First lets discuss what fossils are, this is probably redundant but I’ve got to fill my word count somehow. You likely already know the postage stamp version of this, fossils are the mineralized remains of dead animals, but how are they formed and why are there so few of them? Actually the answers to those to questions are the same, the way fossils are formed leads to the natural paucity of the fossil record that those who would dispute evolution decry. Fossils require very particular circumstances in order to be created, the remains of the animal or plant need to be buried quickly in fine sediment such as clay, silt or sand. If the animal is buried fast enough then this may stop scavengers as well as exclude oxygen from the remains and so reduce decomposition. In this case we may get a cast of the exterior features of the animal.

The ever popular Trilobite
The ever popular Trilobite

Once the bones are under a good amount of sediment then the process of fossilization can commence. It’s not a quick process by any means and can happen in several ways. Crystals may develop in the original structure and eventually replace it with a crystallized copy, or the  remains may rot away leaving a hollow that is subsequently filled with rock creating a cast of the original or finally the organic material may be slowly replaced with minerals from the surrounding rock. As I said this process requires that the remains be cover relatively quickly after death and so most fossilization occurs near water where sediments can be washed over the body in a fairly short time. Death inland is usually swiftly followed by the carcass being either torn apart by scavengers, broken and scattered to the wind or decomposition.

Now that we have our fossils what do they tell us? Well the fossil record is a remarkably complete (considering it’s limitations) archive of the gradual change of organisms of the eons for one form to another. The phrase Transitional Fossil is something of a misnomer and gives the wrong impression. As evolution is occurring all the time (visibly or not) every species can be viewed as transitional, including those that survive today. Or to put it another way species can only be viewed as transitional with hind sight and the knowledge that one species preceded another and that was followed by a third. Each species was/is evolved for the environment it live(d)s in and it is only our passion for putting things in order and telling a story that makes it seem natural for us to indulge in the conceit of calling one or another species “transitional”.

Having said that I shall now disregard my own rant and say what are examples of transitional fossils? We one lineage that is now particularly understood is that of horses.There is a list of about 23 intermediate species identified ranging from something like a rodent to deerlike animals and into the modern horse now so familiar. The sequence was first sketched out in the 1800’s and so is now very well understood.

Hyracotherium, early horse ancestor

Below is a graphic representation of horse evolution that is quite remarkable and yet does not do the knowledge we have gained justice, as neither does this humble post. But hopefully you will be inspired to look into these examples for yourself.

Horse evolutionary line
Horse Evolution in living colour

Yesterday: Evolution Journal Tomorrow: Evolutionary “Gems”