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 66.8 to 65 million years ago[1]). 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 and Wyoming, and the Canadian province of Saskatchewan. It was a large animal, up to approximately 12 metres (39 ft) 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 that includes the genera Anatosaurus, Anatotitan, Claosaurus, Diclonius, Thespesius, and Trachodon.

Description
The skull and skeleton of E. annectens are well-known. Edward Drinker Cope estimated the length of one specimen as about 38 feet (12 m) long, with a skull 3.87 feet (1.18 m) long.[2]* This body length estimate was later revised down to a length of 29 feet (8.8 m),[3] 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.[4] A second skeleton currently exhibited next to Cope's specimen, but in a standing posture, is estimated at 30 feet (9.1 m) long, with its head 17 feet (5.2 m) above the ground.[4] The hip height of this specimen is estimated as approximately 2.1 metres (6.9 ft).[5] Other sources have estimated the length of E. annectens as approximately 12 metres (39 ft).[6][7] Most specimens are somewhat shorter, representing individuals that are not fully grown.[8] Two well-known mounted skeletons, USNM 2414 and YPM 2182, measure 8.00 meters (26.25 ft) long and 8.92 meters (29.3 ft) long, respectively.[3][9] E. annectens may have weighed about 3 metric tons (3.3 tons).[5]

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.[2] 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.[10] The extreme length and breadth did not appear until an individual reached maturity, so many specimens lack the distinctive shape.[8] 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.[11] The ridge on the lower jaw may have reinforced the long, slender structure.[12]

As mounted, the vertebral column of E. annectens includes twelve neck, twelve back, nine sacral, and at least thirty tail vertebrae.[11] 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.[10] 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.[13] Henry Fairfield Osborn used the skeletons in the American Museum of Natural History to portray both quadrupedal and bipedal stances for E. annectens.

Classification
E. annectens was a saurolophine or "flat-headed" hadrosaurid; this group was historically known as Hadrosaurinae.[14] Species now considered to be synonyms of Edmontosaurus annectens were long recognized as closely related to both the genus[15] and the species.[16] However, the skull of the sub-adult type specimen of E. annectens differs noticeably from fully mature remains, so many researchers had classified the two growth stages as different species or even genera. On the other side of the issue, other authors, from John Bell Hatcher in 1902,[16] to Jack Horner, David B. Weishampel, and Catherine Forster in 2004,[13] and most recently Nicolás Campione and David Evans,[8] have proposed that the large, flat-headed specimens most recently classified as Anatotitan copei belong to E. annectens.

E. annectens was also historically classified in an independent genus, Anatosaurus, following the influential 1942 revision of Hadrosauridae by Richard Swann Lull and Nelda Wright, until it was reclassified as a species of Edmontosaurus by Michael K. Brett-Surman.[10] With the discovery that A. copei and E. annectens likely represent the same species, some paleontologists have proposed using Anatosaurus as a valid genus name for E. annectens.