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Field guide
Fossil Field GuideBy Peter Sheldon
Using this Field Guide
The purpose of the guide is to help someone finding a fossil to identify the group to which it belongs. The guide will have achieved its key objective if you are able to say that your find is, for example, a brachiopod, a trilobite or a coral. This should get you started, and more detailed identification can then be made with the help of books, the internet, a visit to a museum, or discussion with someone more experienced at fossil identification. Don’t be disheartened if you can’t identify your fossil with this guide – it covers only the most common groups, and each group contains many more fossils than those illustrated here. Bear in mind too, that even if a fossil is represented in the guide, it may look very different in the rock, where it may be seen in cross-section, viewed from any angle, crushed or deformed, or just a fragment.
The illustrations for each group show fossils that span the group’s typical range of appearance. Vertebrates and plants, although highly diverse, are relatively rare as fossils compared with invertebrates, and their subdivision into major groups is beyond the scope of this guide. Similarly, microfossils, though very common, require a microscope to study, and are not covered here.
The names given in italics beside each illustration are, in most
cases, the genus (plural genera). Most fossil (and modern) genera contain several closely-related species, and species are usually harder to identify correctly than the genus. Our own genus, Homo, has only one species today – ourselves, Homo sapiens. Genus and species names are always printed in italics, and the species name (e.g. sapiens) always begins with a lower case letter.
Underside of a Jurassic sea urchin, Paracidaris. The tooth-like plates in the centre are part of its jaw apparatus for scraping up food. 7 cm.
Introduction2
The typical size of each fossil is given beside its illustration, or stated in the caption, and, unless otherwise indicated, this represents the longest measurement in any direction (excluding soft parts).
A geological period (e.g. Jurassic) is given for each genus. Many genera range through more than one period, in which case the period given is typical for the genus. Geological time since the start of the Cambrian Period is divided into three eras – the Palaeozoic, Mesozoic and Cenozoic – each containing a number of periods of unequal duration. The age, in millions of years, of the start and finish of each period can be seen from the stratigraphic column repeated throughout the guide. These ages are continually refined in the light of new information and improved dating techniques.
What is a fossil?A fossil is simply any evidence of ancient life, naturally preserved within the materials that make up the Earth. Usually, the evidence is found within a sedimentary rock – originally loose sediment such as mud, silt, and sand – but other possibilities for entombment include natural tars and resins (amber), or even ice.
There is no strict dividing line in terms of age between recent organic remains and fossils. As a rough guide, most palaeontologists (people who study fossils) would probably consider any evidence of life over about 10 000 years old to be a fossil. The question of definition is not usually an issue, however, as most fossils are millions of years old. Fossils are only common in sedimentary rocks younger than the start of the Cambrian Period, 542 million years ago, when organisms first acquired the ability to produce hard parts.
The fossil record is dominated by invertebrate animals with durable shells or skeletons that lived in shallow seas (e.g. ammonites, trilobites and corals). The land tends to be a site of net erosion, so the opportunity for long-term burial is less than in shallow seas on the edges of continents, where most sediment accumulates. In general, fast but gentle burial, particularly in oxygen-poor environments, favours good preservation. Vertebrates tend to be less abundant in living populations than invertebrates, and so are relatively rare as fossils, whether they lived in the sea (e.g. ichthyosaurs) or on land (e.g. dinosaurs). Plants, living mainly on land, tend to be scarce as
fossils, as are animals from freshwater rivers and lakes. Insects too, despite their abundance and diversity, are rarely fossilised.
The Jurassic ammonite Psiloceras. 5.5 cm.
3 Introduction
Body fossils preserve something of the bodily remains of animals or plants, such as shells, bones and leaves, or their impression in the enclosing sediment. Parts of the body often become altered in chemical composition and physical structure. Hard parts of organisms, such as bones, teeth and shells often have tiny pores (open spaces). When buried in sediment, these pores tend to be filled with minerals, such as calcite and quartz, that crystallise out from water seeping through the sediment, making the structure denser than in life. The original hard parts of organisms, and more rarely the soft parts, may be completely replaced by the growth of new minerals.
Both the filling up of pores (permineralisation) and the replacement of biological materials by minerals may occur in a single fossil. Neither of these processes, which together are called petrifaction – ‘turning into stone’ – has to occur for something to be called a fossil; sometimes the fossil is still composed of the original, barely altered shell or bone. The bodies of ancient plants are often preserved as thin films of carbon, whereas, in life, plant tissues contain in addition many other chemical elements.
The surface of a sedimentary rock surrounding or infilling a fossil shell (or other body part) is called a mould. Usually, both internal and external moulds are formed, recording impressions of the inside and outside of the shell, respectively. If the shell becomes completely dissolved away, a space is left between the internal mould and the external mould. New minerals may fill this space, forming a crude cast of the shell that lacks details of the shell’s original structure. In general, casts are rarer than moulds.
