| Edmontosaurus | |
|---|---|
 | |
| Mounted cast of a fossilE. annectens skeleton,Rocky Mountain Dinosaur Resource Center | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Family: | †Hadrosauridae |
| Subfamily: | †Saurolophinae |
| Tribe: | †Edmontosaurini |
| Genus: | †Edmontosaurus Lambe,1917 |
| Type species | |
| †Edmontosaurus regalis Lambe, 1917 | |
| Other species | |
| Synonyms | |
| |
Edmontosaurus (/ɛdˌmɒntəˈsɔːrəs/ed-MON-tə-SOR-əs) (meaning "lizard fromEdmonton"), often colloquially and historically known asAnatosaurus orAnatotitan (meaning "duck lizard" and "giant duck"), is agenus ofhadrosaurid (duck-billed)dinosaur. It contains two knownspecies:Edmontosaurus regalis andEdmontosaurus annectens.Fossils ofE. regalis have been found in rocks of western North America that date from the lateCampanian age of theCretaceous period 73 million years ago, while those ofE. annectens were found in the same geographic region from rocks dated to the end of theMaastrichtian age, 66 million years ago.Edmontosaurus was one of the last non-avian dinosaurs ever to exist, and lived alongside dinosaurs likeTriceratops,Tyrannosaurus,Ankylosaurus, andPachycephalosaurus shortly before theCretaceous–Paleogene extinction event.
Edmontosaurus included two of the largest hadrosaurid species, withE. annectens measuring up to 12 metres (39 ft) in length and weighing around 5.6 metric tons (6.2 short tons) in average asymptotic body mass. The exceptionally large specimens ofE. annectens measured around 15 metres (49 ft) long and weighed around 15.9 metric tons (17.5 short tons). Several well-preserved specimens are known that include numerous bones, as well as extensive skin impressions and possible gut contents.Edmontosaurus is classified as a genus ofsaurolophine (orhadrosaurine) hadrosaurid, a member of the group of hadrosaurids that lacked large, hollow crests and instead had smaller, solid crests or fleshy combs.
The first fossils namedEdmontosaurus were discovered in southernAlberta (named afterEdmonton, the capital city), in theHorseshoe Canyon Formation (formerly called the lower Edmonton Formation). Thetype species,E. regalis, was named byLawrence Lambe in 1917, although several other species that are now classified inEdmontosaurus were named earlier. The best known of these isE. annectens, named byOthniel Charles Marsh in 1892. This species was originally known as a species ofClaosaurus, known for many years as a species ofTrachodon, and later known asAnatosaurus annectens.Anatosaurus,Anatotitan, and probablyUgrunaaluk are now generally regarded as synonyms ofEdmontosaurus.
Edmontosaurus was widely distributed across western North America, ranging fromColorado to the northern slopes ofAlaska. The distribution ofEdmontosaurus fossils suggests that it preferred coasts andcoastal plains. It was aherbivore that could move on both two legs and four. Because it is known from severalbone beds,Edmontosaurus is thought to have lived in groups and may have been migratory as well. The wealth of fossils has allowed researchers to study itspaleobiology in detail, including its brain, how it may have fed, and its injuries andpathologies, such as evidence for tyrannosaur attacks on a few specimens.
In anEdmontosaurus fossil, Tuinstra et al. (2025) made the first clear detection of actual original dinosaur organic material, showing the presence of ancient hydroxyproline (a building block of collagen) in the fossil, and refuting the hypothesis that organic matter present in fossils must be due to contamination.[1][2]
Edmontosaurus has had a very long and complicated history in paleontology, having spent decades with various species classified in other genera. Itstaxonomic history intertwines at various points with the generaAgathaumas,Anatosaurus,Anatotitan,Claosaurus,Hadrosaurus,Thespesius, andTrachodon,[3][4] with references predating the 1980s typically usingAnatosaurus,Claosaurus,Thespesius, orTrachodon for edmontosaur fossils (excluding those assigned toE. regalis) depending on the author and the date. AlthoughEdmontosaurus was only named in 1917, its oldest well-supported species (E. annectens) was named in 1892 as a species ofClaosaurus.
The first well-supported species ofEdmontosaurus was named in 1892 asClaosaurus annectens byOthniel Charles Marsh. This species is based onUSNM 2414, which is a partial skull-roof and skeleton, with a second skull and skeleton,YPM 2182, designated as theparatype. Both were collected in 1891 byJohn Bell Hatcher from the late Maastrichtian-age Upper CretaceousLance Formation ofNiobrara County (then part ofConverse County),Wyoming.[5] 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.[4][6] YPM 2182 and UNSM 2414 are, respectively, the first and second essentially complete mounted dinosaur skeletons in the United States.[7] YPM 2182 was put on display in 1901[4] and USNM 2414 was put on display in 1904.[7]
Because of the incomplete understanding of hadrosaurids at the time, following Marsh's death in 1897,Claosaurus annectens was variously classified as a species ofClaosaurus,Thespesius orTrachodon. Opinions varied greatly, as textbooks and encyclopedias drew a distinction between the "Iguanodon-like"Claosaurus annectens and the "duck-billed"Hadrosaurus (based on remains now known as adultEdmontosaurus annectens), while Hatcher explicitly identifiedC. annectens as synonymous with the hadrosaurid represented by those same duck-billed skulls.[4] Hatcher's revision, published in 1902, was sweeping, as he considered almost all hadrosaurid genera then known as synonyms ofTrachodon. This includedCionodon,Diclonius,Hadrosaurus,Ornithotarsus,Pteropelyx, andThespesius, as well asClaorhynchus andPolyonax, which are fragmentary genera now thought to beceratopsians.[8] Hatcher's work led to a brief consensus until post-1910, when new material from Canada and Montana showed a greater diversity of hadrosaurids than previously suspected.[4]Charles W. Gilmore, in 1915, reassessed hadrosaurids and recommended thatThespesius be reintroduced for hadrosaurids from the Lance Formation and rock units of equivalent age and thatTrachodon, based on inadequate material, should be restricted to a hadrosaurid from the olderJudith River Formation and its equivalents. In regards toClaosaurus annectens, he recommended that it be considered the same asThespesius occidentalis.[9] His reinstatement ofThespesius for Lance-age hadrosaurids would have other consequences for the taxonomy ofEdmontosaurus in the following decades.
