Wednesday, April 29, 2009

Hadrosaurs

The most important plant-eaters at the end of the age of dinosaurs were the hadrosaurids, or duck-billed dinosaurs.

Hadrosaurus was one of the first dinosaurs discovered in North America, and one of the first nearly complete skeletons ever found. First noticed in a marl pit in 1838 near Haddonfield, New Jersey, it was nearly twenty years before the fossilized remains were recognized, named, and excavated.
Few remains of hadrosaurus have been found since this original specimen (now on display at the Philadelphia Academy of Science), but a host of similar dinosaurs with duck-bill shaped skulls are now categorized as hadrosaurids. Several distinct features set hadrosaurs apart from other plant-eaters of the time, especially their beaks and unusual head crests.

Hadrosaurs evolved along with the development of flowering plants, from 100 to 65 million years ago, and may have risen to prominence because they were able to eat this new food source.

It was originally believed that the duckbill shaped beak suggested that hadrosaurs lived in water. The tail, which has a finned shape like a crocodile's, would seem to support this idea. But with few other body parts appearing to be adapted to an aquatic existence, this theory has now been widely discredited. Still, hadrosaurs would have had no problem crossing a river or wading through a swamp. Their limbs allowed them to walk on all fours or on just the two rear legs.
The hadrosaur neck is S-shaped, consistent with a low feeder, like a modern horse or buffalo. Different animals have different shaped beaks, however: narrow-beaked dinosaurs would have been selective eaters, while broad-mouthed dinosaurs would have eaten great mouthfuls of plants indiscriminately.

Hadrosaurs had teeth only in the cheek region, near the back of the jaw. They were closely packed into sections of fifty teeth or more, with columns of "back ups" as teeth became broken by cutting through rough plant material. Some hadrosaurs had more than a thousand teeth at a time, which were regularly replaced, like all dinosaur teeth, or like modern crocodiles and sharks.

The hadrosaur group evolved rapidly, and into many varied forms. They have been found in every continent, except for Australia and Africa. Some were more than 12 meters long, and weighed up to 5 tons. Animals this large had to be fast and agile enough to avoid predators, which required massive amounts of energy from food. Evidence suggests that hadrosaurs lived in groups, and when they ate, hadrosaur groups must have denuded entire forests to satisfy their need for food.

Many hadrosaurs had impressive crests, which consisted of hollow tubes connected to the nasal passages. There are many theories about how these crests developed.

One idea is that the hadrosaurs may have relied heavily on their sense of smell, and that these tubes amplified this sense, helping the animal to detect slight variations in the scents in the air. Another possibility is that the crest was a sort of communication device. Air snorted through the tube would have made a loud honking sound, audible for miles. This may have been used for long distance communication, to warn others of the species when a predator was approaching. There is no reason why these theories could not both be true.

The discovery of hadrosaurs, around the time of the US Civil War, helped establish dinosaurs as a distinctive group of animals. It also reinforced the idea that some animals had lived on earth and become extinct before the first humans. After all, there are thousands of hadrosaur remains that have been found all over the world, yet no one ever mentions seeing a living one. It was around this time that Charles Darwin published his theory of natural selection, explaining that life on earth might be much older than anyone had previously suspected.

For my previous posts on dinosaurs, click here.

There are a lot of great books about dinosaurs, filled with lovely pictures, perfect for any coffee table. Two of my favorites are The Illustrated Encyclopedia of Dinosaurs, by Dougal Dixon, and Dinosaurus: The Complete Guide to Dinosaurs, by Steve Parker.

3 comments:

Anonymous said...

Great stuff man. Hadrosaurs are an extraordinary family of animals that just don't seem to get the love some of their more famous cousins do.

Dave said...

I actually helped restore the original Hadrosaur skeleton in the New Jersey State museum in Trenton. That'd be around 1965...

Hansisgreat said...

Wow, that's really cool. That skeleton is world famous.
This whole area was filled with hadrosaurs during the Cretaceous, according to this book "When Dinosaurs Roamed New Jersey". If you're not familiar, it's worth checking out, although I haven't reviewed it on the site.
In what capacity did you work for the State Museum, Dave? Are you a paleontologist?