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Anatotitan

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Anatotitan (meaning "large duck") is a genus of flat-headed or hadrosaurine hadrosaurid ornithopod dinosaur (a "duck-billed dinosaur") from the very end of theCretaceous Period, in what is now North America. Remains of Anatotitan have been preserved in the Hell Creek and Lance Formations, which are dated to the late Maastrichtian stage of the Late Cretaceous Period, representing the last three million years before the extinction of the dinosaurs(68 to 65 million years ago). This dinosaur is known from at least six specimens pertaining to two species, discovered in the U.S. states of South Dakota and Montana. Several of these specimens are extremely complete skeletons with well-preserved skulls. It was a large animal, up to approximately 12 meters (39 ft) in length, with an extremely long and low skull. Anatotitan exhibits one of the most striking examples of the "duckbill" snout common to hadrosaurs. It has a long taxonomic history, including decades classified with the genera Anatosaurus, Diclonius, andTrachodon. The skull and skeleton of Anatotitan are well-known.Edward Drinker Cope estimated the length of what is now the type specimen as about 12 meters (38 ft) long, with a skull 1.18 meters long (3.87 ft).[1] This estimate was later revised downward to a length of 8.8 meters (29 ft),[2] although to be fair a dozen vertebrae, the hips, and thigh bones had been carried away by a stream cutting through it, and the tip of the tail was incomplete.[3] A second skeleton currently exhibited next to the type, but in a standing posture, is estimated at 9.1 meters (30 ft) long, with its head 5.2 meters (17 ft) above the ground.[3] The hip height of this specimen is estimated as approximately 2.1 meters (7 ft).[4] Other sources have estimated the length of Anatotitan as approximately 12 meters (39 ft).[5] Anatotitan may have weighed about 3 metric tons (3.3 tons).[4]

The skull of Anatotitan is known for its long, wide muzzle. Cope compared it to that of a goose in side view, and to a short-billed spoonbill in top view.[1] 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.[6] 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.[7] The ridge on the lower jaw may have reinforced the long, slender jaw.[8]

As mounted, the vertebral column of Anatotitan includes twelve neck, twelve back, nine sacral, and thirty+ tail vertebrae.[7] The limb bones were longer and more lightly built than those of other hadrosaurids of comparable size. Anatotitan had a distinctive pelvis, based on the proportions and form of the pubis bone.[6] Anatotitan, 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.[9] Henry Fairfield Osborn used the skeletons in the American Museum of Natural History to portray both quadrupedal and bipedal stances for Anatotitan.[3] Anatotitan was a hadrosaurine hadrosaurid, as it lacked a hollow crest on its head.[10] It has long been recognized as close toEdmontosaurus,[11] particularly the species now named E. annectens.[12] Anatotitan and Edmontosaurus are so similar that several authors, from John Bell Hatcher in 1902,[12] to Jack Horner, David B. Weishampel, and Catherine Forster in 2004,[9] have proposed that what is now known as the type species of Anatotitan (Anatotitan copei) is a synonym of E. annectens. A. copei and E. annectens also spent several decades in the same genus, Anatosaurus, following the influential 1942 revision of Hadrosauridae by Richard Swann Lull and Nelda Wright, until they were formally put into different genera by Michael K. Brett-Surman.[6] Shantungosaurus is a gigantic hadrosaurine from China which may also be related to these North American dinosaurs.[10] Like many dinosaurs, Anatotitan has a long and somewhat confusing taxonomic history. The holotype, or specimen on which the genus is based, was a complete skull and most of a skeleton collected in 1882 by Dr. J. L. Wortman and R. S. Hill[4] for famous American paleontologistEdward Drinker Cope. This specimen, found in Hell Creek Formation rocks,[13] came from northeast of the Black Hills of South Dakota and originally had extensive skin impressions. It was missing most of the pelvis and part of the torso due to a stream cutting through it. The bill had impressions of a horny sheath with a tooth-like series of interlocking points on the upper and lower jaws.[3] When describing this specimen (now the holotype of Anatotitan copei, AMNH 5730), Cope assigned it to Diclonius mirabilis. This combination was created by combining Diclonius, a hadrosaurid genus based on teeth and described by Cope, with Trachodon mirabilis, an older name based on teeth and published by Joseph Leidy. Cope, believing that Leidy had abandonedTrachodon, assigned the old species to his genus.[1] Leidy had come to recognize that hisTrachodon was based on the remains of multiple kinds of dinosaurs, and had made some attempts to revise the genus, but did not make a formal declaration of his intentions.[14]

