Edmontosaurus

Edmontosaurus is a hadrosaurid (also referred as duck-billed) dinosaur genus that lived during the late Cretaceous Period. The meaning of its generic name is "lizard from Edmonton". Two valid species belongs to the genus: Edmontosaurus regalis, and Edmontosaurus annectens, both from western North America.

Edmontosaurus, together with Tyrannosaurus, Triceratops, and other taxa, was one of those last dinosaurs that lived shortly before the Cretaceous-Paleogene extinction. It was one of the largest hadrosaurids, and belonged to the saurolophine group within hadrosaurids - this gruop can be characterized by the lack of a large and hollow crest on top of their heads. They rather had a smaller and solid one, or just some fleshy combs. Edmontosaurus was one of the first dinosaurs which received a skeletal restoration.

Edmontosaurus annectens. By Nobu Tamura (http://spinops.blogspot.com) - Own work, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=19460128

Known occurrences of Edmontosaurus fossils.

Discovery and taxonomy

Edmontosaurus is one of those dinosaurs which has the longest and most complicated paleontological and taxonomical history. This latter interwaves with the genera Hadrosaurus, Anatotitan, Anatosaurus, Thespesius, Claosaurus, Trachodon, and Agathamus^[1][2], and before the 1980s, Edmontosaurus was usually referred under these names . The first well-supported species which later proved to belong to Edmontosaurus was described by Othniel Charles Marsh in 1892 under the name Claosaurus annectens^[3].

The holotype of the species was a skeleton with a partial skull-roof, and an other skull and skeleton was designated as paratype, both found by John Bell Hatcher in the late Maastrichtian-age Upper Cretaceous Lance Formation, Wyoming. These fossil remains become in 1901 and 1904 the first two essentially complete mounted dinosaur skeletons in the United States^[4]. In the dawn of the 20^th century, in 1908 and 1910, two new fossil skeleton was found by Charles Hazelius Sternberg and his sons in the Lance Formation, one with skin impressions. These were purchased by the British Natural History Museum in the U.K. and the Senckenberg Museum in Germany. The genus Edmontosaurus itself was described by Lawrence Lambe in 1917 based on two partial skeletons from the Edmonton Formation (now Horseshoe Formation), southern Alberta, Canada, and its type species was Edmontosaurus regalis^[5].

In 1942 a new genus, Anatosaurus was erected in order to include those crestless and duck-billed hadrosaurids, that did not fit well into the previous genera. Claosaurus annectens was the type species of the new genus, as Anatosaurus annectens^[6]. This new genus was widely accepted until the results of Michael K. Brett-Surman, who re-examined the fossil material of these hadrosaurian dinosaurs. He found that Anatosaurus annectens, amongst some other species, actually belonged to Edmontosaurus, while the remaining species of Anatosaurus were split between other genera^[7].

Skull of Edmontosaurus annectens. By Ballista at the English-language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4497014

Valid species

Edmontosaurus currently has two valid species: Edmontosaurus regalis, which is the type species, and Edmontosaurus annectens^[1].

Edmontosaurus regalis was discovered in the Horseshoe Formation, Alberta. Since its discovery, fossil remains of several further specimens were found^[8]. Two former species, Anatosaurus edmontoni and Thespesius edmontoni were synonymyzed with E. regalis^[8]. Edmontosaurus regalis specimens were found in older formations, dated to late Campanian stage of the late Cretaceous period, than its congeneric species, E. annectans, known from the late Maastrichtian.

We know more than twenty fossil skulls and some postcranial remains of Edmontosaurus annectens, which is known from the Hell Creek Formation, Montana, and the Lance Formation in South Dakota and Wyoming^[8]. This species had a less robust, lower, and longer skull than the related species E. regalis^[8]. Two former species, Anatosaurus copei and E. saskatchewanensis were synonymyzed with E. annectens^[8].

