Edmontosaurus annectens

''Edmontosaurus annectens'' (meaning "connected lizard from Edmonton") is a species of flat-headed and duck-billed (hadrosaurid) 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 68 and 66 million years agoHoltz, Thomas R. Jr. (2012) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2011 Appendix.
/ref>). ''E. annectens'' is also found in the Laramie Formation, and magnetostratigraphy suggests an age of 69-68 Ma for the Laramie Formation.*Hicks, J.F., Johnson, K.R., Obradovich, J. D., Miggins, D.P., and Tauxe, L. 2003. Magnetostratigraphyof Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin, Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. II., Rocky Mountain Geology 38: 1-27. ''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, Wyoming and Colorado and the Canadian province of Saskatchewan. It had an extremely long and low skull and was a large animal, up to approximately in length and in average asymptotic body mass, and may have been even larger. ''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''.

Discovery and history

''E. annectens'' has a complicated taxonomic history, with various specimens having been classified in a variety of genera. Its history involves ''Anatosaurus'', ''Anatotitan'', '' Claosaurus'', '' Diclonius'', '' Hadrosaurus'', ''Thespesius'', and '' Trachodon'', as well as ''Edmontosaurus''. References predating the 1980s typically use ''Anatosaurus'', ''Claosaurus'', ''Diclonius'', ''Thespesius'', or ''Trachodon'' for ''E. annectens'' fossils, depending on author and date.

Cope's ''Diclonius mirabilis''

The history of ''E. annectens'' predates both ''Edmontosaurus'' and ''annectens''. The first good specimen, the former holotype (specimen on which a taxonomic name is based) of ''Anatosaurus copei'' (''Anatotitan''), was a complete skull and most of a skeleton collected in 1882 by Dr. J. L. Wortman and R. S. Hill for American paleontologist Edward Drinker Cope. This specimen, found in Hell Creek Formation rocks, 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. When describing this specimen AMNH 5730, Cope assigned it to the species ''Diclonius mirabilis''. This species name was created by combining '' Diclonius'', a hadrosaurid genus Cope had named earlier from teeth, with '' Trachodon mirabilis'', an older name based on teeth and published by Joseph Leidy. Cope believed that Leidy had failed to properly characterize the genus ''Trachodon'' and later abandoned its use, so he assigned the old species to his newer genus. Leidy had come to recognize that his ''Trachodon'' was based on the remains of multiple kinds of dinosaurs, and though he had made some attempts to revise the genus, he had not yet made any formal declaration of his intentions.

Cope's description promoted hadrosaurids as amphibious, contributing to this long-time image. 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. However, aside from misidentifying several of the skull bones, by chance the lower jaws were missing the walls supporting the teeth from the inside; the teeth were actually well-supported.Lull and Wright, ''Hadrosaurian Dinosaurs of North America'', pp. 43. Cope intended to describe the skeleton as well as the skull, but his promised paper never appeared. It was purchased for the American Museum of Natural History in 1899, where it acquired its present designation AMNH 5730.

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. Marsh named this partial jaw ''Trachodon longiceps''. 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, that is long, whereas Marsh's dentary is estimated at long.

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. This specimen had a nearly complete vertebral column, permitting the restoration of Cope's specimen. In 1908, these two specimens were mounted side by side in the American Museum of Natural History, under the name ''Trachodon mirabilis''. Cope's specimen is positioned on all 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 ginkgo leaves, '' Sequoia'' cones, and horsetail rushes.

Marsh's ''Claosaurus annectens''

The species now known as ''Edmontosaurus annectens'' was named in 1892 as ''Claosaurus annectens'' by Othniel Charles Marsh. This species is based on USNM 2414, a partial skull-roof and skeleton, with a second skull and skeleton, YPM 2182, designated the paratype. Both were collected in 1891 by John Bell Hatcher from the late Maastrichtian-age Upper Cretaceous Lance Formation of Niobrara County (then part of Converse County), Wyoming. This species has some historical footnotes attached: it is among the first dinosaurs to receive a skeletal restoration, and is the first hadrosaurid so restored; and YPM 2182 and UNSM 2414 are, respectively, the first and second essentially complete mounted dinosaur skeletons in the United States. YPM 2182 was put on display in 1901, and USNM 2414 in 1904.

