Edmontosaurus

Edmontosaurus (meaning "lizard from Edmonton") is a genus of hadrosaurid (duck-billed) dinosaur. It had two known species: Edmontosaurus regalis and Edmontosaurus annectens. Fossils of E. regalis were found in rocks from 73 million years ago, while those of E. annectens came from rocks about 66 million years old. Edmontosaurus lived among famous dinosaurs like Triceratops, Tyrannosaurus, and Ankylosaurus right before a big event changed Earth forever.

Edmontosaurus was one of the largest duck-billed dinosaurs. E. annectens could grow up to 12 metres (39 feet) long and weigh as much as 5.6 metric tons. Some very big examples were even longer, around 15 metres (49 feet), and weighed about 15.9 metric tons. Scientists have found many well-preserved fossils, including bones, skin, and even some gut contents. This helps them learn a lot about how these dinosaurs lived.

The first Edmontosaurus fossils were found in southern Alberta, near the city of Edmonton. The species E. regalis was named in 1917 by Lawrence Lambe, while E. annectens was named earlier in 1892 by Othniel Charles Marsh. Edmontosaurus lived along coasts and coastal plains from Colorado to Alaska. It ate plants and could walk on two legs or four. Because many fossils were found together, scientists think Edmontosaurus lived in groups and may have moved around a lot. Researchers have even found evidence of original dinosaur material in one fossil, helping us understand these ancient creatures better.

Discovery and history

See also: Edmontosaurus mummy S.M. R4036 and Edmontosaurus mummy AMNH 5060

Claosaurus annectens

Edmontosaurus has a long and complicated history in the study of dinosaurs, with many of its species being placed in different groups over time. The oldest well-known species, E. annectens, was first named in 1892 as a type of Claosaurus. It was found in rocks from the late Maastrichtian age of the Cretaceous period in Wyoming.

This species was among the first dinosaurs to have a skeleton put together for display. One of the most famous specimens is known as a "mummified" dinosaur, found with skin preserved. These fossils helped scientists learn a lot about how these dinosaurs looked and lived.

Because scientists didn’t fully understand these dinosaurs at first, Claosaurus annectens was later called by many different names. Over time, more fossils were found, showing that there were several kinds of these duck-billed dinosaurs. Important discoveries included more "mummified" specimens with skin impressions, helping to clarify their appearance.

During this time, two more important fossils of C. annectens were found. The first, a very well-preserved specimen, was discovered in 1908. Another similar specimen was found in 1910, also with skin impressions.

Canadian discoveries

The name Edmontosaurus was created in 1917 for two partial skeletons found in Alberta, Canada. These fossils came from rocks older than those where Claosaurus annectens was found. The species was named after the Edmonton Formation where the fossils were discovered.

Two more species were later named from Canadian fossils in the 1920s. These were initially placed in another genus called Thespesius but are now considered part of Edmontosaurus.

Anatosaurus to the present

In 1942, scientists tried to simplify the many names by creating a new group called Anatosaurus. This group included several species that didn’t fit well into other names. Over time, studies showed that many of these species actually belonged to Edmontosaurus.

By the 1970s and 1980s, scientists concluded that the main species of Anatosaurus was really a type of Edmontosaurus. Today, scientists agree that there are two main species of Edmontosaurus: E. regalis from the late Campanian and E. annectens from the late Maastrichtian.

In 2007, another well-preserved specimen nicknamed “Dakota” was found in North Dakota, adding more information about these dinosaurs. A study in 2011 confirmed that only two species of Edmontosaurus are valid, with some previously named species actually being mature examples of E. annectens.

Species and distribution

Edmontosaurus has two known species: E. regalis and E. annectens. E. regalis fossils were found in Alberta and date from the late Campanian age of the Cretaceous period. We know about at least a dozen individuals of this species.

E. annectens fossils were found in Saskatchewan, Montana, South Dakota, and Wyoming. This species lived during the late Maastrichtian age. Scientists once thought other names like Anatosaurus copei were separate species, but now they believe these were just different growth stages of E. annectens.

