The Fossil Detectives: Discovering Prehistoric Britain

What are fossils?

TODAY, MOST CHILDREN know that fossils are things such as the shells of trilobites, ammonites and dinosaur bones. But these ancient discoveries are not the whole story – witness the recent furore about a small tubular shape found in a Martian meteorite. Was it really a fossil?

The current consensus is that the meteorite material is not a fossil, but for some time, many scientists were convinced that it was. Similar arguments have been around for centuries. This is partly because the process of fossilization can alter life forms so they take on inorganic attributes. There are also natural processes in the formation of inorganic materials that simulate organic forms. Which of the following would you consider to be a fossil, for example: a dinosaur footprint, an insect embedded in amber, a flint replica of a sea-urchin, a flint shaped like a finger, and a lump of coal?

All but the finger-shaped flint are fossils. The dinosaur footprint is known as a trace fossil and the amber insect is a body fossil. The flint replica is also a body fossil but an internal mould as well, and the coal is a chemical fossil. The finger-shaped flint is a ‘sport of nature’ with just an accidental resemblance to a finger. It is the kind of thing that was once regarded as a fossil, since the original meaning of the word referred to anything ‘dug up’. The word ‘fossil’ is derived from the Latin fossa, meaning ‘ditch’.

Historical problems

In the past, the discovery and interpretation of fossils was often very confusing and a constant source of argument. The English mediaeval chronicler Ralph of Coggeshall in Essex, recounts how, in 1171, the collapse of a river bank uncovered some huge bones that had been buried within the sediment. From his examination, Ralph concluded they were human leg bones that were so big they belonged to a giant who ‘must have been fifty foot high’. We now know that the bones were almost certainly the limb bones of an extinct relative of the elephant, such as a mammoth (Mammuthus primigenius) or straight-tusked elephant (Palaeoloxodon antiquus), that lived in the region during the recent Quaternary ice ages.

An ammonite’s coiled shell was once mistaken for the remains of a snake.

While many fossils have shapes and forms that closely resemble those of living organisms, their preserving mineral material is often radically different from the substances the living organisms were originally made from. For example, the early excavation of coal often turned up long, curved ‘rods’ of stone with repeated scale-like patterns on the surface. Superficially they looked snake-like and were generally called ‘serpent stones’. Although the shape seemed organic, the substance was clearly stony. A splendid example is preserved in the fossil cabinet of Dr John Woodward (1665–1722), who was a well-known London physician, fellow of the Royal Society and collector of natural antiquities. Woodward donated his collection and money to the University of Cambridge to found a professorship. His collection can still be seen in the University’s Sedgwick Museum.

Today, we know that these Carboniferous-age fossils, called Stigmaria, are actually the sediment-filled internal moulds of the ‘roots’, or more strictly speaking underground branches, of extinct tree-sized clubmosses (lycopods) that grew to some 30 metres high in the equatorial Coal Measure forests, over 300 million years ago.

Defining a fossil today

Today, fossils are defined as the remains of any once-living organism. They are therefore the main evidence for past life, with its complex evolutionary history of origination, adaptation, speciation and extinction. Without fossils we would have no idea of the extraordinary diversity of prehistoric life and the innumerable strange groups of animals and plants that have come and gone – life forms such as the trilobites, ammonites, and graptolites as well as the dinosaurs, pterosaurs, ichthyosaurs and mammoths.

Fossil remains vary enormously, from fragments of ancient DNA to carbonized feathers and flowers, fossil tracks and footprints, but most of these fossils are exceptionally rare because they require special circumstances for their preservation. Technically, fossils include body fossils – the remains of some part of the original body tissue or skeleton; trace fossils – marks such as footprints, burrows and borings, made by organisms and preserved in the rock record; and chemical fossils – chemical residues from organisms (for example, organic-derived hydrocarbons such as coal and oil).

The first museums were collections of weird and wonderful curiosities that often included fossils.

How to make a fossil

Fossilization is not as easy as you might think. Take a garden snail, for example. Since it is a plant-eater, there is a very remote chance that a trace fossil of this common mollusc might be preserved if a half-eaten leaf was fossilized with the telltale bite marks of the snail. When the snail dies, the soft tissue will decay or be scavenged, but the hard shell, mineralized with calcium carbonate, has a chance of surviving.

Empty snail shells are not uncommon in terrestrial soils. Even if the shells have been broken by birds, the fragments are often quite recognizable. When a snail dies, however, the original colour and surface shine of the shell is lost because the organic surface-coating degrades and the pigments are bleached out. This makes the calcium carbonate of the shell more vulnerable to further chemical alteration, especially by acids present in rainwater or the soil. Eventually, the shell may dissolve completely. However, if the shell has been buried in a fine-grained sediment such as a mud, the sediment may form quite an accurate mould of the shell, both externally and internally. If the sediment is then lithified (turned into stone), the mould may persist and preserve sufficient detail for future fossil detectives to identify it.

