Discovery
Hadrosaurs were the first family of dinosaurs to be identified in North America, beginning with the
Joseph Leidy's eponymous Hadrosaurus in 1858. Anatosaurus ("duck lizard") was a closely related hadrosaur,
whose best specimens were recovered in 1908 by the fossil collector Charles Hazelius Sternberg and his three
sons. Sternberg was a provider to the great Edward Drinker Cope during his famous competition to name new
species with Othniel Charles Marsh. Edmontosaurus was a victory for Cope in the race.
However, Lawrence M. Lambe named Edmontosaurus ("lizard from Edmonton") in 1917 from a find in the Edmonton
Rock Formation, Alberta. Anatosaurus was later reclassified as a species of Edmontosaurus, E. copei
- the original sample was probably a young Edmontosaurus.
The 1942 study by Lull & Wright is the source for most of our understanding of Edmontosaurus.
Description
The hadrosaurs are known as "duck-billed" dinosaurs due to the similarity of their head to that of a modern
duck. The whole front of the skull was flat and broadened out to form a beak, ideal for clipping leaves and
twigs from the tropical forests of North America. However, the back of the mouth contained literally
thousands of teeth suitable for grinding food before it was swallowed. Thus Edmontosaurus could pass the
toughest food back and forth across the teeth with its muscular cheek pouches. To fit so many teeth into its
mouth, they were packed into tight "banks" of up to sixty rows, and new teeth constantly grew to replace lost
teeth - like a modern shark. The bones of the upper jaw would flex outwards as lower jaw came up, so the
mandible could grind against it. Typical food would have included conifer needles, seeds and twigs, and
these have been found in the body cavities of fossilized Edmontosaurs. It was evidently a tree-browser.
Edmontosaurus lived in the Cretaceous period, 73 to 65 million years ago. A fully-grown adult could have
been up to nine metres long, and some of the larger species reached thirteen metres. Weight was in the region
of 3.5 tonnes, making it one of the largest hadrosaurs.
The 1908 discovery in Wyoming was especially remarkable in that paleontologists actually recovered
fossilized imprints of Edmontosaurus' skin. The impression must have been left by the skin drying very quickly
and fixing its shape into the mud. It is from these impressions that we know the skin was scaly and leathery,
and the thigh muscle was under the skin of the body. This would have given the impression that Edmontosaurus'
leg left its body at the knee, and the whole thigh was under the skin. This only contributes to its resemblance
to a duck. It also had a number of tubercles (bumps) along its neck and down its back and tail.
Edmontosaurus was bipedal dinosaur but could certainly have walked on four legs. The forelimbs are shorter
than the hinds but not sufficiently that four-legged motivation was unfeasible. The front feet also had
hooves on two fingers, and weight-bearing pads like those of Camarasaurus. The rear feet had two toes
and all were hoofed. The bone structure in the lower limbs suggests that both the legs and feet were attached
to very powerful muscles. The spine curved downwards at the shoulders, so Edmontosaurus would have had a low
posture and would have browsed close to the ground. Despite the power of the limbs, Edmontosaurus would only
have been slow-moving and had few defensive features. To survive, it must have had keen eyesight, hearing and
smell to get early warning of predators.
The structure of the skull suggests it may have had loose skin around its nose, like the inflatable pouch of
a bullfrog. This could have been inflatable, in order to intimidate other dinosaurs or as part of the mating
ritual.
Edmontosaurus existed in the same place and time period as Tyrannosaurus rex and one specimen on display
in the Denver Museum of Nature and Science has evidence of a T. rex bite in the tail. This vicious attack on
the bone suggests that the Edmontosaurus was alive at the time and hence T. rex was probably not a scavenger,
as had been suggested in the 1970s. A mass graveyard discovery in Alberta, Canada suggests that
Edmontosaurus lived in herds. These herds may have migrated with the seasons, from the North Slope of Alaska,
where plantlife would have been scarce during the dark winter months, to the richer pastures of Alberta. If
this is the case, T. rex would probably have gone with it, in search of a relatively easy meal.
Classification
The
type species is Lambe's
E. regalis. As has been mentioned,
the dinosaur formally known as Anatosaurus was really a young
E. copei. Marsh named
Claosaurus in 1892, but this has now been
reclassified as
E. annectens. Likewise, C.M. Sternberg named
Thespesius in 1926, but this is also a
species of Edmontosaurus,
E. saskatchewanensis.