Edmontosaurus annectens
Edmontosaurus annectens | |
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Mounted cast of a fossil E. annectens skeleton, Oxford University Museum of Natural History | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | †Ornithischia |
Clade: | †Ornithopoda |
Family: | †Hadrosauridae |
Subfamily: | †Saurolophinae |
Genus: | †Edmontosaurus |
Species: | †E. annectens
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Binomial name | |
†Edmontosaurus annectens (
Marsh , 1892) | |
Synonyms | |
|
Edmontosaurus annectens (meaning "connected lizard from
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.[8][9] References predating the 1980s typically use Anatosaurus, Claosaurus, Diclonius, Thespesius, or Trachodon for E. annectens fossils, depending on the author and date.
Cope's Diclonius mirabilis
The history of E. annectens predates the naming of both the genus Edmontosaurus and the species annectens. The first quality specimen, the former
Cope's description promoted hadrosaurids as amphibious animals, 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 eat terrestrial food; he described the beak as weak, too.[14] However, aside from misidentifying several of the skull bones,[16] by chance, the lower jaws were missing the walls supporting the teeth from the inside, and the teeth were actually very well-supported.[15][17] Cope intended to describe the skeleton and skull, but his promised paper never appeared.[9] It was purchased for the American Museum of Natural History in 1899, where it acquired its present designation: AMNH 5730.[18]
Several years after Cope's description, his arch-rival,
A second mostly complete skeleton, AMNH 5886, was found in 1904 in the Hell Creek Formation rocks at Crooked Creek in central
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, being designated as 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.[22] This species has some historical footnotes attached, as it is among the first dinosaurs to receive a skeletal restoration, and is the first hadrosaurid so restored.[9][23] YPM 2182 and UNSM 2414 are, respectively, the first and second essentially complete mounted dinosaur skeletons in the United States.[21] YPM 2182 was put on display in 1901,[9] and USNM 2414 was put on display in 1904.[21]
In the first decade of the twentieth century, two additional important specimens of C. annectens were recovered. The first, the "
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.[26] The Horseshoe Canyon Formation is older than the rocks in which Claosaurus annectens was found.[27] Lambe found that his new dinosaur compared best to Cope's Diclonius mirabilis.[26]
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,[28] now the Frenchman Formation.[8] NMC 8509 included an almost complete skull, numerous vertebrae, partial shoulder and hip girdles, and partial back legs, 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.[28] At the time, T. saskatchewanensis was unusual because of its small size, estimated at 7 to 7.3 meters (23 to 24 ft) in length.[29]
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, with textbooks and encyclopedias drawing a distinction between the "Iguanodon-like" Claosaurus annectens and the "duck-billed" Hadrosaurus (based on Cope's Diclonius mirabilis); conversely, Hatcher explicitly identified C. annectens as synonymous with the hadrosaurid represented by those same duck-billed skulls,[9] the two differentiated only by individual variation or distortion from pressure.[30] Hatcher's revision, published in 1902, was sweeping, as 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,[30] fragmentary genera now thought to be ceratopsians. 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.[9] In 1915, Charles W. Gilmore reassessed hadrosaurids, and recommended that Thespesius should 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.[31] A multiplicity of names resumed, with the AMNH duckbills being known as Diclonius mirabilis, Trachodon mirabilis, Trachodon annectens, Claosaurus, or Thespesius.[9]
Anatosaurus to the present
This confusing situation was temporarily resolved in 1942 by
This state of affairs persisted for several decades until
Of the remaining species of Anatosaurus, A. saskatchewanensis and A. edmontoni were assigned to Edmontosaurus as well,[41] and A. longiceps went to Anatotitan, as either a second species[42] or as a synonym of A. copei.[41] A. longiceps may be a synonym of E. annectens,[8] though it has also been treated as a nomen dubium by some.[43]
The conception of Edmontosaurus that emerged included three valid species: the type species E. regalis; E. annectens (including Anatosaurus edmontoni, emended to edmontonensis); and E. saskatchewanensis.[41] The debate about the proper taxonomy of the A. copei specimens continues to the present day. 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.[8] 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.[44][45]
In a 2011 study by Nicolás Campione and David Evans, the authors conducted the first-ever
Description
The skull and skeleton of E. annectens are very well-known.