In the first decade of the twentieth century, two additional important specimens of ''C. annectens'' were recovered. The first, the "Trachodon mummy" (AMNH 5060), was discovered in 1908 by Charles Hazelius Sternberg and his sons in Lance Formation rocks near Lusk, Wyoming. Sternberg was working for the British Museum of Natural History, but Henry Fairfield Osborn of the American Museum of Natural History was able to purchase the specimen for $2,000. The Sternbergs recovered a second similar specimen from the same area in 1910, not as well-preserved but also found with skin impressions. They sold this specimen (SM 4036) to the Senckenberg Museum in Germany.

Canadian discoveries

''Edmontosaurus'' itself was coined in 1917 by Lawrence Lambe for two partial skeletons found in the Horseshoe Canyon Formation (formerly the lower Edmonton Formation) along the Red Deer River of southern Alberta, Canada. The Horseshoe Canyon Formation is older than the rocks in which ''Claosaurus annectens'' was found. Lambe found that his new dinosaur compared best to Cope's ''Diclonius mirabilis''.

In 1926, Charles Mortram Sternberg named ''Thespesius saskatchewanensis'' for NMC 8509, a skull and partial skeleton from the Wood Mountain plateau of southern Saskatchewan. He had collected this specimen in 1921, from rocks that were assigned to the Lance Formation, now the Frenchman Formation. NMC 8509 included an almost complete skull, numerous vertebrae, partial shoulder and hip girdles, and partial hind limbs, representing the first substantial dinosaur specimen recovered from Saskatchewan. Sternberg opted to assign it to ''Thespesius'' because that was the only hadrosaurid genus known from the Lance Formation at the time. At the time, ''T. saskatchewanensis'' was unusual because of its small size, estimated at in length.

Early classifications

Because of the incomplete understanding of hadrosaurids at the time, following Marsh's death in 1899 ''Claosaurus annectens'' was variously classified as a species of ''Claosaurus'', ''Thespesius'' or ''Trachodon''. Opinions varied greatly; textbooks and encyclopedias drew a distinction between the "''Iguanodon''-like" ''Claosaurus annectens'' and the "duck-billed" ''Hadrosaurus'' (based on Cope's ''Diclonius mirabilis''), while Hatcher explicitly identified ''C. annectens'' as synonymous with the hadrosaurid represented by those same duck-billed skulls, the two differentiated only by individual variation or distortion from pressure. Hatcher's revision, published in 1902, was sweeping: he considered almost all hadrosaurid genera then known as synonyms of ''Trachodon''. This included '' Cionodon'', ''Diclonius'', ''Hadrosaurus'', '' Ornithotarsus'', ''Pteropelyx'', and ''Thespesius'', as well as '' Claorhynchus'' and ''Polyonax'', fragmentary genera now thought to be horned dinosaurs. Hatcher's work led to a brief consensus until about 1910, when new material from Canada and Montana showed a greater diversity of hadrosaurids than previously suspected. Charles W. Gilmore in 1915 reassessed hadrosaurids and recommended that ''Thespesius'' be reintroduced for hadrosaurids from the Lance Formation and rock units of equivalent age, and that ''Trachodon'', based on inadequate material, should be restricted to a hadrosaurid from the older Judith River Formation and its equivalents. In regards to ''Claosaurus annectens'', he recommended that it be considered the same as ''Thespesius occidentalis''. A multiplicity of names resumed, with the American Museum duckbills being known as ''Diclonius mirabilis'', ''Trachodon mirabilis'', ''Trachodon annectens'', ''Claosaurus'', or ''Thespesius''.