Snail fossils are common but are often preserved as moulds because the calcium carbonate of their shells, and those of most molluscs, is in the unstable aragonite form, rather than the more stable calcite form, of which brachiopod and echinoderm shells are made. Most snail fossils are marine in origin, but they may be very common in certain ancient terrestrial environments, such as freshwater deposits of Cenozoic age. Generally, however, terrestrial remains are less well preserved because of the constant weathering and erosion of landmasses and the recycling of their materials. The ideal circumstances for the preservation of organic remains in the fossil record are:

• a great abundance and widespread distribution of the organism;

• a longevity through geological time as a species;

• occurrance in an environment commonly preserved in the rock record where there is rapid burial and deposition, such as continental shelf seas, river deltas or lake shores;

• the presence of a tough and thick shell or skeleton made of a stable mineral such as calcite, apatite (calcium phosphate), silica or organic material such as cellulose or chitin;

• the lithification of the sediment with its organic remains to form a fossiliferous rock that will be resistant to weathering and erosion.

The fossilized three-toed footprints of large, two-legged dinosaurs were first thought to be those of giant birds.

The most common fossils

The fossils most often seen in Britain are those of ancient shellfish such as clams (bivalve molluscs), snails (gastropod molluscs), ammonites (extinct cephalopod molluscs), trilobites (extinct arthropods), sea lilies (crinoid echinoderms) and brachiopods. (For a more complete list of common fossils see here.) Strictly speaking, however, the most common fossils are not readily visible to the naked eye. They include microscopic plant pollen and spores, and the shells and skeletons of single-celled organisms. A few grammes of Cretaceous Chalk, for example, contains millions of fossil coccoliths.

Most visible fossils are marine shellfish because the vast majority of British fossil strata are the deposits of ancient shallow seas. These ancient sandstones, shales and limestones are a rock record of successive beach and sea-bed deposits that have accumulated over more than 540 million years.

Even if the snail’s hard shell is left intact, it has little chance of fossilization in soil.

Beachcombing today

Beachcombing around Britain today can yield a bounty of seashells, especially bivalved molluscs (such as the well-known cockles, mussels and razor shells), snails (such as whelks and winkles), bits of sea urchins and occasional starfish. The remains of other creatures that live within marine waters are surprisingly rare on beaches unless there has been an unusual mass mortality. This is partly because their protein-rich flesh is likely to be scavenged, and their skeletons fall apart before their bones or shells are washed up on the shore or buried in the sediment. Seaweed and other soft plant material also have little chance of preservation.

Mixtures of marine and land-derived remains are important for fossil detectives because they help to make links between the kinds of organisms that lived in these different environments. Luckily, rivers flow into coastal waters and bring with them remains of terrestrial life, ranging from whole trees and animal corpses to the abundant but microscopic spores and pollen of land plants. Estuarine and deltaic waters are important sites of significant sediment deposition, within which a great deal of organic material gets trapped. These are often important sources of hydrocarbons and fossil remains.

A beachcomber finds only a very small portion of the life that lives in and around coastal environments. Many organisms live within the sediment for protection in these hostile places. These include worms and shrimp-like crustaceans, along with many molluscs and sea urchins. The bodies of the worms and crustaceans are not normally preserved, but traces of their activity through the sediment are commonly seen as ancient burrows. In addition, shelled creatures may be fossilized in the sediment.

The remains vary from place to place, depending on factors such as the type of sediment, the presence of rocks or cliffs nearby, the depth of the water offshore, the temperature and the time of year. The size of sediment particles and the energy of the environment are important, too. Coarse sand and pebbles agitated by wind and wave currents rapidly grind down shells to such an extent that just a ‘shell hash’ remains. To find well-preserved fossils, it is best to target sedimentary rocks laid down in quiet waters such as muds, silts and fine-grained limestones.

Fossil-hunting

Collecting fossils from ancient marine strata is not unlike beachcombing. If you were able to wander along a Silurian beach, over 400 million years ago, you might see some familiar-looking mollusc-type shells of clams and snails. To identify them you would need the Silurian equivalent of a modern field guide. You might also see several unfamiliar shells and skeletons classified as brachiopods, trilobites, graptolites and orthoconic nautiloids.

The brachiopods could be mistaken for clam shells, but on close examination they show some important differences. One shell is bigger than the other and the bigger shell has a hole in it through which a bit of fleshy stalk still protrudes. Brachiopods are commonly known as lampshells because some look like Roman oil lamps. When alive, the animal would be anchored by the fleshy stalk to another shell, to stone or to the sea-bed sediment.