Trace fossils preserve evidence of the activity of animals, such as their tracks, trails, burrows, borings or droppings. They are often the only evidence we have of extinct organisms whose bodies lacked any hard parts (e.g. many types of worm). Unlike body fossils, in which the body may have been transported after death a long way from where the original organism lived, most trace fossils are direct, in situ evidence of the environment at the time and place the organism was living.
Sometimes one can be deceived into thinking an object is a fossil when it is not. Pseudofossils are misleading structures, produced by inorganic processes, that by chance look as if they are evidence of ancient life.
Fossils are very useful for giving us information about ancient environments and climates, for revealing the evolution of life through time, and for matching up rocks of similar age in different parts of the world. And were it not for ancient life we would not have any fossil fuels – coal, oil and natural gas.
4 Introduction
A pseudofossil. This plant-like pattern was formed by crystals of manganese oxide growing along a crack in limestone. 3.5 cm.
Responsible fossil collectingFossils form a major scientific, educational and recreational resource, and are part of any country’s heritage. Only a responsible approach to fossil hunting will ensure this resource remains viable for future generations to enjoy. It may be best just to take a photograph, and leave a specimen for others to see, as the context in which fossils occur can be the main scientific interest. Collect only a few representative specimens and, unless you have time to make a detailed scientific study of fossils in situ and publish your findings, obtain them from loose, fallen or scree material where possible. Encourage responsible collecting in others too. If you think you have made an important find, seek specialist advice from a museum, university, geological society or conservation agency.
Here are some key points to bear in mind. Safety is, of course, paramount.
Always seek permission first if you intend to enter private land.
On the coast, beware of tides, cliff falls and mudflows, especially during wet and stormy weather. Collect when the tide is going out.
The cliffs on more rapidly eroding coastlines, and recently blasted quarry faces, are often exceedingly dangerous – never be tempted to go too close. A hard hat may give protection from small pebbles, but is useless against a more significant rockfall. Be careful, too, not to dislodge rock onto others below.
Follow the Country Code, e.g. avoid disturbance of wildlife and do not leave the collecting site in an untidy or dangerous condition for those that follow. Be considerate to others.
Even if you take a mobile phone, reception may be poor, so tell someone where you are going and what time you expect to return.
Many of the most important fossil sites in Britain are protected by law as Sites of Special Scientific Interest (SSSIs) or designated as Regionally Important Geological Sites (RIGS) by local RIGS groups. Collecting fossils from these sites may be prohibited, except for bona fide research purposes. In such cases special permission is required from the relevant government conservation agency – Natural England, Scottish Natural Heritage, the Countryside Council for Wales or (in Northern Ireland) the Environment and Heritage Service.
Fossils can be found in most places where sedimentary rocks of Cambrian age and younger are exposed. Clays, shales and limestones tend to be more richly fossiliferous than sandstones, though the latter may yield abundant trace fossils.
Although a geological hammer is often useful, much study of fossils can be done without one. Use only a geological hammer
made of specially hardened steel, as an ordinary DIY hammer is too brittle. Always protect your eyes with safety spectacles, and never hammer indiscriminately. Hammering is forbidden or even illegal at some sites, so check the situation first.
5 Introduction
A metal chisel can be used with a hammer for prizing out pieces of rock and trimming matrix from specimens. A spade and sieve may be helpful in extracting fossils from soft clays or uncemented sands and silts. It is usually easier to remove the majority of matrix around the fossil in the field, rather than back at home, and there is less weight to carry too.
A hand lens can enhance your understanding of fossils and the rocks containing them. These can be bought from stamp shops (philatelists) and some hobby shops. A magnification of ×10 is recommended.
A notebook is essential, and annotated sketches, supplemented by photographs, are often the best way to record your observations. Geographic location is especially important. Photos and drawings can be a more desirable alternative to collecting, but if you do collect, you will need plastic bags, a non-smearing felt pen, and some suitable wrapping material (e.g. kitchen roll) to protect more delicate specimens. When home, be sure to label and organise your finds before you forget information about them (a computer can be helpful here). For one reason or another, most fossil collecting sites do not last forever, and a well-organised collection of even common fossils may later prove of much scientific value. Further tips about preparing and curating specimens are beyond the scope of this guide.
Remember, if you think you have made a rare find, take your specimen, or send an image of it, to an expert. New species, or exceptional specimens of poorly known ones, can be found by complete beginners. Sometimes a new species is named after the finder!
Finally, there are many benefits in joining one of the numerous societies where you can meet others with a similar interest in fossils and rocks, discuss your finds, take part in organised field trips, and learn a great deal more.
Jurassic strata rich in fossils – but always keeP yoUR dIsTance from unstable cliffs like this.
ReferencesHere is just a small selection from the huge range of excellent books available.
Fossils. Cyril Walker and David Ward. 2000. Dorling Kindersley. ISBN 0751327964.
Fossils - the Key to the Past. Richard Fortey. 2002. Natural History Museum. ISBN 0565091638.
Minerals, Rocks and Fossils. A. Bishop, A. Woolley, & W. Hamilton. 1999. Philip’s. ISBN 0540074292.