During this time frame (1902–1915), two additional important specimens ofC. annectens were recovered. The first, the"mummified" specimen AMNH 5060, was discovered in 1908 byCharles Hazelius Sternberg and his sons in Lance Formation rocks nearLusk, Wyoming. Sternberg was working for theBritish Museum of Natural History, but Henry Fairfield Osborn of theAmerican Museum of Natural History was able to purchase the specimen for $2,000.[10] The Sternbergs recovered asecond similar specimen from the same area in 1910,[11] which was not as well preserved. However, it was also found with skin impressions. They sold the specimen, SM 4036, to theSenckenberg Museum in Germany.[10]
As a side note,Trachodon selwyni, described byLawrence Lambe in 1902 for a lower jaw from what is now known as theDinosaur Park Formation ofAlberta,[12] was erroneously described by Glut (1997) as having been assigned toEdmontosaurus regalis by Lull and Wright.[13] It was not, instead being designated "of very doubtful validity."[14] More recent reviews of hadrosaurids have concurred.[3][15]
Edmontosaurus itself was coined in 1917 by Lawrence Lambe for two partial skeletons found in theHorseshoe Canyon Formation (formerly the lower Edmonton Formation) along theRed Deer River of southern Alberta.[16] These rocks are older than the rocks in whichClaosaurus annectens was found.[17] The Edmonton Formation lendsEdmontosaurus its name.[4] Thetype species,E. regalis (meaning'regal', or, more loosely,'king-sized'),[4] is based onNMC 2288, which consists of a skull, articulated vertebrae up to the sixth tail vertebra, ribs, partial hips, an upper arm bone, and most of a leg. It was discovered in 1912 by Levi Sternberg. The second specimen, paratype NMC 2289, consists of a skull and skeleton lacking the beak, most of the tail, and part of the feet. It was discovered in 1916 byGeorge F. Sternberg. Lambe found that his new dinosaur compared best toDiclonius mirabilis (specimens now assigned toEdmontosaurus annectens) and drew attention to the size and robustness ofEdmontosaurus.[16] Initially, Lambe only described the skulls of the two skeletons, but returned to the genus in 1920 to describe the skeleton of NMC 2289.[18] Thepostcrania of the type specimen remains undescribed, still in its plaster jackets to this day.[13]
Two more species that would come to be included withEdmontosaurus were named from Canadian remains in the 1920s, but both would initially be assigned toThespesius. Gilmore named the first,Thespesius edmontoni, in 1924.T. edmontoni also came from the Horseshoe Canyon Formation. It was based on NMC 8399, another nearly complete skeleton lacking most of the tail. NMC 8399 was discovered on the Red Deer River in 1912 by a Sternberg party.[19] Its arms, ossified tendons, and skin impressions were briefly described in 1913 and 1914 by Lambe, who at first thought it was an example of a species he had namedTrachodon marginatus,[20] but then changed his mind.[21] The specimen became the first dinosaur skeleton to be mounted for exhibition in a Canadian museum. Gilmore found that his new species compared closely to what he calledThespesius annectens, but left the two apart because of details of the arms and hands. He also noted that his species had more vertebrae than Marsh's in the back and neck, but proposed that Marsh was mistaken in assuming that theannectens specimens were complete in those regions.[19]
In 1926,Charles Mortram Sternberg namedThespesius saskatchewanensis for NMC 8509, which is a skull and partial skeleton from the Wood Mountain plateau of southernSaskatchewan. He had collected this specimen in 1921 from rocks that were assigned to the Lance Formation,[22] now theFrenchman Formation.[3] NMC 8509 included an almost complete skull, numerous vertebrae, partial shoulder and hip girdles, and partial legs, representing the first substantial dinosaur specimen recovered from Saskatchewan. Sternberg opted to assign it toThespesius because that was the only hadrosaurid genus known from the Lance Formation at the time.[22] At the time,T. saskatchewanensis was unusual because of its small size, estimated at 7 to 7.3 metres (23 to 24 ft) in length.[23]
In 1942, Lull and Wright attempted to resolve the complicated taxonomy of crestless hadrosaurids by naming a new genus,Anatosaurus, to take in several species that did not fit well under their previous genera.Anatosaurus, meaning'duck lizard', because of its wide, duck-like beak (Latinanas'duck' +Greeksauros'lizard'), had as its type species Marsh's oldClaosaurus annectens. Also assigned to this genus wereThespesius edmontoni,T. saskatchewanensis, a large lower jaw that Marsh had namedTrachodon longiceps in 1890, and a new species namedAnatosaurus copei for two skeletons on display at the American Museum of Natural History that had long been known asDiclonius mirabilis (or variations thereof). Thus, the various species becameAnatosaurus annectens,A. copei,A. edmontoni,A. longiceps, andA. saskatchewanensis.[23]Anatosaurus would come to be called the "classic duck-billed dinosaur."[24]
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 ofAnatosaurus,A. annectens, was actually a species ofEdmontosaurus and thatA. copei was different enough to warrant its own genus.[25][26][27] Although theses anddissertations are not regarded as official publications by theInternational 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.[28][29] Brett-Surman and Ralph Chapman designated a new genus forA. copei (Anatotitan) in 1990.[30] Of the remaining species,A. saskatchewanensis andA. edmontoni were assigned toEdmontosaurus as well[15] andA. longiceps went toAnatotitan as either a second species[31] or as a synonym ofA. copei.[15] Because the type species ofAnatosaurus (A. annectens) was sunk intoEdmontosaurus, the nameAnatosaurus is abandoned as ajunior synonym ofEdmontosaurus.
