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Edmontosaurus annectens
''Edmontosaurus annectens'' is a species of flat-headed or saurolophine hadrosaurid ornithopod dinosaur (a "duck-billed dinosaur") from the very end of the Cretaceous Period, in what is now North America. Remains of ''E. annectens'' have been preserved in the Frenchman, Hell Creek, and Lance Formations. All of these formations are dated to the late Maastrichtian stage of the Late Cretaceous Period, representing the last three million years before the extinction of the dinosaurs (between 67 to 66 million years ago〔Holtz, Thomas R. Jr. (2012) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'' (Winter 2011 Appendix. )〕). ''Edmontosaurus annectens'' is known from numerous specimens, including at least twenty partial to complete skulls, discovered in the U.S. states of Montana, South Dakota, North Dakota and Wyoming, Colorado and the Canadian province of Saskatchewan. It was a large animal, up to approximately in length, with an extremely long and low skull. ''E. annectens'' exhibits one of the most striking examples of the "duckbill" snout common to hadrosaurs. It has a long taxonomic history, and specimens have at times been classified in the genera ''Diclonius'', ''Trachodon'', ''Hadrosaurus'', ''Claosaurus'', ''Thespesius'', ''Anatosaurus'', and ''Anatotitan'', before being grouped together in ''Edmontosaurus''.
==Description==
The skull and skeleton of ''E. annectens'' are well-known. Edward Drinker Cope estimated the length of one specimen as about long, with a skull long. This body length estimate was later revised down to a length of , although to be fair to Cope a dozen vertebrae, the hips, and thigh bones had been carried away by a stream cutting through the skeleton, and the tip of the tail was incomplete. A second skeleton currently exhibited next to Cope's specimen, but in a standing posture, is estimated at long, with its head above the ground.〔 The hip height of this specimen is estimated as approximately . Other sources have estimated the length of ''E. annectens'' as approximately . Most specimens are somewhat shorter, representing individuals that are not fully grown. Two well-known mounted skeletons, USNM 2414 and YPM 2182, measure long and long, respectively.〔 ''E. annectens'' may have weighed about 3 metric tons (3.3 tons).〔
The skull of ''E. annectens'' is known for its long, wide muzzle. Cope compared this feature to that of a goose in side view, and to a short-billed spoonbill in top view.〔 The skull was longer and lower proportionally than in any other known hadrosaurid. The toothless portion of the anterior mandible was relatively longer than in any hadrosaur. The extreme length and breadth did not appear until an individual reached maturity, so many specimens lack the distinctive shape.〔 The bones surrounding the large openings for the nostrils formed deep pockets around the openings. The eye sockets were rectangular and longer front to back than top to bottom, although this may have been exaggerated by postmortem crushing. The skull roof was flat and lacked a bony crest, and the quadrate bone that formed the articulation with the lower jaw was distinctly curved. The lower jaw was long and straight, lacking the downward curve seen in other hadrosaurids, and possessing a heavy ridge running its length. The predentary was wide and shovel-like.〔Lull and Wright, ''Hadrosaurian Dinosaurs of North America'', pp. 157-159.〕 The ridge on the lower jaw may have reinforced the long, slender structure.〔Lull and Wright, ''Hadrosaurian Dinosaurs of North America'', pp. 163-164.〕
As mounted, the vertebral column of ''E. annectens'' includes twelve neck, twelve back, nine sacral, and at least thirty tail vertebrae.〔 The limb bones were longer and more lightly built than those of other hadrosaurids of comparable size. ''E. annectens'' had a distinctive pelvis, based on the proportions and form of the pubis bone.〔 ''E. annectens'', like other hadrosaurids, could move both on two legs and on four legs. It probably preferred to forage for food on four legs, but ran on two. Henry Fairfield Osborn used the skeletons in the American Museum of Natural History to portray both quadrupedal and bipedal stances for ''E. annectens''.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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