Brachiopods are one of the most common and widespread seashells of Palaeozoic times. Biologically, they are placed in their own phylum, Brachiopoda, and were filter-feeding animals that lived on the sea bed. Most were small – up to 4 cm long and, rarely, as long as 15 cm. Some 4000 fossil genera are known and there are still over 300 surviving species, some of which live in offshore waters around Britain.

Trilobites are extinct sea-dwelling arthropods, covered with a crab-like carapace on the upper surface and with numerous jointed legs and other appendages on the lower surface. The skeleton is divided into articulating pieces that allowed the animal to roll up into a ball when threatened. Most trilobites lived on the sea bed and fed upon small organic particles from the sediment surface and seawater. Some 15,000 species are known to have lived between early Cambrian and late Permian times.

After a storm, a beach strandline is a good place to sample the potentially preservable ‘protofossil’ remains of marine life.

Graptolites are perhaps the strangest of the creatures to be found on a Silurian beach and may be quite difficult to spot. Looking like fret-saw blades, their remains consist of thin but quite stiff 1–3 mm-wide organic tubes with a series of tiny interconnected cups on one side. Growing to several centimetres long, their skeletons have a curious geometric, plant-like form, but graptolites are in fact colonial animals. Some grow into elegant spiral shapes with regularly spaced branches. Free-living in the water column, they drifted wherever currents took them.

The long, straight, conical shells of early Palaeozoic nautiloids were commonly washed up onto Silurian beaches. The chambered cone floated for some time after the animal died and may even have been colonized by other sea creatures before finally sinking to the sea bed or being washed ashore. The nautiloids were free-swimming animals, many of which may have lived in localized shoals. Larger forms up to a few metres long were more solitary and widely distributed. Some were so big that they preferred to live on the sea bed. In earliest Palaeozoic times, they probably had few predators except other nautiloids, at least until the jawed fish evolved in Silurian times.

Fish remains are as rare on Silurian beaches as the remains of modern fish are on today’s beaches. Even if you found a Silurian fish, it might be difficult to recognize it as such because so many fish of this period were quite unlike modern jawed, bony fish. Most Silurian fish were jawless forms (agnathans) covered with a strange armour of leathery plates over the head and trunk. Only the flexible tail had more familiar scales. The fish lived on the sediment surface and fed on organic debris and bacterial mats that covered the surface. As the jawed fish evolved and diversified through Silurian times and became increasingly predatory, the jawless fish moved into fresh waters. They survived there throughout Devonian times, only to decline through the Carboniferous period as the jawed fish invaded fresh waters.

These jawless fish of Devonian age are well preserved as fossils because of their tough body armour.

Silurian rocks in Britain

The rocks of the British landscape preserve a surprising surface outcrop of Silurian-age strata, despite the fact that the period only lasted for some 26 million years (from 443 to 417 million years ago). Some of the most extensive strata are found in the Southern Uplands of Scotland, in Cumbria and throughout the Welsh Borderlands. They are especially well exposed along Wenlock Edge in Shropshire, where there are many legally protected sites (SSSIs) and a number of Regionally Important Geological Sites (RIGS) that are more accessible to the general public. In these locations, breaking open any of the slabs of muddy limestone can reveal a glimpse of the life of the deep past. Each freshly broken slab will reveal a sight that no human eye has ever seen before. If the rock is carefully broken along a bedding plane, it will reveal an ancient sea bed and, if it has fossils on it, a moment in the geological history of the evolution of life.

Britain’s life and times

A geological history

GEOLOGISTS NOW RECOGNIZE that Britain is made up of several different underlying structural units known as terranes, some of which had very different histories before being amalgamated into the British Isles. This plate tectonic history might seem strange and unbelievable to the uninitiated, but there are independent lines of evidence that support this extraordinary story of changing places on the grand scale.

The geological story can be picked up in the Precambrian eon (4560 to 542 million years ago), some 600 million years ago. At this time, southeast Britain (today’s England), Wales, southeast Ireland and parts of France and Germany, were all part of a terrane known as Avalonia, which lay close to North Africa and the Gondwanan supercontinent (today’s South America, Africa, Australia, Antarctica and India).

Scottish rocks record a Precambrian glaciation. Scottish tillites were the first in the world to be recognized as ‘fossilized’ glacial deposits of Precambrian age by James Thomson (1822–92) in 1871. The Scottish scientist James Croll (1821–90) predicted that such an ice age might have promoted the explosion of life in the later Cambrian period (542 to 488 million years ago).