The conception ofEdmontosaurus that emerged included three valid species: the type speciesE. regalis,E. annectens (includingAnatosaurus edmontoni, amended toedmontonensis), andE. saskatchewanensis.[15] The debate about the proper taxonomy of theA. copei specimens continues to the present day. Returning to Hatcher's argument of 1902, Jack Horner,David B. Weishampel, and Catherine Forster regardedAnatotitan copei as representing specimens ofEdmontosaurus annectens with crushed skulls.[3] In 2007, another "mummy" was announced. Nicknamed "Dakota", it was discovered in 1999 byTyler Lyson and came from theHell Creek Formation ofNorth Dakota.[32][33]
In a 2011 study by Nicolás Campione and David Evans, the authors conducted the first ever morphometric analysis to compare the various specimens assigned toEdmontosaurus. They concluded that only two species are valid:E. regalis, from the late Campanian, andE. annectens, from the late Maastrichtian. Their study provided further evidence thatAnatotitan copei is a synonym ofE. annectens. Specifically, the long, low skull ofA. copei is the result of ontogenetic change and represents matureE. annectens individuals.[17]
Edmontosaurus is currently regarded as having two valid species: the type speciesE. regalis andE. annectens.[3][17]E. regalis is known only from the Horseshoe Canyon Formation of Alberta, dating from the late Campanian age of the late Cretaceous period. At least a dozen individuals are known,[17] including seven skulls with associated postcrania and five to seven other skulls.[3][15] The species formerly known asThespesius edmontoni orAnatosaurus edmontoni represents immature individuals ofE. regalis.[17][34][35]
E. annectens is known from the Frenchman Formation of Saskatchewan, the Hell Creek Formation of Montana, and the Lance Formation of South Dakota and Wyoming. It is limited to late Maastrichtian rocks and is represented by at least twenty skulls, some with postcranial remains.[17] One author, Kraig Derstler, has describedE. annectens as "perhaps the most perfectly-known dinosaur to date [1994]."[36]Anatosaurus copei andE. saskatchewanensis are now thought to be growth stages ofE. annectens, withA. copei as adults andE. saskatchewanensis as juveniles.[17]Trachodon longiceps may be a synonym ofE. annectens as well.[3]Anatosaurus edmontoni was mistakenly listed as a synonym ofE. annectens in both reviews of Dinosauria,[3][15] but this does not appear to be the case.[17][35]
E. annectens differed fromE. regalis by having a longer, lower, and less robust skull.[13][17] Although Brett-Surman regardedE. regalis andE. annectens as potentially representing males and females of the same species,[25] allE. regalis specimens come from older formations thanE. annectens specimens.[35] Edmontosaurine specimens from thePrince Creek Formation of Alaska formerly assigned toEdmontosaurus sp. were given their own genus and species name,Ugrunaaluk kuukpikensis, in 2015.[37] However, the identification ofUgrunaaluk as a separate genus was questioned by a 2017 study from Hai Xing and colleagues, who regarded it as anomen dubium that was indistinguishable from otherEdmontosaurus.[38] In 2020, Ryuji Takasaki and colleagues agreed that the Prince Creek remains should be classified asEdmontosaurus, though species designation is unclear because the specimens are juveniles.[39] Another study found the Alaskan material to be referable toEdmontosaurus cf.regalis based on craniomandibular anatomy.[40]Edmontosaurus was also reported from theJavelina Formation ofBig Bend National Park, westernTexas based on TMM 41442-1,[41] but was later referred toKritosaurus cf.navajovius by Wagner (2001), before being assigned toKritosaurus sp. by Lehmanet al. (2016).[42][43][44]
Edmontosaurus has been described in detail from numerous specimens.[18][19][22][45][46][47] Traditionally,E. regalis has been regarded as the largest species, though this was challenged by the hypothesis that the larger hadrosauridAnatotitan copei is a synonym ofEdmontosaurus annectens, as put forward byJack Horner and colleagues in 2004,[3] and supported in studies by Campione and Evans in 2011.[17]
Edmontosaurus was among the largest hadrosaurids ever to exist. Like other hadrosaurids, it was a bulky animal with a long, laterally flattened tail and an expanded, duck-like beak. The arms were not as heavily built as the legs, but were long enough to be used for standing or for quadrupedal movement. Depending on the species, previous estimates suggested that a fully grown adult could have been 9–12 metres (30–39 ft) long and some of the larger specimens reached the range of 12–13 metres (39–43 ft)[13][48][49] with a body mass on the order of 4 metric tons (4.4 short tons).[3]
E. annectens is often seen as smaller. Two mounted skeletons,USNM 2414 andYPM 2182, measure 8.00 metres (26.25 ft) long and 8.92 metres (29.3 ft) long, respectively.[49][7] However, these are probably subadult individuals.[17] There is also at least one report of a much larger potentialE. annectens specimen that is almost 12 metres (39 ft) long.[50] Two specimens still under study in the collection of theMuseum of the Rockies - a 7.5 m (25 ft) tail labelled as MOR 1142 and another labelled as MOR 1609 - indicate thatEdmontosaurus annectens could have grown to much larger sizes and reach nearly 15 metres (49 ft) in length,[51][52] and weighed up to 15.87 metric tons (17.49 short tons),[53] but such large individuals were likely very rare.[51]
A 2022 study on the osteohistology and growth ofE. annectens suggested that previous estimates might have underestimated or overestimated the size of this dinosaur and proposed that a fully grown adultE. annectens would have measured up to 11–12 metres (36–39 ft) in length and approximately 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) when based on the comparison between various specimens of different sizes from the Ruth Mason Dinosaur Quarry and other specimens from different localities. According to this analysis,E. regalis may have been heavier, but not enough samples exist to provide a valid estimate and examination on its osteohistology and growth, so the results forE. regalis aren't statistically significant.[54]