The following handbooks from the Natural History Museum are exceptionally useful, with drawings of the most commonly found British fossils from each era: British Palaeozoic Fossils. (2001 reprint). Intercept. ISBN 1898298718. British Mesozoic Fossils. (2001 reprint). Intercept. ISBN 1898298734. British Caenozoic Fossils. (2001 reprint). Intercept. ISBN 1898298777.
acknowledgements. The author would like to thank Dr Colin Scrutton for helpful comments on this guide, and Dr Paul Taylor for advice about bryozoans.
6 Introduction
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ageCambrian Period to present day.
environmentMainly marine, on the sea floor; some live in freshwater.
descriptionSponges have a skeleton of calcium carbonate, silica, or, as in some modern bath sponges, horny organic material. Water passes in through the sponge’s many surface pores, often to the central cavity of a sack-like body, and out through a large hole at the top. Sponges vary greatly in shape. Some have a stalk, others are encrusting and irregular. Sponges feed by filtering off minute organic particles from the water.
Interesting factSponges are the most common fossils in pieces of flint from the Chalk. You can often find them by looking in flint gravel drives and paths in central and south-east England.
sponges
RhizopoterionCretaceous
pores for taking in water
stem
roots
10 cm 12 cm
roots
SiphoniaCretaceous
Sponges from the chalk preserved in flint. The largest round specimen is nearly 4 cm in diameter.
Sponges are the simplest multicellular animals. They lack definite tissues and organs, e.g. they have no nervous system.
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ageOrdovician Period to present day.
environmentMost live in shallow seas, some in freshwater. All aquatic.
descriptionColonies range from millimetres to 1 m across, but the individuals (called zooids) that make up the colonies are tiny, usually less than a mm long. Each zooid builds a tube or box (zooecium) of calcium carbonate, with an aperture (opening). Colonies vary greatly in form, e.g. encrusting sheets (‘sea mats’), net-like fronds or branching twigs. Some bryozoans look like small corals. The zooids’ tentacles filter plankton from the water. Bryozoans often occur in limestones. Most require microscopic study to identify correctly.
Interesting factBryozoans can often be seen on modern beaches, encrusting sea-weed, rocks and shells.
BryozoansBryozoans are a separate phylum of colonial animals. They are common fossils, but being rather small and often delicate, are relatively unfamiliar.
FenestellaCarboniferous
HalloporaSilurian
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The apertures, which occur in two rows along the branches, are not visible at this magnification.
3 cm across
fan-shaped, net-like colony
2 cm
A cylindrical, branching colony with large round apertures.
StomatoporaJurassicAn encrusting, thread-like colony. irregular, leaf-like
colony
apertures 0.1-0.2 mm across
MetrarabdotosNeogene
5 mm
4 cm
1.5 mm across
aperture
close up
one zooecium
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Graptolites
ageCambrian Period to Carboniferous Period. Graptolites are relatively common in Ordovician and Silurian rocks, and are very useful in dating them as many species were widespread and short-lived.
environmentEntirely marine. A few lived attached to the sea floor, but most lived in the open sea, drifting with currents or possibly swimming feebly as part of the zooplankton.
descriptionMany look like little saw blades a few centimetres long, with ‘teeth’ on one or both sides of the ‘saw’. The ‘teeth’ were actually tiny cups (thecae) that housed the individuals with filter-feeding tentacles which made up the colony. A few colonies were fan-shaped with many branches. Only the resistant skeletons (originally made of collagen-like proteins) occur as fossils. They are often found in fine-grained rocks such as dark shales laid down in quiet, oxygen-poor conditions on the sea floor.
Interesting factThe name ‘graptolite’ means ‘writing on stone’ because they resemble pencil markings.
Graptolites are an extinct group of colonial animals. They were hemichordates, a phylum with few species today.
ClimacograptusOrdovician
2 cm
DicellograptusOrdovician
TetragraptusOrdovician
CyrtograptusSilurian
MonograptusSilurian
1.5 cm
3 cm
2 cm
1.5 cm
tiny flattened cups (thecae) in which individuals of the colony lived
first-formed part of colony
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Worms and Trace fossils10
WormsWorm is an informal name for various invertebrate groups belonging to different phyla. Most types of worm are entirely soft-bodied, and trace fossils may be the only evidence of their existence. Some worms, especially a group of polychaetes (bristle-worms) called serpulids (in the Phylum Annelida), secrete a tubular shell for living in, usually made of calcium carbonate (calcite or aragonite). Some tube-secreting worms are free-living (not attached to anything), whilst others cement themselves to hard surfaces such as shells, shell fragments and pebbles. Many worm tubes are rather irregular in shape.
Serpula. Jurassic. This form of worm tube is often found attached to large oyster shells on which the worm grew. Typical length 2-5 cm.
Rotularia. Palaeogene. This spirally coiled form, about 2 cm across, was free-living on the sea floor.