''Anatosaurus'' to the present

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", because of its wide, duck-like beak; Latin ''anas'' = duck + Greek ''sauros'' = 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''), and Cope's ''Diclonius mirabilis''.Lull and Wright, ''Hadrosaurian Dinosaurs of North America'', pp. 154–164. 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. ''Anatosaurus'' would come to be called the "classic duck-billed dinosaur."

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. The name ''Edmontosaurus annectens'' was first coined some time in the 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. 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. His replacement name, ''Anatotitan'' (Latin ''anas'' ("duck") and the Greek ''Titan'', meaning large), was known and published as such in the popular literature by 1990. 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). Because the type species of ''Anatosaurus'' (''A. annectens'') was sunk into ''Edmontosaurus'', the name ''Anatosaurus'' is abandoned as a junior synonym of ''Edmontosaurus''.

Of the remaining species of ''Anatosaurus'', ''A. saskatchewanensis'' and ''A. edmontoni'' were assigned to ''Edmontosaurus'' as well, and ''A. longiceps'' went to ''Anatotitan'', as either a second species or as a synonym of ''A. copei''. ''A. longiceps'' may be a synonym of ''E. annectens'', though it has also been treated as a ''nomen dubium'' by some.Lund, E. & Gates, T. (2006). "A historical and biogeographical examination of hadrosaurian dinosaurs." Pp. 263-276 in Lucas, S.G. and Sullivan, R.M. (eds.), ''Late Cretaceous vertebrates from the Western Interior''. New Mexico Museum of Natural History and Science Bulletin 35.

The conception of ''Edmontosaurus'' that emerged included three valid species: the type ''E. regalis'', ''E. annectens'' (including ''Anatosaurus edmontoni'', emended to ''edmontonensis''), and ''E. saskatchewanensis''. 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. In 2007 another "mummy" was announced; nicknamed "Dakota", it was discovered in 1999 by Tyler Lyson, and came from the Hell Creek Formation of North Dakota.

In a 2011 study by Nicolás Campione and David Evans, the authors conducted the first-ever morphometric analysis of the various specimens assigned to ''Edmontosaurus''. They concluded that only two species are valid: ''E. regalis'', from the late Campanian, and ''E. annectens'', from the late Maastrichtian. Their study provided further evidence that ''Anatotitan copei'' is a synonym of ''E. annectens''; specifically, that the long, low skull of ''A. copei'' is the result of ontogenetic change and represents mature ''E. annectens'' individuals. ''E. saskatechwanensis'' represents young ''E. annectens,'' and ''Anatosaurus edmontoni'' specimens belong to ''E. regalis'', not ''E. annectens''. The reassessment of ''Edmontosaurus'' assigns twenty skulls to ''E. annectens''. Adult skulls can be distinguished from skulls of ''E. regalis'' by the elongate snout and other details of skull anatomy.

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 6.6 metric tons (7.3 tons) when fully grown.

Recently-found specimens that are still under study at the Museum of the Rockies, namely MOR 1142 ("X-rex") and MOR 1609 ("Becky's Giant"), suggest that ''E. annectens'' may have reached lengths of nearly and weighed , potentially making it one of the largest hadrosaurids; however, Horner and his colleagues suggested that such large individuals would have been extremely rare. The 2022 study on the osteohistology and growth of ''E. annectens'' suggested that previous estimates might have underestimated or overestimated the size of this dinosaur and argued that a fully grown adult ''E. annectens'' would have measured up to in length and in average asymptotic body mass, while the largest individuals measured more than , even up to , based on the comparison between various specimens of different sizes from the Ruth Mason Dinosaur Quarry and other specimens from different localities.

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''.

Classification

''E. annectens'' was a saurolophine or "flat-headed" hadrosaurid; this group was historically known as Hadrosaurinae. Species now considered to be synonyms of ''Edmontosaurus annectens'' were long recognized as closely related to both the genus and the species. 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, to Jack Horner, David B. Weishampel, and Catherine Forster in 2004, and most recently Nicolás Campione and David Evans, 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