The skull of a fully grownEdmontosaurus could be over a metre long. One skull ofE. annectens (formerlyAnatotitan) measures 3.87 feet (1.18 m) long.[55] The skull was roughly triangular in profile,[18] with no bony cranial crest.[23] Viewed from above, the front and rear of the skull were expanded, with the broad front forming a duck-bill orspoon-bill shape. The beak was toothless, and both the upper and lower beaks were extended bykeratinous material.[3] Substantial remains of the keratinous upper beak are known fromthe "mummy" kept at theSenckenberg Museum.[13] In this specimen, the preserved nonbony part of the beak extended for at least 8 centimetres (3.1 in) beyond the bone, projecting down vertically.[56] The nasal openings ofEdmontosaurus were elongate and housed in deep depressions surrounded by distinct bony rims above, behind, and below.[34]
In at least one case (the Senckenberg specimen), rarely preservedsclerotic rings were preserved in the eye sockets.[57] Another rarely seen bone, thestapes (the reptilian ear bone), has also been seen in a specimen ofEdmontosaurus.[3] It has been suggested thatEdmontosaurus may have hadbinocular vision based on the 3D scan of a nearly complete skull ofE. regalis (CMN 2289).[58]
Teeth were present only in the maxillae (upper cheeks) and dentaries (main bone of the lower jaw). The teeth were continually replaced, taking about half a year to form.[59] They were composed of six types of tissues, rivaling the complexity of mammal teeth.[60] They grew in columns, with an observed maximum of six in each, and the number of columns varied based on the animal's size.[45] Known column counts for the two species are: 51 to 53 columns per maxilla and 48 to 49 per dentary (teeth of the upper jaw being slightly narrower than those in the lower jaw) forE. regalis; and 52 columns per maxilla and 44 per dentary forE. annectens (anE. saskatchewanensis specimen).[23]
The number ofvertebrae differs between specimens.E. regalis had thirteen neck vertebrae, eighteen back vertebrae, nine hip vertebrae, and an unknown number of tail vertebrae.[23] A specimen once identified as belonging toAnatosaurus edmontoni (now considered to be the same asE. regalis) is reported as having an additional back vertebra and 85 tail vertebrae, with an undisclosed amount of restoration.[23] Other hadrosaurids are only reported as having 50 to 70 tail vertebrae,[3] so this appears to have been an overestimate. Theanterior back was curved toward the ground, with the neck flexed upward and the rest of the back and tail held horizontally.[3] Most of the back and tail were lined byossified tendons arranged in alatticework along theneural spines of the vertebrae. This condition has been described as making the back and at least part of the tail "ramrod" straight.[61][62] The ossified tendons are interpreted as having strengthened the vertebral column against gravitational stress, incurred through being a large animal with a horizontal vertebral column otherwise supported mostly by the hind legs and hips.[61]
Theshoulder blades were long flat blade-like bones, held roughly parallel to the vertebral column. Thehips were composed of three elements each: an elongateilium above the articulation with the leg, anischium below and behind with a long thin rod, and apubis in front that flared into a plate-like structure. The structure of the hip hindered the animal from standing with its back erect, because in such a position thethigh bone would have pushed against the joint of the ilium and pubis, instead of pushing only against the solid ilium. The nine fused hip vertebrae provided support for the hip.[45]
The fore legs were shorter and less heavily built than the hind legs. Theupper arm had a large deltopectoral crest for muscle attachment, while theulna andradius were slim. The upper arm and forearm were similar in length. Thewrist was simple, with only two small bones. Each hand had four fingers, with no thumb (first finger). The index (second), third, and fourth fingers were approximately the same length and were united in life within a fleshy covering. Although the second and third finger had hoof-likeunguals, these bones were also within the skin and not apparent from the outside. The little finger diverged from the other three and was much shorter. The thigh bone was robust and straight, with a prominentflange about halfway down theposterior side.[45] This ridge was for the attachment of powerful muscles attached to the hips and tail that pulled the thighs (and thus the hind legs) backward and helped maintain the use of the tail as a balancing organ.[63] Each foot had three toes, with no big toe or little toe. The toes had hoof-like tips.[45]
Multiple specimens ofEdmontosaurus annectens have been found with preservedskin impressions. Several have been well-publicized, such as the "Trachodon mummy" of the early 20th century,[64][65] and the specimen nicknamed "Dakota",[32][33][66] the latter apparently including remnantorganic compounds from the skin.[66] Because of these finds, thescalation ofEdmontosaurus annectens is known for most areas of the body. Skin impressions are less well known forE. regalis; a mummified specimen (UALVP 53722) from theWapiti Formation which preserves a soft tissue cranial crest or wattle on the head has been referred toE. regalis by Phil R. Bell and colleagues in 2014,[67] but a re-examination by Henry Sharpe and colleagues in 2025 suggested that this specimen cannot be confidently referred to any known species ofEdmontosaurus, and thus it probably represents a distinct taxon belonging to the Edmontosaurini, leading to a tentative assignment to asE. sp.[68]
A preservedrhamphotheca present in theE. annectens specimen LACM 23502, housed in theLos Angeles County Museum, indicates the beak ofEdmontosaurus was more hook-shaped and extensive than many illustrations in scientific and public media have previously depicted. Whether or not the specimen in question preserved the true rhamphotheca or just a cast of the inner structure attached to the bone is not known at present.[69][70][50]
Edmontosaurus was ahadrosaurid (a duck-billed dinosaur), a member of afamily of dinosaurs which to date are known only from theLate Cretaceous. It is classified within theSaurolophinae (alternately Hadrosaurinae), aclade of hadrosaurids which lacked hollow crests. Other members of the group includeBrachylophosaurus,Gryposaurus,Lophorhothon,Maiasaura,Naashoibitosaurus,Prosaurolophus, andSaurolophus.[3] It was either closely related to[15] or includes the speciesAnatosaurus annectens (alternatelyEdmontosaurus annectens),[3] a large hadrosaurid from various latest Cretaceousformations of western North America. The giant Chinese hadrosaurineShantungosaurus giganteus is also anatomically similar toEdmontosaurus; M. K. Brett-Surman found the two to differ only in details related to the greater size ofShantungosaurus, based on what had been described of the latter genus.[25]