Ditrupa. Palaeogene. This gently curving, tusk-like form was free-living on the sea floor. Typical length 2 cm.
Trace fossilsTrace fossils are evidence of animal activity, such as footprints, trails, burrows, borings, bite marks or droppings. They are often the only evidence we have of extinct organisms whose bodies lacked any hard parts. Even if the organism that made the trace had hard parts, the culprit is rarely found at the scene. Most trace fossils are classified by their shape, or by the type of behaviour represented, not the trace-maker, which can rarely be identified with certainty. Sometimes an individual may make several different-looking traces, and the same-looking trace may be made by several different types of animal. See also the introduction (p.4).
Droppings from an unknown Jurassic animal. This coprolite (fossil dung) is preserved in iron pyrites. 13 cm.
Trace fossils made by unknown animals, probably mostly worms making trails across mud, preserved in relief on the base of a sandstone bed. Palaeogene. 5 cm across.
A horizontal ‘U’ shaped burrow, called Rhizocorallium, probably made by a crustacean. Jurassic. 20 cm.
Trace fossils made by various different types of animal moving over, sitting on, or burrowing through, sediment, mixing light-coloured sand with darker mud. Jurassic. 15 cm across.
4 cm
GibbithyrisCretaceous
TetrarhynchiaJurassic
ribs
growth lines
A modern brachiopod in life position
pedicle (stalk)
sea floor
hole for the pedicle during life
EpithyrisJurassic
shell 4 cm
2 cm3 cm
Magellania
Brachiopods
LingulaCarboniferous
GigantoproductusCarboniferous
DolerorthisSilurian
AntiquatoniaCarboniferous
4 cm 18 cm
1.5 cm
3 cm
plane of symmetry
calcite shell
dark, phosphatic shell (unlike most brachiopod shells which are made of calcite)
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Brachiopods (pronounced ‘bracky-o-pods’) are a separate phylum.
ageCambrian Period to present day. Brachiopods were much more abundant and diverse during the Palaeozoic and Mesozoic Eras than they are today.
environmentEntirely marine. They live on the sea floor.
descriptionBrachiopods all have a shell enclosing the soft tissues, including a feeding device which filters off food particles. The shell has two sides (called valves) which are usually found closed together in fossils. Unlike in most bivalves (molluscs), one side of a brachiopod shell is almost always larger than the other. In brachiopods, the plane of symmetry runs through the two sides of the shell, whereas in bivalves it runs between the two valves. In most brachiopods the shell is composed of calcite, though some (e.g. Lingula, overleaf) are phosphatic.
Many brachiopods are attached to the sea floor by a stalk called the pedicle. This stalk is not preserved in fossils, but its presence is indicated by a hole passing through the larger of the two valves.
Brachiopods are the commonest fossil in many Palaeozoic shallow marine limestones and shales.
Interesting factThey are sometimes called ‘lamp-shells’ after their resemblance to Roman oil lamps.
Brachiopods
A, B: Terebratula. Cretaceous. Two views of the same specimen. 4 cm. The hole through which the pedicle emerged can clearly be seen in A.
A B
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Gastropods
ViviparusCretaceous Fusinus
Palaeogene
CornulinaPalaeogene
SymmetrocapulusJurassic
NaticaNeogene
NeptuneaNeogene
3 cm
4 cm
5 cm
5.5 cm
3.5 cm
4 cm
7 cmlimpet-like form, indicating specialisation for clinging to rocks with large, sucker-like foot
aperture through which head and foot emerged
apex (first-formed part of shell)
long canal for siphon along which clean water was drawn to the gills
central rod (columella)
unusually, the aperture is on the left in this species (N. contraria)
spines
aperture
canal for siphon
cross-section through the shell of an undetermined genus, showing internal structure
spirally coiled tube into which body withdrew
growth lines
PalaeoxestinaPalaeogene
2 cm
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ryGastropods are a group of molluscs that includes slugs and snails. The molluscs form a very diverse phylum, and are some of the commonest fossils. Many molluscs have shells composed of calcite and/or aragonite, but some are only soft-bodied. Three mollusc groups are particularly important, both as fossils and today: gastropods, bivalves and cephalopods. There are several other mollusc groups, some of which are extinct.
ageCambrian Period to present day. Gastropods first became really abundant in the Cenozoic Era, exceeding other molluscs in numbers and diversity, as they do today.
environmentMost gastropods are marine, in shallow seas, but many inhabit rivers, lakes or ponds; others live on dry land.
descriptionFamiliar shelled gastropods include garden snails and, by the sea, whelks, winkles, limpets, cowries and abalones. The shell is absent in forms such as garden slugs.
The shell is usually a tapering tube coiled in a screw-like spiral. At rest, the animal’s body is pulled into the shell, but when moving, the head and muscular foot (used for creeping around) extend from the aperture. The shell is usually made of aragonite, rather than calcite, and in fossils the aragonite has often dissolved away, leaving a hole. Some gastropods with rather flat shells may look at first like ammonites, but gastropod shells are never divided into separate chambers as are ammonite shells.
