Fossilization

How Fossils are Formed:

It is worth noting that fossils are actually quite rare - most things that die simply rot away, most footprints are not preserved, etc. However, over millions of years, the specific conditions required for fossils to form will occasionally be met. Here's a simplified version of how a typical body fossil (like a bone) might come to be:

1. The animal dies.
2. Soft tissues start to decompose, and/or get eaten.
3. Before the body completely disappears, it is buried. This is why bones and teeth fossilize more often than soft tissue.
4. More and more sediment builds up, and eventually the weight and pressure causes the sediment around the bone to be compacted and turned to sedimentary rock.
5. Over time, water seeps through the rock and into the bones, leaving minerals behind and "petrifying" the bone (essentially turning it to stone).

How are footprints/tracks fossilized?

These fossils are, again, rarely formed - but over millions of years, rare events will occasionally happen. For a footprint to fossilize, the dinosaur had to step on ground that was not too wet, too dry, too soft, or not too hard. If the ground was overly soft or saturated, the impression would collapse or wash away; if it was too firm or dry, no clear print would form. After the track was made, the sediment needed time to dry and harden before being eroded. Finally, it had to be buried by sediment that was, in some way, distinguishable from the material the print was pressed into (for example, volcanic ash).

How are Fossils Found?

If fossils are buried under so much sediment, how do we find them, and how do we age them? Fossils are found because, over time, erosion from ice, wind, water, etc. can expose them. Rocks can also be "uplifted" by tectonic activity - this is why fossils are sometimes found high in mountains.
To find fossils, paleontologists look for exposed sedimentary rocks of the right age. Ideal hunting grounds include:

• Hills
• River Valleys
• "Badlands" - areas of heavily eroded land with ridges & ravines, and with very little vegitation, making it easier to see the fossils.

One famous fossil hostpot is Dinosaur Provincial Park in Alberta, Canada - where over 500 fossils have been recovered and over 50 new dinosaur species discovered. When looking for suitable sites paleontologists often use geological maps, which show the rock layers exposed around the earth & their ages. Many fossils are also discovered accidentally by non-paleontologists when they have surfaced, or are accidentally unearthed during construction.

What Happens When a Fossil Is Found?

Once a fossil is found, paleontologists carefully document the site - recording its location, taking photos, and sometimes collecting surrounding rock samples. Following this, the fossils are carefully extracted. Initially, heavy tools may be used (in some cases including explosives!), and as they get closer to the fossil they will use smaller tools like chisels, brushes, etc. The method can also vary depending on the rock - heavier/harder rocks can require heavier tools. To protect the fossil, it is covered with plaster or similar material, like we use for broken bones.
Back at the lab, tools like dental picks, small power tools, and fine brushes help remove remaining rock. Broken bones may be glued back together. Finally, the fossil is catalogued and may be for research or prepared for museum display.

How Do We Date Fossils?

While most people have heard of carbon dating, it's only effective for materials up to about 50,000 years old - so not useful for dinosaur fossils. Fortunately however, there are other isotopes which are stable for much longer periods and can be used for radiometric dating. There is one big limitation though: we can only do this for volcanic rocks, and as mentioned earlier fossils typically form in sedimentary rock. Therefore, for dinosaur fossils we generally use radiometric dating on igneous rock layers above & below the fossil, giving a range of time the animal could have lived in. We can also use comparions with other, already dated, rocks and fossils.

Identifying a New Species

Figuring out whether a fossil belongs to a new species is tricky. Many people will be familiar with the definition of a species they learned in school - "a group of organisms capable of successfully interbreeding to produce fertile offspring" - but this cannot be tested with fossils. Instead, paleontologists use something called the morphological species concept - basically, looking at physical traits & shape. This unfortunately is not as simple as it might sound - paleontologists are often working on incomplete specimens, and even animals in the same species can have significant variations. For example, males & females of many species look very different, and unfortunately with dinosaurs we usually don't know the sex.

If it does appear that the fossil belongs to a new species, the species will be given a name. The fossil specimen will then be known as the holotype - a name-bearing specimen that other fossils can be compared with to check if they belong to that species. For this reason, the specimen will be available for study. A holotype can be an incomplete specimen, and often is, as long as there is enough to clearly show it belongs to a new species. If the holotype is later lost or destroyed, a new specimen may be selected and is known as a neotype. For example, the holotype of Spinosaurus (a dinosaur featured in the Jurassic Park movies) was unfortunately destroyed by an allied bombing raid in WW2.