While the status ofEdmontosaurus as a saurolophine has not been challenged, its exact placement within the clade is uncertain. Earlyphylogenies, such as that presented inR. S. Lull and Nelda Wright's influential 1942monograph, hadEdmontosaurus and various species ofAnatosaurus (most of which would be later considered as additional species or specimens ofEdmontosaurus) as one lineage among several lineages of "flat-headed" hadrosaurs.[71] One of the first analyses usingcladistic methods found it to be linked withAnatosaurus (=Anatotitan) andShantungosaurus in an informal "edmontosaur" clade, which was paired with the spike-crested "saurolophs" and more distantly related to the "brachylophosaurs" and arch-snouted "gryposaurs".[15] A 2007 study by Terry Gates and Scott Sampson found broadly similar results, in thatEdmontosaurus remained close toSaurolophus andProsaurolophus and distant fromGryposaurus,Brachylophosaurus, andMaiasaura.[72] However, the most recent review of Hadrosauridae, byJack Horner and colleagues (2004), came to a noticeably different result:Edmontosaurus was nested betweenGryposaurus and the "brachylophosaurs", and distant fromSaurolophus.[3]Edmontosaurus is the namesake genus of the saurolophine tribeEdmontosaurini, which also includes taxa likeShantungosaurus,Kerberosaurus andLaiyangosaurus.[73]
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As a hadrosaurid,Edmontosaurus was a large terrestrialherbivore. 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.[3] It used its broad beak to cut loose food, perhaps by cropping,[3] or by closing the jaws in a clamshell-like manner over twigs and branches and then stripping off the more nutritiousleaves andshoots.[61] Because the tooth rows are deeply indented from the outside of the jaws, and because of other anatomical details, it is inferred thatEdmontosaurus and most other ornithischians had cheek-like structures, muscular or non-muscular. The function of the cheeks was to retain food in the mouth.[74][75] The animal's feeding range would have been from ground level to around 4 metres (13 ft) above.[3]
Before the 1960s and 1970s, the prevailing interpretation of hadrosaurids likeEdmontosaurus was that they were aquatic and fed on aquatic plants.[76] An example of this is William Morris's 1970 interpretation of an edmontosaur skull with nonbony beak remnants. He proposed that the animal had a diet much like that of some modern ducks, filtering plants and aquatic invertebrates likemollusks andcrustaceans from the water and discharging water via V-shaped furrows along the inner face of the upper beak.[50] This interpretation of the beak has been rejected, as the furrows and ridges are more like those of herbivorous turtle beaks than the flexible structures seen in filter-feeding birds.[76]
Because scratches dominate the microwear texture of the teeth, Williamset al. suggestedEdmontosaurus was agrazer instead of abrowser, which would be predicted to have fewer scratches due to eating less abrasive materials. Candidates for ingested abrasives includesilica-rich plants likehorsetails and soil that was accidentally ingested due to feeding at ground level.[77] When compared to the dental microwear textures of other hadrosaurids, however, those ofEdmontosaurus do not suggest that the abrasiveness of food items it consumed was drastically greater than what other hadrosaurids consumed.[78] The tooth structure indicates combined slicing and grinding capabilities.[60]
Reports ofgastroliths, or stomach stones, in the hadrosauridClaosaurus is actually based on a probable double misidentification. First, the specimen is actually ofEdmontosaurus annectens.Barnum Brown, who discovered the specimen in 1900, referred to it asClaosaurus becauseE. annectens was thought to be a species ofClaosaurus at the time. Additionally, it is more likely that the supposed gastroliths represent gravel washed in during burial.[4]
Both of the "mummy" specimens collected by the Sternbergs were reported to have had possible gut contents. Charles H. Sternberg reported the presence ofcarbonized gut contents in the American Museum of Natural History specimen,[79] but this material has not been described.[80] The plant remains in the Senckenberg Museum specimen have been described, but have proven difficult to interpret. The plants found in the carcass included needles of theconiferCunninghamites elegans, twigs from conifer and broadleaf trees, and numerous small seeds or fruits.[81] Upon their description in 1922, they were the subject of a debate in the German-language journalPaläontologische Zeitschrift. Kräusel, who described the material, interpreted it as the gut contents of the animal,[81] while Abel could not rule out that the plants had been washed into the carcass after death.[82]
At the time, hadrosaurids were thought to have been aquatic animals, and Kräusel made a point of stating that the specimen did not rule out hadrosaurids eating water plants.[61][81] The discovery of possible gut contents made little impact in English-speaking circles, except for another brief mention of the aquatic-terrestrial dichotomy,[83] until it was brought up byJohn Ostrom in the course of an article reassessing the old interpretation of hadrosaurids as water-bound. Instead of trying to adapt the discovery to the aquatic model, he used it as a line of evidence that hadrosaurids were terrestrial herbivores.[61] While his interpretation of hadrosaurids as terrestrial animals has been generally accepted,[3] the Senckenberg plant fossils remain equivocal.Kenneth Carpenter has suggested that they may actually represent the gut contents of a starving animal, instead of a typical diet.[84][85] Other authors have noted that because the plant fossils were removed from their original context in the specimen and were heavily prepared, it is no longer possible to follow up on the original work, leaving open the possibility that the plants were washed-in debris.[80][86]
