Fossil life - on different scales

The fossil fish Pholidophorus; the circle shows the dark area where the fossil bacteria are preserved

A close up of the fossilised bacterial colony, at a magnification of x20,000

Fossils are the remains of ancient life. Most animal fossils are made of the hard parts of the body, such as bones, teeth, scales and shells. On rare occasions however, soft tissues, or even bacteria, can became fossilised.

A well preserved fossil fish from Somerset

The fossil fish illustrated here was found on Kilve Beach in north Somerset and although it is missing its head, the rest of it is well preserved. The fossil is about 11cm long and its scales outline the shape of the body. Faint traces of the front fins are also visible.

The fish is named Pholidophorus; it is an extinct animal that looked similar to a modern-day herring. Almost 200 million years ago, this fish died while swimming in the Jurassic sea that covered Somerset. As scavengers did not disturb the carcass after death, it became buried in fine sediment. This sediment eventually hardened to form the rock in which the fish became fossilised.

Microscopic detail reveals further fossilised remains

Although it is a fine example, this fossil fish preserves something even more remarkable. Scientists at Amgueddfa Cymru removed tiny fragments from the central area of the fossil to study them using an electron microscope. Zooming in on these fragments 20,000 times, some fine details started to emerge - a layer of tiny, rounded, elongate structures. These are the fossilised remains of bacteria.

Fossilised bacteria have been found in many localities around the world, most famously from fossils in South America and Germany. The bacteria are often preserved in calcium phosphate, because calcium is a common element in sediment, and phosphorus is found in the decaying tissues of animals.

How do we know these are fossil bacteria?

First, they are clustered together in colonies, and are a similar size and shape to modern-day bacteria.

Secondly, other very fragile cellular tissues, such as the cells of embryos, as found in the Cambrian rocks of China, have been documented .Fossilised muscle tissues can even preserve the internal details of cells, for example in Jurassic rocks in Brazil and Miocene rocks from Spain. The fossilisation of cells, including bacteria, is being observed more frequently as scientists look ever closer at the fossils in their collections.

Finally, on rare occasions, a thin dark layer of fossil bacteria occasionally traces the body outline of a fossil, revealing the shape of the fleshy tissues surrounding the skeleton, as in Eocene rocks from Germany. This tells us that fossilisation of the bacteria took occurred rapidly after the time of death.

It is becoming increasingly clear that bacteria play an important part in the processes of fossilisation, with the bacteria themselves sometimes being preserved.

Looking at a fossil, it is natural to wonder what kind of animal or plant it is, but some interesting discoveries can be made by asking what the fossil is actually made of. The answer, on close inspection, is often surprising and is the key to finding out how an organism became fossilised in the first place.