Many bones possess distinctive characteristics related to function. Some of these characteristics reflect where certain muscles attached, creating scars. Others reflect fusions of old bones from deep within the evolutionary history of the clade (a group of organisms believed to have evolved from a common ancestor, according to the principles of cladistics).

It is true that many of the post-cranial bones (those of the skeleton behind the skull) often lack good characters - The long bones of the legs in particular are often not too distinctive. Some aspects of bone cell structure allow you to distinguish fish from reptile from mammal. However, certain bones are very distinctive, particularly those associated with the skull, teeth, and to a lesser degree, the pelvis. Much of this takes a trained eye to spot, and one of the things a vertebrate paleontologist must do is spend months as a student learning how to identify bone scraps.

That is a tough question if a scientist finds a bone that looks like nothing, like anything they have ever seen. It is like putting a puzzle together without having the final picture to work with. As you find a single bone, you look for others like it and hope that more bones are found.

For example, one leg bone may be found all by itself in China, but a similar leg bone may be found in Canada but with more of the skeleton with it. You can then infer that the one leg bone from China is the same animal and estimate the size and shape from that.

Identifying an animal from just a few bones can range anywhere from very easy to impossible. For each animal, there are certain bones that you could use to identify it in minutes, and certain bones that will never be able to give you a positive identification. Teeth are among the best bones to find,since they provide a lot of information, such as diet, size, and often are unique enough to be able to identify a species from just one tooth. Skulls in general are unique enough when studied in great detail so that we can compare a new skull to tons of old skulls preserved in museums to figure out what we have.

We also can use museum collections to identify other bones, but this is often difficult to do without teeth or a skull. Sometimes clues like the other bones found in a site, the age of the rocks around the fossils, and the geographic location can help us narrow down the possibilities. From most bones, we can guess a lot about an animal, but can't necessarily say EXACTLY what it is--for example, maybe we could say it was a porcupine, but we couldn't say precisely which type of porcupine.

First, it's important to remember that scientists can only theorize about the animals who left the fossilized bones, since nobody has actually seen these animals. Some of these theories end up being disprove, but most of the theories are almost certainly true.

There are a couple of reasons why I say that. The first is that all theories and findings made by scientists that are published in scientific journals(which is how everyone else gets to hear about them) have to undergo a process called "peer review". This is where other, objective scientists go over the theories and findings before they're published to be sure that they're accurate and sensible. So everything that's published in a scientific journal has been double- and triple-checked by a number of other scientists.

The other reason why most theories about dinosaur bones are probably true is that the people who make those theories know an awful lot about bone structure, about which bones do what, about what kinds of features identify a bone as coming from this or that kind of animal, and about how big or small bones need to be to perform different tasks. For example, certain features of a bone can tell them what kind of animal it came from (bird bones are hollow, for instance), and what body part. So now they know ,maybe, that a certain bone was from a bird-like animal whose arm (or wing)was 6 feet long. The thickness of the bone can give them a good guess of how much weight it needed to move around and, therefore, how much this animal probably weighed. They can identify the part of the bone that attached to muscle, and this can tell them how much the animal moved the limb and with how much force. That might give them a big clue about whether or not the limb was a wing used for flight, or a little-used front limb like those on T. Rex.

There are many other details scientists examine about the bones that can give them surprising amounts of information about the animal. Plus, scientists might find fossilized impressions in mud that can show them feathers, fins, and all kinds of other soft tissues.

I hope that answers your question! Remember that science is all about making good theories, since very few things can be proven beyond all doubt. The theories you hear about dinosaurs, though, are probably quite reliable. Keep asking such great questions!

Do you really like cars, or a certain style of music, or something else? You might be able to identify particular cars, songs, or other things apart without any trouble. To someone who is not a fan, each one is just another car, or another (country/salsa/rap) song. We pay attention to the details of what interests us.

My dad can look briefly at a car and say "57 Chevy Bel-Aire" or "42 Ford." If you found the skull of a mammal and showed it to me, I could probably tell you what it is without any trouble. That's because I think skulls are very cool, so I study them and learn about them. Skulls tell a whole story about an animal; where it lived, what it ate, how old it was, and more. So to a scientist, they're not just "some bones."

If you are interested in questions like this, you might want to look into a field called "forensic anthropology". These scientists learn all about crimes by looking at skeletons.

Just as different living animals look distinct from the outside (you'd never confuse a dog for a cat, or a starfish for a lobster), the parts of those animals that become fossilized (bones, teeth, and such) can be equally distinctive. It is as easy to tell horse teeth, cow teeth, and turtle hips apart, as it is living and breathing cows, horses, and turtles.

Different animals have different skeletons. There are attributes of a skeleton that define a group of animals, and if we find the remains of an extinct animal, we can figure out what group(s) it is in by comparing the shape and nature of its bones to living animals. Sometimes, of course, we discover that it is not a member of a group living today - it is a member of a group of which all are extinct.