Interesting factOccasionally fossil gastropod shells show patterns of colour banding. Some Palaeogene ones from southern England 35 million years old show purples and browns, but the colour itself has probably altered.
Gastropods
A B
A: Internal mould of Aptyxiella, a Jurassic gastropod common in Portland Stone (and known as a ‘Portland Screw’). The aragonite shell has dissolved away. 7 cm.
B: Volutospina. Palaeogene. 9 cm.
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Bivalves
PleuromyaJurassic
5.5 cm
growth lines (successive edges of shell during growth)
LophaJurassic
zig-zag margin
9 cm
plane of symmetry
an oyster with typical irregular form, lacking symmetry
VenericorPalaeogene
8 cm
ribs growth lines
interlocking ‘teeth’ and sockets to guide valves back into a tight fit as shell closed
muscle scars marking site of muscles that closed shell
internal view
MyophorellaJurassic
PseudopectenJurassic Inoceramus
Cretaceous
8.5 cm
9 cm
ribs
hinge line
umbo (first-formed part of shell)
conspicuous growth lines
small bumps (tubercles)
7 cm
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Examples of these molluscs include cockles, mussels, scallops and oysters.
ageCambrian Period to present day. More common in the Mesozoic and Cenozoic than the Palaeozoic.
environmentEntirely aquatic. Most are marine, living on shallow sea floors, though some inhabit freshwater. Many burrow into sediment, some cement onto, or bore into, objects, whilst others attach by threads. A few can swim intermittently.
descriptionMost bivalves have a shell with two parts (called valves) of equal size and shape, one a mirror image of the other (unlike in brachiopods). However, some bivalves such as oysters lack any symmetry. Shells may be made of aragonite (which often dissolves away) or calcite, or a mixture of both. The shell is opened at the hinge by an elastic ligament (not fossilised) and closed by one or two muscles (which leave attachment scars on internal surfaces). Single, detached valves are common as fossils (unlike in brachiopods).
Interesting factOne very common Jurassic oyster with a thick, curved shell, a species of Gryphaea, is often known in English folklore as the ‘Devil’s toenail’. It is unclear whether the shells were once believed to be the actual toenails of devils, or whether people thought they were what a devil’s toenail ought to look like.
Bivalves
A: Gryphaea arcuata. The Jurassic oyster species nicknamed the ‘Devil’s toenail’. Side view. 5 cm.
B: Gryphaea dilatata. Above: complete shell of this Jurassic oyster. Note the lack of symmetry (compare brachiopods). Below: internal view of a detached upper valve showing the single, central muscle scar. 8 cm.
C: Glycymeris. Neogene. Two muscle scars are clearly visible on the inside of this valve. 5.5 cm.
A
C
B
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Belemnites
Reconstruction of a living belemnite. The soft tissue is shown as if partially removed to reveal the internal skeleton (the bit found fossilised) at the rear.
phragmocone
guard
The radiating calcite crystals distinguish belemnites from burrows, bones, wood and other structures that lack them.
Chambered phragmocone. This often falls out or dissolves away to leave a cone-shaped hole.
CylindroteuthisJurassic
radiating calcite crystals
cross-section
faint concentric growth lines
18 cm
NeohibolitesCretaceous
AcrocoelitesJurassic
3 cm
9 cm8 cm
GonioteuthisCretaceous
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Belemnites are an extinct group of cephalopods (molluscs) that in many ways were probably rather like squid.
ageJurassic Period to the end of the Cretaceous Period.
environmentEntirely marine. Belemnites were carnivores that swam in the open sea.
descriptionBelemnites had a unique, bullet-shaped, internal shell called a guard, which being made of calcite was easily fossilised. At the wider (head) end of the guard was a chambered structure made of aragonite called the phragmocone. In fossils this has often fallen out or dissolved away, leaving a cone-shaped hole.
The whole living animal was several times longer than the guard, which was entirely surrounded by soft tissue. The guard at the rear is thought to have counterbalanced the weight of the head at the front, keeping the body level when swimming.
Belemnites are often found in Jurassic and Cretaceous clays, from which they easily get washed out. Sometimes a very large number occur together in the same bed of rock, possibly representing post-mating death events like those which occur in modern squid. Some small, isolated patches of belemnite guards are probably the regurgitated, indigestible remains of belemnites eaten by marine reptiles.
Interesting factIn mediaeval times, belemnites were thought to be petrified thunderbolts (lightning strikes). The word ‘belemnite’ comes from the Greek for dart or javelin.
Belemnites
Pachyteuthis. Jurassic. 11 cm. Note the hole at the end on the right where the phragmocone has fallen out or dissolved away.