The diet andphysiology ofEdmontosaurus have been probed by usingstable isotopes ofcarbon andoxygen as recorded intooth enamel. When feeding, drinking, and breathing, animals take in carbon and oxygen, which become incorporated into bone. The isotopes of these two elements are determined by various internal and external factors, such as the type of plants being eaten, the physiology of the animal,salinity, and climate. If isotope ratios in fossils are not altered by fossilization and laterchanges, they can be studied for information about the original factors;warmblooded animals will have certain isotopic compositions compared to their surroundings, animals that eat certain types of plants or use certain digestive processes will have distinct isotopic compositions, and so on. Enamel is typically used because the structure of the mineral that forms enamel makes it the most resistant material to chemical change in the skeleton.[59]
A 2004 study by Kathryn Thomas and Sandra Carlson used teeth from the upper jaw of three individuals interpreted as a juvenile, a subadult, and an adult, recovered from a bone bed in the Hell Creek Formation ofCorson County, South Dakota. In this study, successive teeth in columns in the edmontosaurs' dental batteries were sampled from multiple locations along each tooth using a microdrilling system. This sampling method takes advantage of the organization of hadrosaurid dental batteries to find variation in tooth isotopes over a period of time. From their work, it appears that edmontosaur teeth took less than about 0.65 years to form, slightly faster in younger edmontosaurs. The teeth of all three individuals appeared to show variation in oxygen isotope ratios that could correspond to warm/dry and cool/wet periods; Thomas and Carlson considered the possibility that the animals were migrating instead, but favored local seasonal variations because migration would have more likely led to ratio homogenization, as many animals migrate to stay within specific temperature ranges or near particular food sources.[59]
The edmontosaurs also showed enriched carbon isotope values, which for modern mammals would be interpreted as a mixed diet ofC3 plants (most plants) andC4 plants (grasses); however, C4 plants were extremely rare in the Late Cretaceous if present at all. Thomas and Carlson put forward several factors that may have been operating, and found the most likely to include a diet heavy ingymnosperms, consuming salt-stressed plants from coastal areas adjacent to theWestern Interior Seaway, and a physiological difference between dinosaurs and mammals that caused dinosaurs to form tissue with different carbon ratios than would be expected for mammals. A combination of factors is also possible.[59]
Between the mid-1980s and the 2000s, the prevailing interpretation of how hadrosaurids processed their food followed the model put forward in 1984 by David B. Weishampel. He proposed that the structure of the skull permitted motion between bones that resulted in backward and forward motion of the lower jaw, and outward bowing of the tooth-bearing bones of the upper jaw when the mouth was closed. The teeth of the upper jaw would grind against the teeth of the lower jaw likerasps, processing plant material trapped between them.[3][87] Such a motion would parallel the effects ofmastication in mammals, although accomplishing the effects in a completely different way.[88] Work in the early 2000s has challenged the Weishampel model. A study published in 2008 by Casey Holliday and Lawrence Witmer found that ornithopods likeEdmontosaurus lacked the types of skull joints seen in those modern animals that are known to have kinetic skulls (skulls that permit motion between their constituent bones), such assquamates and birds. They proposed that joints that had been interpreted as permitting movement in dinosaur skulls were actuallycartilaginous growth zones.[89] An important piece of evidence for Weishampel's model is the orientation of scratches on the teeth, showing the direction of jaw action. Other movements could produce similar scratches though, such as movement of the bones of the two halves of the lower jaw. Not all models have been scrutinized under present techniques.[89] Vincent Williams and colleagues (2009) published additional work on hadrosaurid tooth microwear. They found four classes of scratches onEdmontosaurus teeth. The most common class was interpreted as resulting from an oblique motion, not a simple up-down or front-back motion, which is consistent with the Weishampel model. This motion is thought to have been the primary motion for grinding food. Two scratch classes were interpreted as resulting from forward or backward movement of the jaws. The other class was variable and probably resulted from opening the jaws. The combination of movements is more complex than had been previously predicted.[77]
Weishampel developed his model with the aid of a computer simulation. Natalia Rybczynski and colleagues have updated this work with a much more sophisticatedthree-dimensional animation model, scanning a skull ofE. regalis with lasers. They were able to replicate the proposed motion with their model, although they found that additional secondary movements between other bones were required, with maximum separations of 1.3 to 1.4 centimetres (0.51 to 0.55 in) between some bones during the chewing cycle. Rybczynski and colleagues were not convinced that the Weishampel model is viable, but noted that they have several improvements to implement to their animation. Planned improvements include incorporating soft tissue and tooth wear marks and scratches, which should better constrain movements. They note that there are several other hypotheses to test as well.[88] Further research published in 2012 by Robin Cuthbertson and colleagues found the motions required for Weishampel's model to be unlikely, and favored a model in which movements of the lower jaw produced grinding action. The lower jaw's joint with the upper jaw would permit anterior–posterior motion along with the usual rotation, and the anterior joint of the two halves of the lower jaw would also permit motion; in combination, the two halves of the lower jaw could move slightly back and forth as well as rotating slightly along their long axes. These motions would account for the observed tooth wear and a more solidly constructed skull than modeled by Weishampel.[90]
