Under rapid-burial conditions, the individual cells in an organism can mineralize and harden individually, preserving microscopic details of the original plant or animal.
So how can we look at the inside of a fossil? There are two methods.
The first, called sectioning, is to use a fine diamond saw to cut through the fossil in the area we wish to study, then polish the surfaces of the cut so that the differences in texture and color show up. This is how the large photograph was made. When done properly, this method uncovers fine interior detail. Unfortunately, this 'uses up' the artifact and creates problems for other types of testing.
The second, less-destructive method is to use some form of radiation, such as X-rays, to penetrate the rock and record on film the variations in interior densities. Simple X-ray techniques cannot clearly show differences between bone and stone, which tend to have similar densities. The C.T. scan is an improvement that overcomes these limitations by focusing on a thin slice of the sample. This was the method used to give us interior pictures of the fossil.
The hand is a wonderful collection of levers (bones), ropes (tendons), and hinges and pulleys (joints). It enables a pitcher to throw a precisely-placed curve ball or enables a violinist to play a Paganini concerto. Some of this machinery is visible in the C.T scans of the fossil finger shown here.
The side view shows dark areas that are interpreted as the interior parts of the bones and bone marrow. These areas have less density than the surrounding stones, and therefore more easily pass X-rays, causing darkening of the image. Also visible in both pictures are nearly-black areas caused by the sectioning.