Hadrosaurus, meaning "big lizard," has the distinction of being the first dinosaur to be discovered in North America. Its bones were found in New Jersey, and it was reconstructed and named in 1858 by the American professor of anatomy, Joseph Leidy, of the University of Pennsylvania. He recognized that Hadrosaurus was structurally related to Iguanodon, whose remains had first been found in southern England, and described in 1825 (see p. 144).
But unlike the early, inaccurate reconstructions of Iguanodon by Mantell, the geologist — as a 4-legged, dragonlike creature — Leidy, the anatomist, knew from the structure of Hadrosaurus that it could also rise up on its hindlegs. He showed it in a running pose as a bipedal animal — standing upright on 2 legs, its short arms dangling and its body bent horizontal to the ground, balanced by the long, outstretched tail.
Hadrosaurus was the typical member of the duckbill family. Like Kritosaurus (above), it had no crest on its long, low head, but there was a large hump on its snout, made of solid bone, and probably covered with thick, hard skin. Its bill, at the front of the jaws, had no teeth, but there were hundreds of teeth at the back of the jaws, in a continual state of replacement.
Hadrosaurs could move their upper and lower jaws against each other vertically and horizontally. This produced strong chewing and grinding actions, which would have thoroughly pulverized the food before it was swallowed.
kritosaurus shantungosaurus edmontosaurus kritosaurus shantungosaurus edmontosaurus name: Maiasaura time: Late Cretaceous locality: North America (Montana) size: 30 ft/9 m long
The discovery of Maiasaura in 1978 has given paleontologists a new insight into the family life of dinosaurs. In that year, a complete nesting site was found in Montana — the remains of an ancient nursery, some 75 million years old, where duckbills laid their eggs, and the young developed in safety.
This exciting find consisted of the skeleton of an adult (the mother, presumably); several youngsters (each about 3 ft 3 in/lm long); a group of hatchlings (each about 20 in/50 cm long) together in a fossilized nest; and several other nests complete with intact eggs, and pieces of broken shell lying around.
The nests themselves had been made of heaped-up mounds of mud, now of course solid rock. Each nest was about 10ft/3 m in diameter and 5 ft/1.5 m high. There was a craterlike depression in the center of each mound, about 6 ft 6 in/2 m in diameter and 2 ft 5 in/ 0.75 m deep. Spacing between the nests was about 23 ft/7 m, which meant that the mothers nested fairly close to each other, since the average length of Maiasaura was some 26 ft/8 m.
The fossilized eggs found in the nests were obviously laid with care. They were arranged in circles within the crater, layer upon layer. The mother probably covered over each layer with earth or sand as she went, and then covered the whole nest with earth when she was finished. This would have kept the eggs warm while they hatched, and well concealed from predators.
These duckbilled hadrosaurs were obviously social animals, as seen by this nursery site. The females nested in groups, probably even returning to the same site each year, as do some modern animals — seabirds, turtles and fish.
The youngsters stayed with their mothers until they were mature enough to fend for themselves — also seen in many modern groups.
name: Shantungosaurus time: Late Cretaceous locality: Asia (China) size: 43 ft/13 m long
This massive, flat-headed duckbill was among the biggest of the hadrosaurs. An almost complete specimen was discovered in Shandong (formerly Shantung) Province of eastern China during the 1970s, and its reconstructed skeleton is now on display in Beijing's (Peking) Natural History Museum.
Shantungosaurus had an extra-long tail, accounting for almost half the total length of its body. This was necessary to counterbalance the animal's greatweight — probably over 5 US tons/4.5 tonnes; when Shantungosaurus walked upright, the tail was held out stiffly behind, to balance the body at the hips.
Like that of other hadrosaurs, this duckbill's tail was deep, and flattened from side to side — rather like the tail of a modern crocodile. Because of this, paleontologists originally thought that duckbills spent most of their time in water, and used their tails for swimming. However, bony tendons lashed together all the vertebrae of the tail, making it much too rigid to have acted as a paddle. Also, the spines above and below the tail vertebrae sloped backward; in true aquatic animals, these spines are vertical, to provide attachment points for strong swimming muscles.
This does not mean that Shantun-gosaurus and its relatives did not venture into water. Most likely they did, to escape the attentions of contemporary predators such as Tarbosaurus and Alioramus in Asia, or Albertosaurus and Tyrannosaurus in North America.
name: Anatosaurus time: Late Cretaceous locality: North America (Alberta) size: 33 ft/10 m long
The popular name for this group of dinosaurs, the "duckbills," was coined after the discovery of the broad, flat skull of this hadrosaur in western North America. The name Anatosaurus, in fact, means "duck lizard," and refers to its horny, toothless bill or beak.
Several well-preserved skeletons of Anatosaurus have been found. From these, paleontologists can tell that this duckbill was over 30 ft/9 m long, stood some 13 ft/4 m tall, and probably weighed about 4 US tons/3.6 tonnes. Two "mummified" specimens were also found — a rare discovery — with dried-up tendons and the contents of their stomachs intact. The last meal eaten by these 2 individuals consisted of pine needles, twigs, seeds and fruits.
Impressions of the animals' skin were also preserved, stamped into the surrounding rocks. Though all the skin had long since rotted, these impressions show that Anatosaurus was covered in a thick, leathery hide.
The same mummified specimens appeared to have had webs of skin between the 3 main fingers of each hand. At first, this seemed to reinforce the theory that duckbills were aquatic animals, using their webbed hands and flattened tails for swimming. But closer examination indicated that the hands could not be stretched widely, and the webs of skin were more likely to be the shriveled-up remains of weight-bearing walking pads, calloused with wear — like those on the feet of modern camels. Such pads tie in with the presence of hooflike nails on 2 of the main fingers of each hand, and support the current theory that duckbills were true land-dwellers that walked mainly on all-fours.
name: Edmontosaurus time: Late Cretaceous locality: North America (Alberta and Montana) size: 43 ft/13 m long Many skulls of this large, flat-headed duckbill have been found. The teeth are particularly well preserved, and show the typical hadrosaur pattern. Behind the toothless beak, banks of tightly packed teeth formed a veritable grinding pavement in both the upper and lower jaws. As those at the top were worn down and discarded, new teeth replaced them from the bottom. At any one time, there may have been over 1000 teeth in Edmontosaurus' mouth.
The outer edge of each tooth was coated with hard enamel, which wore away more slowly than the dentine that made up the rest of the tooth. The result was that each tooth had a cutting ridge of hard, upstanding enamel. Packed so closely together in each jaw, the batteries of teeth presented a coarse, abrasive surface for pulverizing plant food.
The jaw structure was rather like that of lguanodon (see p. 144) in the way that the upper jaw could move over the lower jaw, so that the teeth ground against each other when the mouth was closed. This produced a shearing, grinding action between the tooth rows, capable of shredding the toughest of plant material. Indeed, Edmontosaurus and its relatives lived on coarse food, as the fossilized stomach contents of Anatosaurus (above) showed.
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