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Last updated on April 23, 2014 at 21:24 EDT

Edmontosaurus

Edmontosaurus is a genus of a crestless hadrosaurid, meaning duck-billed, dinosaur. It includes two species: Edmontosaurus regalis and Edmontosaurus annectens. Fossils of E. regalis have been recovered in rocks of western North America that date from the late Campanian stage of the Cretaceous Period 73 million years ago, while those of E. annectens were recovered in the same geographic region but in rocks that dated to the end of the Maastrichtian stage of the Cretaceous, 65.5 million years ago. E. annectens was one of the last non-avian dinosaurs, and lived in conjunction with Triceratops horridus and Tyrannosaurus rex a short time before the Cretaceous-Paleogene extinction event.

Edmontosaurus consisted of some of the largest hadrosaurid species, measuring up to 39 feet long and weighing around 4.4 short tons. Several well-preserved specimens are known that contain not only bones, but in some cases extensive skin impressions and possible gut contents. It’s classified as a genus of saurolophine hadrosaurid, a member of the hadrosaurids group which lacked hollow crests.

The first fossils named Edmontosaurus were discovered in southern Alberta, Canada, and the Horseshoe Canyon Formation. The type species, E. regalis, was named by Lawrence Lambe in the year 1917, though several other species that are now classified in Edmontosaurus were named earlier.

Edmontosaurus was distributed widely across the western part of North America. The distribution of Edmontosaurus fossils suggests that it had a preference for coasts and coastal plains. It was an herbivore that was able to move on both two legs and four. Because it’s known from several bone beds, Edmontosaurus is considered to have lived in groups, and may have been migratory as well. The wealth of fossils has enabled researchers to study its paleobiology in detail, including its brain, how it might have fed, and its pathologies and injuries, such as evidence for a tyrannosaur attack on one edmontosaur specimen.

It had been described in detail from several specimens. Like other hadrosaurids, it was a large animal with a long and literally flattened tail and a head with an expanded beak like a ducks. The skull had no hollow or solid crest, not like many other hadrosaurids. The fore legs weren’t as heavily constructed as the hind legs, but were long enough to be used while standing or in movement. Edmontosaurus was among the largest hadrosaurids; depending on the species, a fully grown adult could have been 30 ft in length, and some of the larger specimens reached the range of 39 feet to 43 feet in length. Its weight was around of 4.4 short tons. The type specimen of E. regalis, NMC 2288, is estimated as 30 to 39 feet in length. E. annectens is frequently seen as smaller. Two famous mounted skeletons, USNM 2414 and YPM 2182, measure 26.25 ft in length and 29.3 feet long, correspondingly. However, these are most likely sub adult individuals, and there is at least one documentation of a much larger potential E. annectens specimen that is almost 39 feet in length.

The skull of a fully grown Edmontosaurus could be over a yard long. A single skull of E. annectens measures 3.87 feet long. It’s roughly triangular in profile, with no bony cranial crest. The view from above, the front and rear of the skull were expanded, with the broad front forming a duck-bill or spoon-bill shape. The beak was without teeth, and both of the upper and lower beaks were extended by keratinous material. Considerable remains of the keratinous upper beak are known from the “mummy” being kept at the Senckenberg Museum. In this specimen, the preserved non-bony part of the beak extended for at least 3.1 inches beyond the bone, sticking out downwards vertically. The nasal openings of Edmontosaurus were elongate and housed in deep depressions surrounded by distinctive bony rims above, below, and behind. In at least one case, rarely preserved sclerotic rings were preserved in their eye sockets. Another rarely seen bone, the stapes, has also been seen in an example of Edmontosaurus.

Some teeth were present only in maxillae, meaning upper cheeks, and dentaries, meaning the main bone of the lower jaw. The teeth were continually replaced, taking about a half of a year to form. They grew in columns, with an observed maximum of six in each one, and the number of columns varied depending on the size of the animal. Known column counts for the two species are: 51 to 53 columns per maxilla and 48 to 49 per dentary, the teeth of the upper jaw being somewhat narrower than those in the lower jaw, for E. regalis; and 52 columns per maxilla and 44 per dentary for E. annectens.

Numerous specimens of Edmontosaurus have been recovered with preserved impressions of the skin. Several have been made known, such as the “Trachodon mummy” of the early 20th century, and the sample nicknamed “Dakota”, the latter appears to have included remnant organic compounds from the skin. Due to these finds, the scalation of Edmontosaurus is known for most of the areas of the body.

AMNH 5060, the “Trachodon mummy”, so named because of its appearance to be a fossil of a natural mummy, is now recognized as a specimen of E. annectens. It was found to contain skin impressions over the snout, much of the neck and torso, and some parts of the arms and legs. The tail and part of the legs had gone through erosion before collection, so these areas are not known for the specimen. In addition, some of the areas with skin impressions, for example, sections that are associated with the neck ridge and hands, were removed on accident during preparation of the specimen. The specimen is though to have dried out in a dry stream bed, most likely near or on a point bar. The conditions of the location and preservation of the body propose that the animal died during a prolonged drought, maybe from starvation. The dried out carcass was finally buried in a sudden flood, surrounded by sediment that had the right amount of fine particles to make a cast of the epidermal structures.

The epidermis was thin, and the scalation composed of small scales that do not overlap, as seen in the Gila monster. Two usual types of scales were present over the majority of the body: small and pointed or convex tubercles, .039 to .12 inches in diameter with no definite positioning; and larger and flat polygonal tubercles typically less than .20 inches in diameter, but up to .39 inches over the forearm. The pavement tubercles were grouped into groups separated by ground tubercles, with transitional scales between the two types. Over the majority of the body, the pavement tubercles were positioned in circular or oval groups, while near the shoulder on the upper arm, they created strips roughly parallel to each other and the shoulder blade. Usually, the groups were larger on the upper surfaces of the body and smaller of the underside of the body. Groups up to 20 inches in length were present above the hips.

The Edmontosaurus was a hadrosaurid, a duck-billed dinosaur member of a family of dinosaurs which, so far, are known only from the Late Cretaceous. It’s classified in the Saurolophinae, a clade of hadrosaurids lacking hollow crests.

The skulls from Edmontosaurus became flatter and longer as the animals grew.

Being a hadrosaurid, Edmontosaurus was a large terrestrial herbivore. The teeth were continually replaced and packed into dental batteries that consisted of hundreds of teeth, only a relative handful of which were utilized at any time. It used its wide beak to cut loose food, maybe by cropping, or by closing its jaws in a clamshell-like way over twigs and branches and then stripping off the more nutritious shoots and leaves. Due to the tooth rows being deeply indented from the outside of the jaws, and because of other anatomical details, it’s inferred that Edmontosaurus and most other ornithischians had structures much like cheeks, muscular or non-muscular. The function of said cheeks was to hold on to food in the mouth. The feeding range would have been from ground level to around 13 feet above.

At that time, hadrosaurids were believed to have been aquatic animals, and Krausel made a point of stating that the sample didn’t rule out hadrosaurids consuming water plants.

Image Caption: Edmontosaurus at the Oxford University Museum of Natural History, Oxford, England. Credit: Kevin Walsh/Wikipedia  (CC BY 2.0)

Edmontosaurus