Edmontosaurus regalis, size comparison with a human. By Hai Xing, Jordan C. Mallon, Margaret L. Currie - Fig. 2 (doi:10.1371/journal.pone.0175253.g002) in: Supplementary cranial description of the types of Edmontosaurus regalis (Ornithischia: Hadrosauridae), with comments on the phylogenetics and biogeography of Hadrosaurinae. PLoS ONE 12(4):e0175253, doi:10.1371/journal.pone.0175253, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=57767844

Description

Edmontosaurus was a massive dinosaur with a long, muscular, and laterally flattened tail, having stronger hindlimbs than forelimbs, and characterised by its duck-like beak. The length of adult Edmontosaurus specimens was between 12-13 meters, and it weighted about 4 tons^[1]. The skull of this dinosaur could reach more than a meter in length^[9]. It possessed a small, bony cranial crest. The beak had no teeth, but it was extended for approximately 8 centimeters over the bone by a keratinous structure^[1][6].

It had elongated nasal openings, placed in a deep depression, and sorrounded by bony rims; these structures might have held inflatable air sacs, which could be used for vocal and visual signaling^[10]. Like many other dinosaurs, Edmontosaurus also possessed sclerotic rings in the eys sockets^[6]. Teeth were present in the maxillae and dentaries, and they were continually replaced^[11]. These teeth grew in coloumns, and one coloumn contained a maximum of 6 teeth^[6]. It is an interesting fact, that the number of vertebrae are different in case of some fossil specimens^[6]. Edmontosaurus kept its tail horizontally, as well as the middle and posterior parts of its back, but the anterior part of the back was inclined downwards, and the neck was curved upwards in order to keep the head high^[1].

This dinosaur had ossified tendons in its tail, and these were also present in most of its back. These structure helped the dinosaur to keep its back and tail tightly in a horizontally straight position^[12].The forelimbs were shorter than the hindlimbs. Edmontosaurus had no thumb on the hand of its forelegs, and had only four fingers - three with similar length, and the little finger much shorter. The feet of hindlegs had three toes, with the big and little toes missing. We know soft-tissue remnants are known in case of Edmontosaurus, such as skin impressions of E. annectens^[13], and the impression of a soft tissue crest on the head of E. regalis^[14]. These skin-impressions suggest that the body of Edmontosaurus was covered by scales.

Paleobiology

Edmontosaurus was definitely a herbivorous dinosaur. A study from 2011 revealed that the shape of Edmontosaurus skull went through considerably changes during its growth - it became longer and flatter along the animals lifespan. This also meant that in fact, many characters based on the skull which were earlier used to support additional taxa in the past, directly correlated with the size of the skull^[8]. Edmontosaurus had little brain compared to its body mass - it had an elongate structure, and lacked neocortex^[6][15].

Edmontosaurus was able to switch between bipedal and quadrupedal stances and could walk both ways. Probably the animal raised to its hindlegs when it needed to move quickly^[1]. Results of computer modelling suggests that it could run in bipedal mode at the maximum speed of maximum 45 km/h. Edmontosaurus was originally believed to be a semi-aquatic dinosaur, whic could flee from predators into the water. However, the bone structur of its hand and tails suggest that this dinosaur was not a good swimmer^[16]. Fossil records suggest, that just like most of the hadrosaurids, Edmontosaurus was also a herding dinosaur^[1][17].

Sexual dimorphism in case of Edmontosaurus is not confirmed^[18]. There are scientific theories supporting both the migratory nature of Edmontosaurus^[19] and the overwintering of the polar populations^[20]. Edmontosaurus was a contemporary of Tyrannosaurus and Triceratops, and they were probably parts of the same ecosystem. There is fossil evidence of Tyrannosaurus bite on the skeleton of Edmontosaurus on one of its tail vertebrae^[21] .

Mounted replica of a composite skeleton of Edmontosaurus annectens, on display at the University of Oxford Museum, Oxford, England. By Kevin Walsh from Bicester, England - edmontosaurusUploaded by FunkMonk, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=20054391

Scientific references

[1] Horner, J.R.; Weishampel, D.B.; Forster, C.A. (2004): Hadrosauridae. In Weishampel, D.B.; Dodson, P.; Osmólska H.(eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 438-463. ISBN 978-0-520-24209-8.

[2] Creisler, B.S. (2007): Deciphering duckbills: a history in nomenclature. In Carpenter Kenneth (ed.). Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Bloomington and Indianapolis: Indiana University Press. pp. 185-210. ISBN 978-0-253-34817-3.