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ammonites and Goniatites
OxynoticerasJurassic
6 cm
Ammonites
Goniatites
broken edge of shell
first-formed part of shell
internal moulds (infillings) of two successive chambers. The chambers often get filled with sediment or calcite crystals.
highly complex suture pattern typical of ammonites. The sutures mark the chamber partitions, or septa.
body chamber missing (crushed or broken off). Sometimes the body chamber is found on its own, as a separate internal mould, especially if the inner chambers have been crushed.
ribs
HildocerasJurassic
6.5 cm
CardiocerasJurassic
6.5 cm
keel (ridge) with grooves either side
strong ribs, varying in length
HarpocerasJurassic
8 cm
keel (ridge) sickle-shaped ribs
ScaphitesCretaceous
4 cm
later-formed part of shell loosely coiled
early-formed part of shell spirally coiled as usual
Some ammonites, especially in the Cretaceous, became uncoiled or coiled into irregular, curious shapes.
GoniatitesCarboniferous
This group of cephalopods lived only in Palaeozoic seas. Their chambered shells had sutures with a complexity between that of nautiloids and ammonites.
body chamber missing
shell mostly broken away, revealing internal chambers filled with sediment
simple zig-zag suture
7.5 cm
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ryAmmonites and their older relatives, the goniatites, are extinct groups of cephalopods (molluscs). Living cephalopods include squid, cuttlefish, octopus and nautilus.
ageAmmonites: Triassic to the end of the Cretaceous.Goniatites: Devonian Period to Permian Period.
environmentEntirely marine, like all other cephalopods past and present. The majority of ammonites inhabited shallow seas. They were predators with an active lifestyle, swimming and catching prey with their tentacles.
descriptionThe shell of an ammonite is a coiled tube, divided into many separate chambers by partitions (called septa). The septa of ammonites are highly complex in shape. When viewed from the side, where the outer shell has been broken off or dissolved away, the edges of ammonite septa can be seen as very wiggly lines called sutures. This distinguishes ammonites from nautiloids, in which the sutures are straight or gently curving. Ammonite shells were made of aragonite, which in fossils has often dissolved away or recrystallised to calcite.
The body of the ammonite was housed in the outermost part of the shell, the body chamber. A thin tube used for regulating buoyancy (the siphuncle) extended back through all the chambers. The soft parts, e.g. tentacles, have never been found as fossils, so exactly what living ammonites looked like is unknown. Ammonites were abundant, diverse and widespread. They rapidly evolved many different species, and so are useful for matching up rocks of the same age in different places.
Interesting factAmmonites were called ‘snakestones’ in English folklore. They were believed to be the petrified remains of snakes that once infested places such as Whitby in Yorkshire.
A B
A: The ammonite Dactylioceras. Jurassic. 8 cm.
B: Close-up of a part of an ammonite to show the complex sutures characteristic of ammonites. Amaltheus. Jurassic. View 3.5 cm across.
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ageCambrian Period to present day.
environmentEntirely marine. Many extinct nautiloids lived in shallow seas, but the few living species of nautilus inhabit deep water. Tentacles are used to catch prey.
descriptionIn the past, nautiloid shells had many different shapes, some straight, some curved, and some irregularly coiled. Except for a few species in the Triassic Period, all straight-shelled nautiloids lived in the Palaeozoic Era.
Like in ammonites, the tubular shell of nautiloids was divided into many chambers by partitions (called septa). Unlike in ammonites, the septa of nautiloids have a very simple shape. When viewed from the side, where the outer shell has been broken off or dissolved away, the edges of nautiloid septa can be seen as straight or gently curving lines, called sutures.
Interesting factSome straight nautiloids were over 5 metres long.
nautiloidsNautiloids are a group of cephalopods (molluscs).
Section through part of a straight nautiloid. Ordovician. 12 cm.
CenocerasJurassic 6 cm
MichelinocerasSilurian
9 cm (incomplete)
body chamber for living animal
simple, gently curving sutures
shell dissolved away, showing chambers filled with sediment
broken edge of shell
straight sutures
siphuncle (tube connecting chambers) septa
body chamber (at head end)
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ThecosmiliaJurassic
7 cm
corals RugoseTabulate
Scleractinian
Scleractinian corals
MontlivaltiaJurassic
ParasmiliaCretaceous
IsastraeaJurassic
FungiastraeaJurassic
4 cm 3 cm
5 cm
9 cm
solitary individual
colonies
septa (radial partitions)
top of aragonite skeleton on which sat the anemone-like soft parts when the coral was alive
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colony composed of a few large individuals
colonies
corals RugoseTabulate
Scleractinian
TryplasmaSilurian Dibunophyllum
Carboniferous
AcervulariaSilurianSiphonodendron
Carboniferous
5 cm
10 cm
tubes 3 mm across
5 cm
wrinkled surface
solitary individuals
septa (radial partitions)
HeliolitesSilurian
FavositesSilurian
SyringoporaCarboniferous
HalysitesSilurian
6 cm 6 cm
4 cm
2 cm
septa (radial partitions) missing or reduced
reduced septa
close-up
top view
Tabulate corals
Rugose corals
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Corals are by far the most important fossil group of the Phylum Cnidaria (pronounced with a silent ‘C’). Sea anemones, jellyfish and hydroids are also cnidarians, but being soft-bodied these groups are very rare as fossils. Cnidarians have a central mouth around which are stinging tentacles for catching prey.
ageOrdovician Period to present day.
environmentAll corals are, and have been, entirely marine. They usually live on the sea floor.
descriptionCorals secrete a skeleton of calcium carbonate below the soft, sea anemone-like parts at the top. Corals may either be solitary individuals, or form colonies in which many genetically identical, linked individuals share a skeleton.