In a 2011 study, Campione and Evans recorded data from all known "edmontosaur" skulls from the Campanian and Maastrichtian and used it to plot amorphometric graph, comparing variable features of theskull with skull size. Their results showed that within both recognizedEdmontosaurus species, many features previously used to classify additional species or genera were directly correlated with skull size. Campione and Evans interpreted these results as strongly suggesting that the shape ofEdmontosaurus skulls changed dramatically as they grew. This has led to several apparent mistakes in classification in the past. The Campanian speciesThespesius edmontoni, previously considered a synonym ofE. annectens due to its small size and skull shape, is more likely a subadult specimen of the contemporaryE. regalis. Similarly, the three previously recognized Maastrichtian edmontosaur species likely represent growth stages of a single species, withE. saskatchewanensis representing juveniles,E. annectens subadults, andAnatotitan copei fully mature adults. The skulls became longer and flatter as the animals grew.[17]
In a 2014 study, researchers proposed thatE. regalis reached maturity in 10-15 years of age.[91] In a 2022 study, Wosik and Evans proposed thatE. annectens reached maturity in 9 years of age based on their analysis for various specimens from different localities. They found the result to be similar to that of other hadrosaurs.[54]
The brain ofEdmontosaurus has been described in several papers and abstracts through the use ofendocasts of the cavity where the brain had been.E. annectens[92][93] andE. regalis,[18] as well as specimens not identified to species,[94][95][96] have been studied in this way. The brain was not particularly large for an animal the size ofEdmontosaurus. The space holding it was only about a quarter of the length of the skull,[18] and various endocasts have been measured as displacing 374 millilitres (13 US fl oz)[96] to 450 millilitres (15 US fl oz),[95] which does not take into account that the brain may have occupied as little as 50% of the space of the endocast, the rest of the space being taken up by thedura mater surrounding the brain.[95][96] For example, the brain of the specimen with the 374 millilitre endocast is estimated to have had a volume of 268 millilitres (9 US fl oz).[96] The brain was an elongate structure,[95] and as with other non-mammals, there would have been noneocortex.[96] LikeStegosaurus, theneural canal was expanded in the hips, but not to the same degree: the endosacral space ofStegosaurus had 20 times the volume of its endocranial cast, whereas the endosacral space ofEdmontosaurus was only 2.59 times larger in volume.[95]
In 2003, evidence oftumors, includinghemangiomas,desmoplastic fibroma,metastatic cancer, andosteoblastoma, was described inEdmontosaurus bones. Rothschildet al. tested dinosaur vertebrae for tumors usingcomputerized tomography andfluoroscope screening. Several other hadrosaurids, includingBrachylophosaurus,Gilmoreosaurus, andBactrosaurus, also tested positive. Although more than 10,000 fossils were examined in this manner, the tumors were limited toEdmontosaurus and closely related genera. The tumors may have been caused by environmental factors orgenetic propensity.[97]
Osteochondrosis, or surficial pits in bone at places where bones articulate, is also known inEdmontosaurus. This condition, resulting fromcartilage failing to be replaced by bone during growth, was found to be present in 2.2% of 224 edmontosaur toe bones. The underlying cause of the condition is unknown. Genetic predisposition, trauma, feeding intensity, alterations in blood supply, excessthyroid hormones, and deficiencies in various growth factors have been suggested. Among dinosaurs, osteochondrosis (like tumors) is most commonly found in hadrosaurids.[98]
Like other hadrosaurids,Edmontosaurus is thought to have been afacultative biped, meaning that it mostly moved on four legs, but could adopt a bipedal stance when needed. It probably went on all fours when standing still or moving slowly, and switched to using the hind legs alone when moving more rapidly.[3] Research conducted by computer modeling in 2007 suggests thatEdmontosaurus could run at high speeds, perhaps up to 45 kilometres per hour (28 mph).[32] Further simulations using a subadult specimen estimated as weighing 715 kilograms (1,576 lb) when alive produced a model that could run or hop bipedally, use atrot,pace, or single foot symmetric quadrupedal gait, or move at agallop. The researchers found to their surprise that the fastest gait waskangaroo-like hopping (maximum simulated speed of 17.3 metres per second (62 km/h; 39 mph)), which they regarded as unlikely based on the size of the animal and lack of hopping footprints in the fossil record, and instead interpreted the result as indicative of an inaccuracy in their simulation. The fastest non-hopping gaits were galloping (maximum simulated speed of 15.7 metres per second (57 km/h; 35 mph)) and running bipedally (maximum simulated speed of 14.0 metres per second (50 km/h; 31 mph)). They found weak support for bipedal running as the most likely option for high-speed movement, but did not rule out high-speed quadrupedal movement.[99]
While long thought to have been aquatic or semiaquatic, hadrosaurids were not as well-suited for swimming as other dinosaurs (particularly theropods, who were once thought to have been unable to pursue hadrosaurids into water). Hadrosaurids had slim hands with short fingers, making their forelimbs ineffective for propulsion, and the tail was also not useful for propulsion because of the ossified tendons that increased its rigidity, and the poorly developed attachment points for muscles that would have moved the tail from side to side.[100][101]
Extensivebone beds are known forEdmontosaurus, and such groupings of hadrosaurids are used to suggest that they were gregarious, living in groups.[3] Three quarries containingEdmontosaurus remains are identified in a 2007 database of fossil bone beds, from Alberta (Horseshoe Canyon Formation), South Dakota (Hell Creek Formation), and Wyoming (Lance Formation). One edmontosaur bone bed, fromclaystone andmudstone of the Lance Formation in eastern Wyoming, covers more than a square kilometre, althoughEdmontosaurus bones are most concentrated in a 40 hectares (0.15 sq mi) subsection of this site. It is estimated that disassociated remains pertaining to 10,000 to 25,000 edmontosaurs are present here.[102]