[3] Marsh, O.C. (1892): Notice of new reptiles from the Laramie Formation. American Journal of Science. 43 (257): 449-453. doi: 10.2475/ajs.s3-43.257.449.

[4] Lucas, F.A. (1904): The dinosaur Trachodon annectens. Smithsonian Miscellaneous Collections, 45: 317-320.

[5] Lambe, L.M. (1917): A new genus and species of crestless hadrosaur from the Edmonton Formation of Alberta. The Ottawa Naturalist, 31 (7): 65-73.

[6] Lull, R.S. & Wright, N.E. (1942): Hadrosaurian Dinosaurs of North America. pp. 151-164.

[7] Brett-Surman, M.K. (1979): Phylogeny and paleobiogeography of hadrosaurian dinosaurs. Nature, 277 (5697): 560-562. doi: 10.1038/277560a0.

[8] Campione, N.E.; Evans, D.C. (2011): Cranial Growth and Variation in Edmontosaurs (Dinosauria: Hadrosauridae): Implications for Latest Cretaceous Megaherbivore Diversity in North America. PLOS ONE. 6 (9): e25186. doi: 10.1371/journal.pone.0025186.

[9] Cope, E.D. (1883): On the characters of the skull in the Hadrosauridae. Proceedings of the Philadelphia Academy of Natural Sciences, 35: 97-107.

[10] Hopson, J.A. (1975): The evolution of cranial display structures in hadrosaurian dinosaurs. Paleobiology, 1 (1): 21-43. doi: 10.1017/S0094837300002165.

[11] Stanton Thomas, Kathryn J.; Carlson, Sandra J. (2004). Microscale ?¹⁸O and ?¹³C isotopic analysis of an ontogenetic series of the hadrosaurid dinosaur Edmontosaurus: implications for physiology and ecology. Palaeogeography, Palaeoclimatology, Palaeoecology, 206 (2004): 257-287. doi: 10.1016/j.palaeo.2004.01.007.

[12] Ostrom, J.H. (1964): A reconsideration of the paleoecology of the hadrosaurian dinosaurs. American Journal of Science, 262 (8): 975-997. doi: 10.2475/ajs.262.8.975.

[13] Osborn, H.F. (1909): The epidermis of an iguanodont dinosaur. Science, 29 (750): 793-795. doi: 10.1126/science.29.750.793.

[14] Bell, P. R.; Fanti, F.; Currie, P. J.; Arbour, V.M. (2013): A Mummified Duck-Billed Dinosaur with a Soft-Tissue Cock's Comb. Current Biology, 24 (1): 70-75. doi: 10.1016/j.cub.2013.11.008.

[15] Jerison, H.J.; Horner, J.R.; Horner, C.C. (2001): Dinosaur forebrains. Journal of Vertebrate Paleontology, 21 (3, Suppl): 64A. doi: 10.1080/02724634.2001.10010852.

[16] Bakker, R.T. (1986): The case of the duckbill's hand. The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and their Extinction. New York: William Morrow. pp. 146-159. ISBN 978-0-8217-2859-8.

[17] Chadwick, A.; Spencer, L.; Turner, L. (2006): Preliminary depositional model for an Upper Cretaceous Edmontosaurus bonebed. Journal of Vertebrate Paleontology, 26 (3, suppl): 49A. doi: 10.1080/02724634.2006.10010069.

[18] Gould, R.; Larson, R.; Nellermoe, R. (2003): An allometric study comparing metatarsal IIs in Edmontosaurus from a low-diversity hadrosaur bone bed in Corson Co., SD. Journal of Vertebrate Paleontology, 23 (3, suppl): 56A-57A. doi: 10.1080/02724634.2003.10010538.

[19] Bell, P.R.; Snively, E. (2008): Polar dinosaurs on parade: a review of dinosaur migration. Alcheringa, 32 (3): 271-284. doi: 10.1080/03115510802096101.

[20] Chinsamy, A.; Thomas, D. B.; Tumarkin-Deratzian, A. R.; Fiorillo, A. R. (2012): Hadrosaurs were perennial polar residents. he Anatomical Record. online preprint (4): 610-614. doi: 10.1002/ar.22428.

[21] Carpenter, K. (1998): Evidence of predatory behavior by theropod dinosaurs. Gaia, 15: 135-144.