There are three main groups of corals:
Rugose corals (solitary individuals or colonies). Age: Ordovician Period to Permian Period.
Tabulate corals (always colonies). Age: Ordovician Period to Permian Period.
Scleractinian corals (solitary individuals or colonies). Age: Triassic Period to present day.
Interesting factAlthough scleractinian corals are much younger than rugose or tabulate corals, their fossils are often much less well preserved because their skeletons are made of aragonite, rather than calcite. Aragonite is unstable over long periods, and tends to dissolve away or recrystallise to calcite.
corals RugoseTabulate
Scleractinian
A B
A: Lithostrotion. Carboniferous. 5 cm. A colonial rugose coral.
B: Kodonophyllum. Silurian. 2.5 cm. A solitary rugose coral.
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sea urchins (echinoids)
MicrasterCretaceous
5 cm
petal-like rays of plates bearing tiny pores through which tube feet projectedtop view
side viewinterlocking calcite plates (edges often hard to see)
EchinocorysCretaceous
5 cm
side view
pores for tube feet
mouth
attachment points for tiny spines (fallen off)
anus
top view underside
HemicidarisJurassic3 cm
top view
side viewanus
ray of pore-bearing plates
ball joint for base of large spines (fallen off)
(mouth is central on underside)
NucleolitesJurassic
3 cm anus
3 cm 3 cm
examples of large detached spines
spines often break across flat, reflective surfaces
base of spine with socket for ball joint
top view
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Sea urchins or echinoids (pronounded ‘eck-in-oids’) and crinoids are the most common fossil echinoderms (‘eck-eye-no-derms’). Other echinoderm groups today include starfish (asteroids) and brittle stars (ophiuroids). There are also several extinct groups. Echinoderm means ‘spiny skin’, referring to the spines or hard, warty bumps that project from the surface in some groups. Many echinoderms have a five-rayed arrangement of calcite plates.
ageOrdovician Period to present day. Sea urchins are much more common in Mesozoic and Cenozoic rocks than in Palaeozoic ones.
environmentEntirely marine, now and in the past, like all echinoderms. Most sea urchins live in shallow seas. Well-rounded, globular ones usually move around over the sea floor, and have a mouth at the centre of the underside. Less rounded, flattened or heart-shaped ones tend to burrow in soft sediment, and may have a mouth placed less centrally.
descriptionSea urchins have a shell (or test) made of many calcite plates thinly covered with soft tissue. Most sea urchins have five petal-like rays of plates with tiny pores through which tube feet project. Tube feet are multipurpose, extendible tentacles used especially in feeding, respiration and locomotion. Some sea urchins have very large spines, others only tiny ones; the spines usually drop off after death.
Interesting factSome Cretaceous sea urchins were called ‘shepherds’ crowns’ in English folklore as they had five rays converging on the apex, like the ribs on a crown. Shepherds may have come across them, weathered out of the Chalk, when tending sheep on the downlands of southern England.
sea urchins (echinoids)
A B
A: Echinocorys. Cretaceous. 5 cm. Typical preservation in flint.
B: Tylocidaris. Cretaceous. 6.5 cm. View of underside. The large spines were defensive.
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crinoids
DictenocrinusOrdovician
cup with central mouth at top
SagenocrinitesSilurian
cup 2.5 cm
flexible stem with many calcite plates (stem ossicles are called columnals)
anal tube
reconstruction on sea floor
root-like holdfast
branched arms for gathering food particles
branched, flexible arms which opened up for feeding
cup (or calyx)
broken stem
Marsupites Cretaceous
Apiocrinites Jurassic
incomplete arm (others broken off)
cup 3.5 cm
globular cup with large, thin plates
no stem
broken bases of arms (seldom preserved)
cup 3.5 cm
calcite plates of cup
broken stem
stem plates (columnals) are usually round or star-shaped with 5 points
central canal for soft tissue
plates typically 3 – 15 mm across
Each plate of a crinoid is called an ossicle.
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Crinoids are echinoderms that are sometimes called ‘sea lilies’ as some look superficially like plants.
ageCambrian Period to present day. Crinoids were much more abundant and diverse during the Palaeozoic and Mesozoic Eras than they are today.
environmentEntirely marine. Most fossil ones lived in shallow seas.
descriptionMost ancient crinoids were attached to the sea floor by a stem or stalk with a root-like base. Most stemmed crinoids feed by bending their flexible, branching appendages (called ‘arms’) outwards and backwards into the current, looking like an umbrella in the wind. Tube feet on the arms gather tiny food particles suspended in the water, and hair-like structures waft the food towards the central mouth situated in a cup (or calyx) at the top of the stem. Some crinoids lack stems and are free-swimming.