Cope's description promoted hadrosaurids as amphibious, contributing to this long-time image.[15] His reasoning was that the teeth of the lower jaw were weakly connected to the bone and liable to break off if used to consume terrestrial food, and he described the beak as weak as well.[1] Unfortunately for Cope, aside from misidentifying several of the bones of the skull,[16] by chance the lower jaws he was studying were missing the walls supporting the teeth from the inside; the teeth were actually well-supported.[15][17] This specimen, AMNH 5730, was purchased by the American Museum of Natural History in 1899.[7] Cope intended to describe the skeleton as well as the skull, but his promised paper never appeared.[14]

[1][2]

Several years after Cope's description, his rival Othniel Charles Marsh published on a sizable lower jaw recovered by John Bell Hatcher in 1889 from Lance Formation rocks in Niobrara County,Wyoming.[8] Marsh named this partial jaw Trachodon longiceps.[18] It is cataloged as YPM 616. As noted by Lull and Wright, this long slender partial jaw shares with Cope's specimen a prominent ridge running on its side. However, it is much larger: Cope's specimen had a dentary, or tooth-bearing bone of the lower jaw, 92.0 centimeters (36.2 in) long, whereas Marsh's dentary was estimated at 110.0 centimeters (43.3 in) long.[8]

A second mostly complete skeleton (AMNH 5886) was found in 1904 in Hell Creek Formation rocks at Crooked Creek in central Montana by Oscar Hunter, a rancher. Upon finding the partially-exposed specimen, he and a companion argued about whether or not the remains were recent or fossil. Hunter demonstrated that they were brittle and thus stone by kicking the tops off the vertebrae, an act later lamented by the eventual collector Barnum Brown. Another cowboy, Alfred Sensiba, bought the specimen from Hunter for a pistol, and later sold it to Brown, who excavated it for the American Museum of Natural History in 1906.[13] This specimen had a nearly complete vertebral column, permitting the restoration of Cope's specimen. In 1907, these two specimens were famously mounted side-by-side in the American Museum of Natural History, under the name Trachodon mirabilis. Cope's specimen is positioned on fours with its head down, as if feeding, because it has the better skull, while Brown's specimen, with a less perfect skull, is posed bipedally with the head less accessible. Henry Fairfield Osborn described the tableau as representing the two animals feeding along a marsh, the standing individual having been startled by the approach of a Tyrannosaurus. Impressions of appropriate plant remains and shells based on associated fossils were included on the base of the group, including ginkgoleaves, Sequoia cones, and horsetail rushes.[3] Taxonomy of hadrosaurids was convoluted by the turn of the 20th century, with different authors proposing schemes splitting and synonymizing genera and species, most of them known from poor material. A consensus briefly emerged around John Bell Hatcher's 1902 work grouping almost all of the hadrosaurs then known (as well as Claorhynchus and Polyonax, now thought to be horned dinosaurs) intoTrachodon. This included Cionodon, Diclonius, Hadrosaurus, Ornithotarsus, Pteropelyx, Thespesius, and a species Marsh had namedClaosaurus annectens in 1892. Hatcher considered Marsh's C. annectens to be the same species as Cope's Diclonius mirabilis, the two differentiated only by individual variation or distortion from pressure.[12] This consensus lasted for a few years, until after 1910 new material from Canada and Montana showed a greater diversity of hadrosaurids than previously suspected.[14] Charles W. Gilmore in 1915 reassessed hadrosaurids and recommended that Thespesius be reintroduced for hadrosaurids from Lance Formation-age rock units, and that Trachodonshould be restricted to a hadrosaurid from the older Judith River Formation and its equivalents. He also recognized that Trachodon was based on inadequate material.[19] A multiplicity of names resumed, with the American Museum duckbills being known as Diclonius mirabilis,Trachodon mirabilis, Trachodon annectens, Claosaurus, or Thespesius.[14]

