In this article we shall discuss the processes by which fossils are formed, and the circumstances under which this occurs.
Formation of fossils
In the process of replacement, the original material of bone, shell, or other tissue is replaced by minerals from the surrounding sediment.
In permineralization minerals (typically silica) fill in the spaces and voids within a fossil, including the interiors of cells.
Permineralization and replacement often go hand in hand (in which case the organism is said to have undergone petrification) but it is possible to have one without the other. The photograph to the right shows a piece of petrified wood.
A fossil mold is produced when sediment is packed around a organic remains which are then destroyed by decay, leaving a void in the shape of the organism.
A cast is produced from a mold when minerals fine enough to percolate through the gaps in the sediment then fill in the void left where the organism used to be.
There are also unaltered fossils; although in popular usage to say that something is a fossil is often to imply that it has been mineralized, in scientific usage "fossil" can refer to any remains which are prehistoric: that is, which are older than any written human history. So, for example, people speak of "fossil mammoths" even though their bones are unaltered by mineralization.
Small organisms such as insects and spiders can become trapped in the sticky resin of trees, which hardens to become amber; the magnified image to the right of a tiny gnat trapped in amber shows the exquisite level of preservation that this process achieves.
Finally, we may note the existence of trace fossils such as footprints or worm casts: these are preserved like other sedimentary structures by the lithification of the sediment in which they are formed.
Conditions for fossilization
Following the death of an organism, several forces contribute to the dissolution of its remains. Decay and predators and/or scavengers will typically rapidly remove the flesh; the hard parts, if they are separable at all (i.e. if the organism doesn't just have one big hard part such as the shell of an ammonite) can be dispersed by predators, scavengers or currents; the individual hard parts are subject to chemical weathering and erosion, as well as to splintering by predators and/or scavengers, which will crunch up bones for marrow and shells to extract the flesh inside. Also, an animal swallowed whole by a predator, such as a mouse swallowed by a snake, will have not just its flesh but some and perhaps all its bones destroyed by the gastric juices of the predator.
It would not be an exaggeration to say that the typical vertebrate fossil consists of a single bone, or tooth, or fish scale. The preservation of an intact skeleton with the bones in the relative positions they had in life requires a remarkable and fortuitous circumstance: burial in volcanic ash; burial in aeolian sand due to the sudden slumping of a sand dune; burial in a mudslide; burial by a turbidity current; and so forth.
The mineralization of soft parts is even less common, and is seen only in exceptionally rare chemical and/or biological conditions. How rare? Well, consider the story of the conodont animals.
From 1856 onwards paleontologists noticed that the fossils record from the Cambrian period right through to the end of the Triassic was littered with microfossils known as conodont structures. They guessed, correctly as it happens, that these were the scattered teeth of some unknown type of otherwise soft-bodied organism. The photomicrograph to the right shows some conodont structures.
In 1934 Schmidt and Scott discovered conodont structures grouped together on the same bedding plane, arranged in symmetric pairs; that is, in this case the conodont structures had presumably not been scattered but lay in the relative positions they would have had in life. But since the soft tissues were not preserved the nature of the conodont animals remained a mystery.
Not until 1983 did paleontologists discover a specimen in which the soft tissues of a conodont animal had been preserved — one hundred and twenty-seven years after the very abundant conodont structures had first been described.
Mineralized fossils: how do we know?
We take it for granted today that mold, cast and mineralized fossils are the relics of organic life. It may surprise the reader to learn that this was once a minority view, verging literally on heresy. Instead, it was widely believed that they were not: the most common view being that the fossils grew in the rocks as the result of a mysterious force known as vis plastica.
This explanation fitted nicely with the religious views of the time. Many fossils, if interpreted as the relics of once-living organisms, would have to represent species that had gone extinct, since no-one could find their modern equivalents. Now theologians argued that God, being perfect, would not have made any species so badly that it would go extinct; dissenting scientists such as Robert Hooke were obliged to guard themselves carefully against accusations of impiety.
It is then at least possible to suppose that mineralized fossils are not in fact mineralized remains of organisms. How would we argue against someone who was inclined to doubt it?
We might point out the existence of partially mineralized fossils: in fact, this was pointed out to supporters of vis plastica, who replied that the sequence was in fact the other way round, from rock to mineralized fossil to unmineralized fossil.
We might point out, as was pointed out at the time, the similarity between some fossils and living organisms. It would be remarkable if a process occurring in the rocks should produce just the same sorts of forms as are also produced by organic processes, giving rise to things that look exactly like sharks' teeth or sea-urchins.
We might also look at the consistency within rock formations. For example, one sort of sandstone will contain terrestrial plants and animals, and the large cross-beds found in aeolian sand; another will contain seashells and the same sort of symmetrical ripples found on a beach. On the basis of the mineralization theory, this is explicable and indeed expected: but how strange it would be if the vis plastica somehow managed to bring forth just those tableaux that would look like (but not be) the relics of former ecosystems.
From our perspective in the twenty-first century we might appeal to actualism. We know the composition of rocks down to the very arrangement of atoms in their constituent minerals, and there seems to be no room for a mechanism for vis plastica. However, there is a mechanism for mineralization: the process of mineralization only requires the sediment and organism to become more chemically homogeneous, which is much more chemically plausible than the reverse.
This seems a fair reply to the proponents of vis plastica. Another view proposed at about the same time was that the fossils were created by God when he created the Earth. This is certainly conceivable (an omnipotent God can do what he wants) but it might be answered in a similar way. From an actualistic view, we would point out that this invokes a miracle where none is apparently necessary, violating the scientific method. And from consideration of the nature of the fossil record we would have to say that in that case God has gone to extraordinary lengths to deceive us by making it look exactly like we are looking at the lithified relics of times gone past; we may imagine this, but it is grossly inconsistent with the traditional view of God, which supposes honesty to be among his virtues.
Since the hypotheses of initial creation or of vis plastica as the origin of fossils are, so far as I know, currently held by no-one whatsoever, the foregoing discussion may seem somewhat in the nature of a needless digression. However, it does emphasize the point, which I feel is worth making, that any statement in a geology textbook, no matter how much we take it for granted today, had to be discovered by someone; evidence had to be produced, and arguments had to be made — often against determined and dogmatic opposition.
So if we now take it for granted that mold, cast, and mineralized fossils are what we think they are, then this is not an unfounded assumption: we can afford to take it for granted because the case has been so well-made that perhaps no textbook except this one takes the trouble to review the question.