E. annectens had a longer, lower skull compared to E. regalis. Some scientists thought they might be male and female versions of the same species, but E. regalis fossils are older than E. annectens fossils. Fossils from Alaska were once called Edmontosaurus, but some scientists think they might belong to a different genus named Ugrunaaluk. More studies are needed to know for sure. There was also a report of Edmontosaurus in Texas, but it was later identified as belonging to a different genus, Kritosaurus.

Prince Creek Formation, Javelina Formation, Big Bend National Park, Texas, Kritosaurus, nomen dubium

Description

Edmontosaurus is one of the largest duck-billed dinosaurs, or hadrosaurids, that ever lived. These big dinosaurs had long, flat tails and wide, duck-like beaks. Their arms were shorter than their legs but could help them move on all fours.

Scientists have studied many fossils of Edmontosaurus. One type, called E. regalis, was thought to be the biggest, but some believe another type, E. annectens, might have been just as large. Some E. annectens fossils suggest these dinosaurs could grow up to almost 15 meters long, though such huge examples were very rare. Most grown adults were around 9 to 12 meters long and weighed several tons.

The skull of Edmontosaurus was very large, over a meter long, with a wide, duck-like front. The beak had no teeth, but the sides of the mouth had many teeth that constantly grew back. These teeth were arranged in columns and could number up to 53 in the upper jaw and 49 in the lower jaw for E. regalis. The body had a strong frame with many bones and muscles that helped it stand and move. Skin impressions from some fossils show that the dinosaur had scaly skin in many places.

Classification

Edmontosaurus was a hadrosaurid (a duck-billed dinosaur), part of a family of dinosaurs known only from the Late Cretaceous. It belonged to the Saurolophinae (also called Hadrosaurinae), a group of hadrosaurids without hollow crests. Other dinosaurs in this group include Brachylophosaurus, Gryposaurus, Lophorhothon, Maiasaura, Naashoibitosaurus, Prosaurolophus, and Saurolophus. It was closely related to or included the species Anatosaurus annectens (also called Edmontosaurus annectens), a large hadrosaurid from western North America.

The exact position of Edmontosaurus within its group has been debated. Early studies grouped it with various species of Anatosaurus, but later analyses using different methods have shown different relationships. Some studies place it close to Saurolophus and Prosaurolophus, while others place it between Gryposaurus and the "brachylophosaurs". Edmontosaurus is the main species of the saurolophine tribe Edmontosaurini, which also includes Shantungosaurus, Kerberosaurus, and Laiyangosaurus.

Paleobiology

Diet and feeding

See also: Hadrosaur diet

As a hadrosaurid, Edmontosaurus was a large land animal that ate plants. Its teeth were constantly replaced and grouped into dental batteries with hundreds of teeth, though only a few were used at any time. It used its wide beak to grab food, possibly by cutting or by closing its jaws like a clamshell over twigs and branches, then stripping off nutritious leaves and shoots. Because the tooth rows were indented from the outside of the jaws, and because of other skull details, it is thought that Edmontosaurus and most other ornithischians had cheek-like structures to keep food in the mouth. The animal's feeding range would have been from ground level up to about 4 metres (13 ft) high.

Before the 1960s and 1970s, some scientists thought hadrosaurids like Edmontosaurus lived in water and ate water plants. An example of this is William Morris's 1970 interpretation of an edmontosaur skull with nonbony beak remnants. He suggested that the animal ate like some modern ducks, filtering plants and small water animals from the water and letting water out through V-shaped furrows along the inner face of the upper beak. This idea about the beak has been rejected, as the furrows and ridges look more like those of herbivorous turtle beaks than the flexible structures seen in filter-feeding birds.