This confusing situation was temporarily resolved in 1942 by Richard Swann Lull and Nelda Wright. In their monograph on hadrosaurian dinosaurs of North America, they opted to settle the questions revolving around the American Museum duckbills, Marsh's Claosaurus annectens, and several other species by creating a new generic name. They created the new genus Anatosaurus ("duck lizard") and made Marsh's species the type species, calling it Anatosaurus annectens. To this genus, they also assigned Marsh's Trachodon longiceps, a pair of species that had been assigned to Thespesius under Gilmore's "Lance Formation hadrosaurid" conception (T. edmontoni from Gilmore in 1924 and T. saskatchewanensis from Charles M. Sternberg in 1926), and Cope's Diclonius mirabilis.[20] Lull and Wright decided to remove the American Museum specimens from Diclonius (or Trachodon) because they found no convincing reason to assign the specimens to either. Because this left the skeletons without a species name, Lull and Wright gave them their own species, Anatosaurus copei, in honor of Cope. Cope's original specimen (AMNH 5730) was made the holotype of the species, with Brown's (AMNH 5886) as the plesiotype. At this time, only two specimens of Anatosaurus copei were known.[7]

This state of affairs persisted for several decades, until Michael K. Brett-Surman reexamined the pertinent material for his graduate studies in the 1970s and 1980s. He concluded that the type species of Anatosaurus, A. annectens, was actually a species of Edmontosaurus, and that A. copei was different enough to warrant its own genus.[21][22][23] Although theses and dissertations are not regarded as official publications by the International Commission on Zoological Nomenclature, which regulates the naming of organisms, his conclusions were known to other paleontologists, and were adopted by several popular works of the time.[24][25] His replacement name, Anatotitan, was known and published as such in the popular literature by 1990.[26] Formal publication of the name Anatotitan copei took place the same year, in an article co-written by Brett-Surman with Ralph Chapman (although the name is sometimes credited as Brett-Surman vide Chapman and Brett-Surman because it came out of Brett-Surman's work). By this time, three additional specimens were known for A .copei, bringing the total to five. The name Anatotitan is a combination of the Latin anas ("duck") and the Greek Titan, meaning large.[6] The debate about the proper taxonomy of the A. copei specimens continues to the present: returning to Hatcher's argument of 1902, Jack Horner, David B. Weishampel, and Catherine Forster regarded Anatotitan copei as representing specimens of Edmontosaurus annectens with crushed skulls.[9]

Potential second species A. longiceps was first added to Anatotitan by George Olshevsky in 1991.[27] This was accepted by Donald F. Glut(1997),[4] although Weishampel and Horner (1990) regarded it as a potential synonym of Anatotitan copei.[10] As with A. copei, the 2004 Horner-Weishampel-Forster review assigned A. longiceps to Edmontosaurus annectens.[9] As a hadrosaurid, Anatotitan would have been a large herbivore, eating plants with a sophisticated skull that permitted a grinding motion analogous to chewing. Its teeth were continually replaced 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 structure. Feeding would have been from the ground up to around 4 meters (13 ft) above. Like other hadrosaurs, it could have moved bothbipedally and quadrupedally.[9]

The extensive depressions surrounding its nasal openings may hosted nasal diverticula. These postulated diverticula would have taken the form of inflatable soft-tissue sacs. Such sacs could be used for both visual and auditory signals.[31]

Anatotitan BW
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