Another dinosaur excavation kit, accompanied by a fine selection of supporting materials, appeared in the house among the Christmas presents. The block with the plastic remains of the inevitable T. rex was larger than usual. And that made all the difference to the usefulness for the kit as a physical model to distinguish among different ways to preserve organisms as fossils.
Taphonomy is a subdiscipline of palaeontology, and allied science-based fields, that deals with the numerous pathways to go from a corpse to a fossil. Ivan Efremov introduced the term in 1949 and it translates from Greek as, ‘the laws of burial’. Efremov worked on plant fossils, and was careful to include parts of organisms, such as leaves, seeds, or lost limbs (tree and animal!) as well as whole bodies. A succinct summary of the scope of the field is given in this article and a particularly good general definition is , ‘The study of the pathways by which remains, parts or products of organisms pass from the biosphere to the lithosphere.’ Or how the squidgy bits become rocks and minerals.
Fossils, whether of whole organisms, pieces of organisms or molecular fossils are the patterns left by the processes of taphonomy. Processes lie beyond the scope of this wee post; we’ll just have a look at patterns with the help of the plastic T. rex and the rock it came from (the matrix, not The Matrix).
Using a section of the skeleton and the block that it came from, it is possible to illustrate the concept of a body fossil, an external mould and a cast made from the mould. In the case of the kit, we have the original ‘bones’. Bone is often preserved by minerals growing within the pore spaces in bones during burial. This process is called permineralization and can make a fossilized bone very, very heavy. So heavy that it makes large exhibits a difficult engineering feat, so the bones on display are often replicas made of lighter materials.
Moulds come in two types. What has been left behind in the matrix of the excavation kit is a sort of external mould. In this case, our removal of the skeleton has done the work but it is more common for the body parts to be dissolved away.
The other type of mould is an internal mould. In these cases, the interior space has been filled in with sediment and then the organic remains have dissolved away, leaving a fossil. The tree remains at Fossil Grove are of this type.
Casts are the final category to consider with the fossil kit. Separating between moulds and casts can be quite difficult. To help distinguish between the two classes, thinking about moulds and casts for human objects can be useful. Below is a mould made to cast an ammonoid model for an abortive hydrodynamics experiment during my Ph. D. Thanks to Mike LaB for fun times in the lab.
A mould is made for a liquid material to be poured into, such as a making jelly in a mould. The jelly that comes out is a cast. By using another of the kids’ toys, kinetic sand, we made a cast from the external mould. As the kinetic sand has much coarser grains that the original matrix, the reproduction has much less detail, which mimics a processes sometimes observed in real fossils. Differences in screen resolution are a good analogue. If you have a high number of pixels per unit area, a sharp image is produced. With a low resolution display, pixellation creeps in.