Unlike many other hadrosaurids,Edmontosaurus lacked a bony crest. It may have had soft-tissue display structures in the skull, though: the bones around the nasal openings had deep indentations surrounding the openings, and this pair of recesses are postulated to have held inflatable air sacs, perhaps allowing for both visual and auditory signaling.[34]Edmontosaurus may have beendimorphic, with more robust and more lightly built forms, but it has not been established if this is related tosexual dimorphism.[103]
Edmontosaurus has been considered a possibly migratory hadrosaurid by some authors. A 2008 review of dinosaur migration studies by Phil R. Bell and Eric Snively proposed thatE. regalis was capable of an annual 2,600 kilometres (1,600 mi) round-trip journey, provided it had the requisitemetabolism and fat deposition rates. Such a trip would have required speeds of about 2 to 10 kilometres per hour (1 to 6 mph), and could have brought it from Alaska to Alberta.[104][105] In contrast to Bell and Snively,Anusuya Chinsamy and colleagues concluded from a study of bone microstructure that polarEdmontosaurus overwintered.[106]
Edmontosaurus was a wide-ranging genus in both time and space. At the southern range of its distribution, the rock units from which it is known can be divided into two groups by age: the older Horseshoe Canyon and St. Mary River formations, and the younger Frenchman, Hell Creek, and Lance formations. The time span covered by the Horseshoe Canyon Formation and equivalents is also known as Edmontonian, and the time span covered by the younger units is also known as Lancian. The Edmontonian and Lancian time intervals had distinct dinosaur faunas.[107] At its northern range,Edmontosaurus is known from a single locality; the Liscomb Bonebed of thePrince Creek Formation.[39]
The Edmontonian land vertebrate age is defined by the first appearance ofEdmontosaurus regalis in the fossil record.[108] Although sometimes reported as of exclusively early Maastrichtian age,[42] the Horseshoe Canyon Formation was of somewhat longer duration. Deposition began approximately 73 million years ago, in the lateCampanian, and ended between 68.0 and 67.6 million years ago.[109]Edmontosaurus regalis is known from the lowest of five units within the Horseshoe Canyon Formation, but is absent from at least the second to the top.[110] As many as three quarters of the dinosaur specimens frombadlands nearDrumheller, Alberta may pertain toEdmontosaurus.[111]
The Lancian time interval was the last interval before theCretaceous–Paleogene extinction event that eliminated non-avian dinosaurs.Edmontosaurus was one of the more common dinosaurs of the interval.Robert T. Bakker reported that it made up one-seventh of the large dinosaur sample, with most of the rest (five-sixths) made up of the horned dinosaurTriceratops.[112] Thecoastal plainTriceratops–Edmontosaurus association, dominated byTriceratops, extended from present-dayColorado toSaskatchewan.[113]
The Lance Formation, as typified by exposures approximately 100 kilometres (62 mi) north ofFort Laramie in eastern Wyoming, has been interpreted as abayou setting similar to theLouisiana coastal plain. It was closer to a large delta than the Hell Creek Formation depositional setting to the north and received much more sediment. Tropicalaraucarian conifers andpalm trees dotted thehardwood forests, differentiating the flora from the northern coastal plain.[114] The climate was humid and subtropical, with conifers,palmettos, and ferns in the swamps, and conifers,ash,live oak, andshrubs in the forests.[36]Freshwater fish,salamanders, turtles, diverse lizards,snakes,shorebirds, and small mammals lived alongside the dinosaurs. Small dinosaurs are not known in as great of abundance here as in the Hell Creek rocks, butThescelosaurus once again seems to have been relatively common.Triceratops is known from many skulls, which tend to be somewhat smaller than those of more northern individuals. The Lance Formation is the setting of two edmontosaur "mummies".[114]
The time span and geographic range ofEdmontosaurus overlapped withTyrannosaurus, and an adult specimen ofE. annectens on display in theDenver Museum of Nature and Science shows evidence of atheropod bite in the tail. Counting back from the hip, the thirteenth to seventeenthvertebrae have damagedspines consistent with an attack from the right rear of the animal. One spine has a portion sheared away, and the others are kinked; three have apparent tooth puncture marks. The top of the tail was at least 2.9 metres (9.5 ft) high, and the only theropod species known from the same rock formation that was tall enough to make such an attack isT. rex. The bones are partially healed, but the edmontosaur died before the traces of damage were completely obliterated. The damage also shows signs of bone infection. Kenneth Carpenter, who studied the specimen, noted that there also seems to be a healed fracture in the left hip which predated the attack because it was more fully healed. He suggested that the edmontosaur was a target because it may have been limping from this earlier injury. Because it survived the attack, Carpenter suggested that it may have outmaneuvered or outrun its attacker, or that the damage to its tail was incurred by the hadrosaurid using it as a weapon against the tyrannosaur.[115] However, more modern studies dispute the idea of an attack but rather other factors unrelated to an attack from a tyrannosaur.[116]
Another specimen ofE. annectens, pertaining to a 7.6 metres (25 ft) long individual from South Dakota, shows evidence of tooth marks from small theropods on its lower jaws. Some of the marks are partially healed. Michael Triebold, informally reporting on the specimen, suggested a scenario where small theropods attacked the throat of the edmontosaur; the animal survived the initial attack but succumbed to its injuries shortly thereafter.[117] Some edmontosaur bone beds were sites of scavenging.Albertosaurus andSaurornitholestes tooth marks are common at one Alberta bone bed,[118] andDaspletosaurus fed onEdmontosaurus and fellow hadrosauridSaurolophus at another Alberta site.[119] However more recent studies suggest that any and all evidence forDaspletosaurus being present in the Horseshoe Canyon Formation is referrable toAlbertosaurus.[120][121]
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