Because scratches are common on the teeth, Williams and others suggested Edmontosaurus was a grazer instead of a browser, which would have fewer scratches from eating less tough materials. Possible foods that could have caused scratches include silica-rich plants like horsetails and soil that was accidentally eaten while feeding at ground level. When compared to the tooth wear of other hadrosaurids, however, the wear on Edmontosaurus teeth does not show that the foods it ate were much tougher than what other hadrosaurids ate. In places like Alaska, conifers would have been the main food, although the diet of Edmontosaurus would have changed with the seasons. The tooth structure shows it could both cut and grind plants.

Reports of stomach stones, or gastroliths, in the hadrosaurid Claosaurus are actually based on a misidentification. The specimen is actually of Edmontosaurus annectens. Barnum Brown, who found the specimen in 1900, called it Claosaurus because E. annectens was thought to be a species of Claosaurus at the time. It is more likely that the supposed gastroliths were gravel washed in during burial.

Gut contents

Both of the "mummy" specimens collected by the Sternbergs were reported to have possible gut contents. Charles H. Sternberg reported the presence of carbonized gut contents in the American Museum of Natural History specimen, but this material has not been described. The plant remains in the Senckenberg Museum specimen have been described, but have been hard to understand. The plants found in the carcass included needles from the conifer Cunninghamites elegans, twigs from conifers and broadleaf trees, and many small seeds or fruits. When described in 1922, they caused debate in a German journal. Kräusel, who described the material, interpreted it as the gut contents of the animal, while Abel thought the plants might have washed into the carcass after death.

At the time, hadrosaurids were thought to live in water, and Kräusel pointed out that the specimen did not rule out hadrosaurids eating water plants. The discovery of possible gut contents did not get much attention in English-speaking areas, except for a brief mention of the water-land debate, until John Ostrom brought it up in an article reevaluating the old idea of hadrosaurids as water animals. Instead of fitting the aquatic model, he used it as evidence that hadrosaurids were land animals that ate plants. While his view of hadrosaurids as land animals is now accepted, the Senckenberg plant fossils are still unclear. Kenneth Carpenter has suggested they might be from a starving animal, not a typical diet. Other authors have noted that because the plant fossils were taken from their original place in the specimen and prepared heavily, it is no longer possible to check the original work, leaving open the possibility that the plants were washed-in debris.

Isotopic studies

The diet and body processes of Edmontosaurus have been studied using stable isotopes of carbon and oxygen found in tooth enamel. When animals eat, drink, and breathe, they take in carbon and oxygen, which become part of their bones. The isotopes of these elements depend on various factors, such as the type of plants eaten, the animal's body processes, salt levels, and climate. If isotope ratios in fossils are not changed by fossilization and later changes, they can be studied for information about the original factors; warm-blooded animals will have certain isotopic compositions compared to their surroundings, animals that eat certain types of plants or use certain digestive processes will have different isotopic compositions, and so on. Enamel is often used because the structure of the mineral that forms enamel makes it very resistant to chemical change in the skeleton.

A 2004 study by Kathryn Thomas and Sandra Carlson used teeth from the upper jaw of three individuals interpreted as a young one, a nearly grown one, and a fully grown one, from a bone bed in the Hell Creek Formation of Corson County, South Dakota. In this study, teeth in columns in the edmontosaurs' dental batteries were sampled from many places along each tooth using a microdrilling system. This method takes advantage of the organization of hadrosaurid dental batteries to find changes in tooth isotopes over time. From their work, it seems that edmontosaur teeth took less than about 0.65 years to form, slightly faster in younger edmontosaurs. The teeth of all three individuals showed changes in oxygen isotope ratios that could match warm/dry and cool/wet periods; Thomas and Carlson considered the possibility that the animals were moving, but favored local seasonal changes because moving would have more likely mixed the ratios, as many animals move to stay within specific temperature ranges or near particular food sources.

The edmontosaurs also showed higher carbon isotope values, which for modern mammals would suggest a mixed diet of C3 plants (most plants) and C4 plants (grasses); however, C4 plants were very rare in the Late Cretaceous if they existed at all. Thomas and Carlson suggested several factors that might have been at work, and found the most likely to include a diet heavy in gymnosperms, eating salt-stressed plants from coastal areas next to the Western Interior Seaway, and a 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.