From around 580 million years ago, a new ocean called Iapetus opened up and Avalonia underwent dramatic geological changes, with periods of folding and faulting and violent island-arc volcanism, like that seen in some of the volcanic islands of Indonesia, Japan and the Caribbean today. Avalonia’s shallow seas were host to some of the earliest organisms, whose fossils have been preserved in the Charnwood Forest region of Leicestershire (see here).

Around 500 million years ago, in late Cambrian times, Avalonia moved away from Gondwana as a new ocean, called the Rheic Ocean, opened up. Over the next 60 million years, sediments and rocks were crumpled into a great fold belt, the remains of which are still familiar to us today as the Caledonian Mountains. The process joined England, Wales and southeast Ireland to Scotland and northwest Ireland for the first time. This was the foundation of the underlying geological substructure of the British Isles, which then lay about 30 degrees south of the equator.

The jawless Devonian Cephalaspis had a tough, leathery headshield below which its mouth was suited to sucking up bits of organic material from the ocean sediment.

From an evolutionary point of view, one of the most important developments of the Palaeozoic era (542 to 251 million years ago) was fish. We now know that the ancestors of fish evolved in the Cambrian seas over 500 million years ago. These curious jawless fish (agnathans) eventually diversified into many different kinds, including some extraordinary armoured forms, by Silurian times (443 to 416 million years ago), when they also colonized fresh water. The other major innovation of Silurian times was the evolution of the vascular, or upright-growing, land plants, accompanied by the first land-going arthropod herbivores.

Late Palaeozoic times

By the beginning of Devonian times (416 to 359 million years ago), the Iapetus Ocean had closed and the British Isles, apart from the extreme south, were part of a large landmass known as Laurussia. Jawed fish had diversified to an extraordinary extent. One of the most important adaptations was that of muscular paired fins that allowed some of them to support their body weight and lift their head. These fins evolved into the first tetrapod limbs while the animals were still primarily aquatic fish-like forms. But life within the harsh environments of land required many other structural and physiological adaptations. Out of water, a fish cannot hear, see or breathe.

By mid-Devonian times, a small terrane from Gondwana, known as Armorica, had crumpled into the southern flank of Avalonia, folding the east–west trending Variscan mountains (named after the Medieval Variscia region of Germany, which is within such a mountain belt) of south Ireland and southwest England. Continued plate movements in early Carboniferous times (359 to 299 million years ago) moved Laurussia toward the equatorial zone. The sea flooded over large tracts of the landscape. These warm shallow seas were ideal for reef development, and organisms included stromatolites and corals along with brachiopods, gastropods, trilobites and diverse fish including sharks. In late Carboniferous times, plate movements brought Gondwana up from the south to join Laurussia, and together they moved to the humid equator. The land re-emerged, drained by large rivers with heavily vegetated swampy floodplains, lakes and deltas. Diversification of the land plants produced vast forests and woodlands with tree-sized clubmosses, horsetails (sphenopsids) and ferns. The accumulated plant debris of these swamps and forests formed the coal seams that fuelled the Industrial Revolution.

DID YOU KNOW? TECTONIC PLATES

Earth’s outer layer of cool and brittle crustal rocks is broken into seven continent-sized plates and six smaller ones that have moved in relation to one another over geological time. Most plates carry both ocean and continent, with new crust being generated at constructive boundaries, known as spreading ridges, where new ocean-floor rocks form and then spread over many millions of years into new oceans.

However, since Earth does not expand, as much crust must be destroyed as is generated: this is mostly old ocean-floor rocks that are pushed back down into Earth’s interior along steeply sloping subduction zones. The subduction process generates intense earthquakes and friction that melts rocks at depth, leading to volcanic eruptions at the surface.

Subduction can also lead to the collision of plates carrying continental crust and the formation of mountain belts such as the Himalayas, when the Indian continent collided with Asia some 15 million years ago.

Devonian times 416–359 million years ago

Jurassic times 200–146 million years ago

Oligocene times 34–23 million years ago

Animal life thrived in these swampy environments, whose waters were filled with fish and newly diversifying tetrapods, including crocodile-like amphibians and the evolving egg-laying amniotes whose novel reproduction allowed them to become independent of water for the first time. The amniotes included the earliest reptiles (mostly lizard-like forms that ate small arthropods) and the abundant insects that lived in and around the lush vegetation.

As Britain and North America moved across the equator into the northern hemisphere and another tropical semi-arid zone, other plate movements had conspired to amalgamate all the continental masses into one gigantic supercontinent known as Pangea. From space, Earth must have looked very strange, with just one landmass occupying a third of its surface. Pangean environments developed wide differences between the dry desert interior and the coast. Britain lay in the dry interior throughout Permian and into Triassic times and the beginning of the Mesozoic era (251 to 65 million years ago).