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 49 feet (15 m) and weighed 11 short tons (10.0 t), potentially making it one of the largest hadrosaurids ever. However, Jack Horner and his colleagues suggested that such large individuals would have been extremely rare.[5][6][7] 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 36–39 feet (11–12 m) in length and 5.6 metric tons (6.2 short tons) in average asymptotic body mass, while the largest individuals measured more than 6 metric tons (6.6 short tons) and even up to 6.6–7 metric tons (7.3–7.7 short tons), based on the comparison between various specimens of different sizes from the Ruth Mason Dinosaur Quarry and other specimens from different localities.[4]
The skull of E. annectens is known for its long, wide muzzle. Cope compared this feature to that of a
As mounted, the vertebral column of E. annectens includes twelve neck, twelve back, nine
Classification
E. annectens was a saurolophine, or "flat-headed", hadrosaurid. This group was historically known as Hadrosaurinae.[48] Species now considered to be synonymous with Edmontosaurus annectens were long recognized as closely related to both the genus[49] and the species.[30] 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 different genera. On the other side of the issue, other authors, from John Bell Hatcher in 1902,[30] to Jack Horner, David B. Weishampel, and Catherine Forster in 2004,[8] and most recently Nicolás Campione and David Evans,[27] 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
The cladogram below follows Godefroit et al. (2012) analysis.[50]
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Paleobiology
As a hadrosaurid, Edmontosaurus annectens was a fairly large
The extensive depressions surrounding its nasal openings may have 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.[51]
A preserved
Growth
In a 2011 study, Campione and Evans recorded data from all known "edmontosaur" skulls from the Campanian and Maastrichtian, and used it to plot a morphometric graph, comparing variable features of the skulls with skull size. Their results showed that, in both recognized Edmontosaurus species, many features previously used to classify additional species or genera were directly correlated to skull size. Campione and Evans interpreted these results as strongly suggesting that the shape of Edmontosaurus skulls changed dramatically as they grew and matured. This has led to several apparent mistakes in past classification. The three previously recognized Maastrichtian edmontosaur species likely represent growth stages of a single species, with E. saskatchewanensis representing juveniles, E. annectens subadults, and Anatotitan copei being fully mature adults. The skulls became longer and flatter as the animals grew.[27] In a 2022 study, Wosik and Evans proposed that E. annectens reached maturity in 9 years, based on their analysis for various specimens from different localities. They found the result to be similar to that of other hadrosaurs.[4]
Paleoecology
True E. annectens remains are known only from latest Maastrichtian rocks of the Hell Creek and Lance Formations of South Dakota, Montana, and Wyoming, alongside the Frenchman Formation of Saskatchewan.[27]
The Lancian time interval was the last interval before the Cretaceous–Paleogene extinction event that killed off the non-avian dinosaurs. Edmontosaurus was one of the most common dinosaurs of the interval. Robert Bakker reports that it made up one-seventh of the large dinosaur sample, with most of the remaining five-sixths made up of Triceratops.[54] The coastal plain Triceratops–Edmontosaurus association, dominated by Triceratops, extended from Colorado to Saskatchewan.[55] Typical dinosaur faunas of the Lancian formations where Edmontosaurus annectens has been found also included: the hypsilophodont Thescelosaurus; the rare ceratopsid Torosaurus; the pachycephalosaurid Pachycephalosaurus; the ankylosaurid Ankylosaurus; and the theropods Ornithomimus, Pectinodon, Acheroraptor, Dakotaraptor, and Tyrannosaurus.[56][57]
The Hell Creek Formation, as typified by exposures in the
The Lance Formation, as typified by exposures approximately 62 miles (100 km) north of
See also
Notes
* Many of the original references deal with specimens or species that were not assigned to E. annectens until later. This is particularly true with the specimens long known, chronologically, as Diclonius mirabilis, Anatosaurus copei, and Anatotitan copei. ^* This toothless section is also known as a diastema.
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