Chewing

E. annectens skull preserving the keratinous beak (partially removed on the right side by accident), Natural History Museum of Los Angeles

Between the mid-1980s and the 2000s, the common idea of how hadrosaurids processed their food followed a model proposed in 1984 by David B. Weishampel. He suggested that the structure of the skull allowed movement between bones that resulted in the lower jaw moving back and forth and the upper jaw's tooth-bearing bones bowing outward when the mouth was closed. The teeth of the upper jaw would grind against the teeth of the lower jaw like rasps, processing plant material caught between them. This movement would be similar to the effects of chewing in mammals, though done in a completely different way. Work in the early 2000s questioned the Weishampel model. A 2008 study by Casey Holliday and Lawrence Witmer found that ornithopods like Edmontosaurus lacked the types of skull joints seen in modern animals that can move their skull bones, such as squamates and birds. They suggested that joints thought to allow movement in dinosaur skulls were actually cartillaginous growth zones. An important piece of evidence for Weishampel's model is the direction of scratches on the teeth, showing the direction of jaw movement. Other movements could create similar scratches, such as movement of the bones of the two halves of the lower jaw. Not all models have been tested with current techniques. Vincent Williams and colleagues (2009) published more work on hadrosaurid tooth wear. They found four types of scratches on Edmontosaurus teeth. The most common type was thought to result from an oblique movement, not a simple up-down or front-back movement, which matches the Weishampel model. This movement is believed to have been the main way of grinding food. Two scratch types were thought to result from front or back movement of the jaws. The other type was variable and probably came from opening the jaws. The mix of movements is more complex than previously thought.

Weishampel developed his model with help from a computer simulation. Natalia Rybczynski and colleagues updated this work with a much more advanced three-dimensional animation model, scanning a skull of E. regalis with lasers. They were able to copy the proposed movement with their model, though they found that extra secondary movements between other bones were needed, with maximum gaps 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 works, but noted that they have several improvements to add to their animation. Planned improvements include adding soft tissue and tooth wear marks and scratches, which should better limit movements. They note that there are several other ideas to test as well. More research in 2012 by Robin Cuthbertson and colleagues found the movements needed for Weishampel's model unlikely, and favored a model in which movements of the lower jaw created grinding action. The lower jaw's joint with the upper jaw would allow front-to-back movement along with the usual rotation, and the front joint of the two halves of the lower jaw would also allow movement; together, the two halves of the lower jaw could move slightly back and forth as well as rotating a little along their long axes. These movements would explain the observed tooth wear and a stronger skull than modeled by Weishampel.

Growth

In a 2011 study, Campione and Evans collected data from all known "edmontosaur" skulls from the Campanian and Maastrichtian and used it to create a morphometric graph, comparing changeable features of the skull with skull size. Their results showed that within both recognized Edmontosaurus species, many features previously used to classify extra species or genera were directly linked to skull size. Campione and Evans interpreted these results as strongly suggesting that the shape of Edmontosaurus skulls changed a lot as they grew. This has led to several misclassifications in the past. The Campanian species Thespesius edmontoni, previously considered a synonym of E. annectens because of its small size and skull shape, is more likely a young specimen of the contemporary E. regalis. Similarly, the three previously known Maastrichtian edmontosaur species likely represent growth stages of one species, with E. saskatchewanensis representing young animals, E. annectens nearly grown animals, and Anatotitan copei fully grown adults. The skulls became longer and flatter as the animals grew.

In a 2014 study, researchers suggested that E. regalis reached maturity in 10-15 years. In a 2022 study, Wosik and Evans suggested that E. annectens reached maturity in 9 years based on their analysis of various specimens from different places. They found the result to be similar to that of other hadrosaurs.

