrupedal gait, or move at a gallop. The researchers found to their surprise that the fastest gait was kangaroo-like hopping (maximum simulated speed of 17.3 metres per second (62 km/h; 39 mph), which they regarded as unlikely based on the size of the animal and lack of hopping footprints in the fossil record, and instead interpreted the result as indicative of an inaccuracy in their simulation. The fastest non-hopping gaits were galloping (maximum simulated speed of 15.7 metres per second (57 km/h; 35 mph)) and running bipedally (maximum simulated speed of 14.0 metres per second (50 km/h; 31 mph)). They found weak support for bipedal running as the most likely option for high-speed movement, but did not rule out high-speed quadrupedal movement.

While long thought to have been aquatic or semiaquatic, hadrosaurids were not as well-suited for swimming as other dinosaurs (particularly theropods, who were once thought to have been unable to pursue hadrosaurids into water). Hadrosaurids had slim hands with short fingers, making their forelimbs ineffective for propulsion, and the tail was also not useful for propulsion because of the ossified tendons that increased its rigidity, and the poorly developed attachment points for muscles that would have moved the tail from side to side.

The time span and geographic range of *Edmontosaurus* overlapped with *Tyrannosaurus*, and an adult specimen of *E. annectens* on display in the Denver Museum of Nature and Science shows evidence of a theropod bite in the tail. Counting back from the hip, the thirteenth to seventeenth vertebrae have damaged spines consistent with an attack from the right rear of the animal. One spine has a portion sheared away, and the others are kinked; three have apparent tooth puncture marks. The top of the tail was at least 2.9 metres (9.5 feet) high, and the only theropod species known from the same rock formation that was tall enough to make such an attack is *T. rex*. The bones are partially healed, but the edmontosaur died before the traces of damage were completely obliterated. The damage also shows signs of bone infection. Kenneth Carpenter, who studied the specimen, noted that there also seems to be a healed fracture in the left hip which predated the attack because it was more fully healed. He suggested that the edmontosaur was a target because it may have been limping from this earlier injury. Because it survived the attack, Carpenter suggested that it may have outmaneuvered or outrun its attacker, or that the damage to its tail was incurred by the hadrosaurid using it as a weapon against the tyrannosaur.

Another specimen of *E. annectens*, pertaining to a 7.6 metres (25 feet) long individual from South Dakota, shows evidence of tooth marks from small theropods on its lower jaws. Some of the marks are partially healed. Michael Triebold, informally reporting on the specimen, suggested a scenario where small theropods attacked the throat of the edmontosaur; the animal survived the initial attack but succumbed to its injuries shortly thereafter.Some edmontosaur bone beds were sites of scavenging. *Albertosaurus* and *Saurornitholestes* tooth marks are common at one Alberta bone bed, and *Daspletosaurus* fed on ***Edmontosaurus*** and fellow hadrosaurid *Saurolophus* at another Alberta site.

Extensive bone beds are known for *Edmontosaurus*, and such groupings of hadrosaurids are used to suggest that they were gregarious, living in groups. Three quarries containing *Edmontosaurus* remains are identified in a 2007 database of fossil bone beds, from Alberta (Horseshoe Canyon Formation), South Dakota (Hell Creek Formation), and Wyoming (Lance Formation). One edmontosaur bone bed, from claystone and mudstone of the Lance Formation in eastern Wyoming, covers more than a square kilometre, although *Edmontosaurus* bones are most concentrated in a 40 hectares (0.15 sq mi) subsection of this site. It is estimated that disassociated remains pertaining to 10,000 to 25,000 edmontosaurs are present here.

Unlike many other hadrosaurids, *Edmontosaurus* lacked a bony crest. It may have had soft-tissue display structures in the skull, though: the bones around the nasal openings had deep indentations surrounding the openings, and this pair of recesses are postulated to have held inflatable air sacs, perhaps allowing for both visual and auditory signaling. *Edmontosaurus* may have been dimorphic, with more robust and more lightly built forms, but it has not been established if this is related to sexual dimorphism.

*Edmontosaurus* has been considered a possibly migratory hadrosaurid by some authors. A 2008 review of dinosaur migration studies by Phil R. Bell and Eric Snively proposed that *E. regalis* was capable of an annual 2,600 kilometres (1,600 mi) round-trip journey, provided it had the requisite metabolism and fat deposition rates. Such a trip would have required speeds of about 2 to 10 kilometres per hour (1 to 6 mph), and could have brought it from Alaska to Alberta. In contrast to Bell and Snively, Anusuya Chinsamy and colleagues concluded from a study of bone microstructure that polar *Edmontosaurus* overwintered.