When sediment covers an animal's body shortly after death, its bones can fossilize. Soft tissues, like skin, muscles and other organs, decompose. Minerals from the soil make their way into the bone, turning it to stone. This doesn't happen very often, so there are gaps in the fossil record -- periods of time when conditions weren't right for fossilization.
Researchers remove fossils from sedimentary rock using a range of tools, from picks to paintbrushes, and lots of patience. It's easy to damage fossils during excavation, and the presence of lots of fossils in the same bed can make it hard to decide which bone belonged to which animal.
After excavating a fossil, researchers typically encase it in plaster and ship it to a research facility. There, scientists can make casts, or reproductions, of the bones and try to recreate the complete skeleton. Scientists can learn a lot from this process:
- The way the bones fit together gives a basic idea of the dinosaur's shape and posture.
- Flat, leaf-shaped teeth mean that the dinosaur ate plants. Sharp, pointed teeth suggest that it ate meat.
- The proportions of the leg bones relate to how fast the dinosaur could run.
- Cavities in the skull suggest how well it could see and hear.
- Bumps called quill barbs on the bones mean the dinosaur had feathers -- some Velociraptor specimens have these barbs.
Technology plays a part, too. Computer simulations help determine how fast a dinosaur could move and how it used its limbs. Researchers can also use computer models to reconstruct the dinosaur digitally, adding virtual layers of muscles, tissue and skin to a 3-D image of the skeleton. With computerized axial tomography (CAT) or CT scans, scientists can also get a detailed view of inaccessible parts of skulls and other bones.
The surrounding rock can offer its own clues. There can be impressions from leaves or feathers, fossil eggs or the remains of nests. Impressions from the dinosaur's skin can give paleontologists an idea of its texture.
But none of this can answer a basic question about dinosaurs' appearance -- what color were they? In everything from movies to play sets, dinosaurs appear in shades of gray, brown and green. But it's just as likely that dinosaurs had the bright coloring of some frogs, snakes and birds.
It can also be challenging to figure out whether a dinosaur had scales or feathers. The absence of quill barbs doesn't prove that an animal was featherless, and feathers themselves decompose long before bone turns to fossil. However, newly-discovered fossils in China have feather impressions in the surrounding rocks. All this evidence suggests that, in terms of appearance, dinosaurs had some avian traits and some reptilian traits.
Next, we'll explore how dinosaurs regulated their body temperature and whether this makes them more like birds or reptiles.