Brain and nervous system

The brain of Edmontosaurus has been described in several papers and reports using endocasts of the space where the brain was. E. annectens and E. regalis, as well as specimens not identified to species, have been studied this way. The brain was not especially large for an animal the size of Edmontosaurus. The space holding it was only about a quarter of the length of the skull, and various endocasts have been measured to hold 374 millilitres (13 US fl oz) to 450 millilitres (15 US fl oz), which does not include that the brain may have taken up as little as 50% of the space of the endocast, the rest of the space being filled by the dura mater surrounding the brain. For example, the brain of the specimen with the 374-millilitre endocast is thought to have had a volume of 268 millilitres (9 US fl oz). The brain was a long shape, and as with other non-mammals, there was no neocortex. Like Stegosaurus, the neural canal was enlarged in the hips, but not to the same level: the endosacral space of Stegosaurus had 20 times the volume of its endocranial cast, while the endosacral space of Edmontosaurus was only 2.59 times larger in volume.

Pathologies and health

In 2003, evidence of tumors, including hemangiomas, desmoplastic fibroma, metastatic cancer, and osteoblastoma, was found in Edmontosaurus bones. Rothschild and others tested dinosaur vertebrae for tumors using computerized tomography and fluoroscope screening. Several other hadrosaurids, including Brachylophosaurus, Gilmoreosaurus, and Bactrosaurus, also tested positive. Although more than 10,000 fossils were examined this way, the tumors were only found in Edmontosaurus and closely related genera. The tumors might have been caused by environmental factors or genetic tendency.

Osteochondrosis, or small pits in bone where bones meet, is also known in Edmontosaurus. This condition, resulting from cartilage not being replaced by bone during growth, was found in 2.2% of 224 edmontosaur toe bones. The cause of the condition is unknown. Genetic predisposition, injury, feeding intensity, changes in blood supply, extra thyroid hormones, and shortages in various growth factors have been suggested. Among dinosaurs, osteochondrosis (like tumors) is most commonly found in hadrosaurids.

Locomotion

Like other hadrosaurids, Edmontosaurus is thought to have been a facultative biped, meaning it mostly moved on four legs, but could stand and walk on two legs when needed. It probably moved on all fours when standing still or moving slowly, and switched to using its back legs alone when moving faster. Research using computer modeling in 2007 suggests Edmontosaurus could run at high speeds, perhaps up to 45 kilometres per hour (28 mph). Further simulations using a young specimen estimated to weigh 715 kilograms (1,576 lb) when alive created a model that could run or hop on two legs, walk with a trot, pace, or single foot symmetric four-legged walk, or move at a gallop. The researchers were surprised to find that the fastest way was kangaroo-like hopping (maximum simulated speed of 17.3 metres per second (62 km/h; 39 mph)), which they thought unlikely based on the size of the animal and lack of hopping footprints in the fossil record, and instead took the result as showing an error in their simulation. The fastest non-hopping ways were galloping (maximum simulated speed of 15.7 metres per second (57 km/h; 35 mph)) and running on two legs (maximum simulated speed of 14.0 metres per second (50 km/h; 31 mph)). Although they found 10 repeats not enough for high-speed four-legged movement analysis, the paper points out that a high-speed four-legged gait cannot be ruled out for hadrosaurs.

While long thought to have lived in water or partly in water, hadrosaurids were not well-made for swimming like other dinosaurs (especially theropods, who were once thought unable to chase hadrosaurids into water). Hadrosaurids had slim hands with short fingers, making their front legs poor for pushing, and the tail was also not useful for pushing because of the ossified tendons that made it stiff, and the weak muscles for moving the tail from side to side.

Social behavior

Many bone beds are known for Edmontosaurus, and such groups of hadrosaurids are used to suggest that they lived together in groups. Three quarries with Edmontosaurus remains are listed 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, from claystone and mudstone of the Lance Formation in eastern Wyoming, covers more than a square kilometre, though Edmontosaurus bones are most crowded in a 40 hectares (0.15 sq mi) part of this site. It is estimated that scattered remains belonging to 10,000 to 25,000 edmontosaurs are present here.

Unlike many other hadrosaurids, Edmontosaurus did not have a bony crest. It may have had soft tissue structures in the skull, though: the bones around the nasal openings had deep indents around the openings, and this pair of indents are thought to have held inflatable air sacs, perhaps allowing for both visual and sound signals. Edmontosaurus may have had different forms, with more robust and more lightly built types, but it has not been proven if this is related to sexual differences.

Edmontosaurus has been considered a possibly migrating hadrosaurid by some scientists. A 2008 review of dinosaur migration studies by Phil R. Bell and Eric Snively suggested that E. regalis could have made an annual round-trip journey of 2,600 kilometres (1,600 mi), given the right metabolism and fat storage. Such a trip would have needed speeds of about 2 to 10 kilometres per hour (1 to 6 mph), and could have taken it from Alaska to Alberta. In contrast to Bell and Snively, Anusuya Chinsamy and colleagues concluded from a study of bone structure that polar Edmontosaurus stayed through the winter.

Paleoecology

Distribution

Edmontosaurus lived in many places and times. In the south, fossils come from rocks split into two groups by age: the older Horseshoe Canyon and St. Mary River formations, and the younger Frenchman, Hell Creek, and Lance formations. These time periods had different kinds of dinosaurs. In the north, Edmontosaurus is known from one place: the Liscomb Bonebed of the Prince Creek Formation.

The Edmontonian time period is marked by the first appearance of Edmontosaurus regalis in fossils. The Horseshoe Canyon Formation started about 73 million years ago in the late Campanian and ended between 68.0 and 67.6 million years ago. Edmontosaurus regalis is found in the lowest layers of this formation but not in the layers above. Many dinosaur fossils from badlands near Drumheller, Alberta may be from Edmontosaurus.

Ecosystem

The Lancian time interval was the last time before the Cretaceous–Paleogene extinction event that ended non-avian dinosaurs. Edmontosaurus was one of the common dinosaurs of this time. Robert T. Bakker noted that it made up one-seventh of large dinosaur samples, with most others being the horned dinosaur Triceratops. The area where Triceratops and Edmontosaurus lived stretched from present-day Colorado to Saskatchewan.

The Lance Formation, near Fort Laramie in Wyoming, looks like a bayou environment similar to Louisiana coastal plains. It had more sediment and was closer to a large delta than the northern Hell Creek Formation. The area had tropical conifers and palm trees in forests, along with ferns and shrubs in swamps. Freshwater fish, salamanders, turtles, lizards, snakes, shorebirds, and small mammals lived there with the dinosaurs. Small dinosaurs were less common here than in the Hell Creek rocks, but Thescelosaurus was relatively frequent. Triceratops skulls from this area tend to be smaller than those farther north. The Lance Formation includes two well-preserved Edmontosaurus fossils.

Predator-prey relationships

Edmontosaurus lived at the same time and place as Tyrannosaurus. One E. annectens specimen at the Denver Museum of Nature and Science shows signs of a theropod bite on its tail. The damaged vertebrae suggest an attack from behind. Though the bones show some healing, the dinosaur died before the damage fully disappeared. Some scientists think the Edmontosaurus might have been limping from an earlier injury, making it a target. However, newer studies question whether this was truly an attack.

Another E. annectens fossil from South Dakota has tooth marks from small theropods on its lower jaws, some of which are healed. One idea is that small theropods attacked the throat, and the Edmontosaurus later died from its injuries. Some Edmontosaurus bone sites show signs of scavenging by other dinosaurs, with teeth from Albertosaurus and Saurornitholestes found among them. Recent studies suggest that any evidence of Daspletosaurus in these rocks may actually belong to Albertosaurus.

A 2026 study described an E. annectens specimen with a tyrannosaurid tooth stuck in its nose, likely from a bite during a predation attempt.