A
B
A: Marsupiocrinites. Silurian. Cup 3 cm across.
B and C: Carboniferous Limestone with crinoid debris – fragments of stems and isolated stem plates. C shows a polished slab of crinoidal limestone with stem fragments cut across at various angles. Largest stems 1 cm in diameter.
C
crinoids
Interesting factSome rocks consist almost entirely of isolated crinoid plates and stem fragments.
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BasilicusOrdovician
8 trunk segments
10 cm
Trilobites
DalmanitesSilurian
CalymeneSilurian
ParadoxidesCambrian
IllaenusOrdovician
TrinucleusOrdovician
PhacopsSilurian
AgnostusCambrian
axisleft lobe right lobe
headshield
trunk
tailpiece
glabellacompound eye
13 trunk segments
5 cm
line of weakness along which headshield split during moulting
very small tailpiece
smooth exoskeleton
4 cm
rows of pits with unknown function (not eyes)
eye
glabella
lenses in large compound eyerolled up
individual viewed from the side
headshield with similar shape to tailpiece
only 2 trunk segments
noTe: Most trilobite fossils are bits and pieces of exoskeleton shed during moulting; complete specimens like these are rare.
large tailpiece
3 cm
20 cm
8 cm
6 cm6 mm
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The extinct trilobites, pronounced ‘try-lo-bites’, are the most important group of fossil arthropods. Arthropods are the largest and most diverse animal phylum; living groups include crustaceans (crabs, lobsters, barnacles and shrimps), insects, millipedes, centipedes, spiders, king crabs, scorpions, and mites.
ageCambrian Period to Permian Period, i.e. trilobites lived only in the Palaeozoic Era.
environmentEntirely marine. Most trilobites lived on or near the floor of shallow seas; some swam higher up in the ocean.
descriptionLike other arthropods, trilobites had a hard outer shell (the exoskeleton) divided into segments, and paired, jointed appendages. They grew during periodic moulting when the exoskeleton was shed and a new, larger one was formed. The trilobite exoskeleton was mostly made of the mineral calcite, so it was easily preserved. Most trilobite fossils represent bits and pieces of the exoskeleton cast off during moulting, rather than dead individuals.
Trilobites varied greatly in shape, but all had three lobes running up and down their length (from which their name is derived). They were also divided cross-ways into a headshield, a trunk and a tailpiece. Many had compound eyes, like those of flies. Some could roll up for defence.
Interesting factOut of many thousands of different trilobite species, only about twenty have been found with any legs and antennae (none in Britain); these parts were normally too soft to get fossilised.
Trilobites
A B
A: Ogygiocarella. Ordovician. 7.5 cm. The eyes can be clearly seen.
B: Platycalymene. Ordovician. 6 cm. Pieces of exoskeleton like this usually represent moulted remains.
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Detail of part of a fossil fish. Note the thick, shiny scales. Palaeoniscus. Permian. 2 cm across.
Shark teeth from the London Clay, Palaeogene. Two on the right have roots replaced by iron pyrites. Longest tooth 4.5 cm.
Shark tooth, adapted for crushing invertebrate shells. Ptychodus. Cretaceous. 4.5 cm.
An ichthyosaur. These marine reptiles lived only in the Mesozoic Era. They averaged 2-4 metres in length; some reached 16 metres.
Four fossils representing ichthyosaurs. Jurassic. A: A vertebra from towards the rear of the animal. Ichthyosaur vertebrae are typically dished in the centre (on both sides, i.e. front and back), unlike those of plesiosaurs, which are flatter. The groove on the top is where the spinal cord was located. 6.5 cm. B: A beach-worn vertebra from the neck region, with rounded projections on left and right where the ribs articulated. 9 cm. C: Fragment of a rib bone. 6 cm. D: One of the many bones of a paddle. 4.5 cm.
A piece of fossil bone (from a reptile), showing the typical, rather irregular grain, lacking the fine, closely parallel lines more typical of fossil wood. Triassic. 7 cm.
One of many bony plates (called scutes) that were embedded in the skin of an alligator. Diplocynodon. Palaeogene. 4.5 cm.
AB
CD
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VertebratesGeological historyThe earliest vertebrates (animals with a bony or cartilaginous skeleton and a skull) were Cambrian fish. The five major groups of vertebrates today, in order of evolutionary appearance, are fish, amphibians, reptiles, mammals and birds. Vertebrate fossils are rare compared with invertebrate fossils, mainly due to smaller populations. Land-dwelling vertebrates, and those of freshwater rivers and lakes, are rarer as fossils than marine vertebrates.
Fish first appeared in the Cambrian, but remained rare until the Devonian, when they diversified and flourished. The earliest fish lacked jaws; some had massive bony armour. Sharks’ teeth are relatively common, especially in soft Mesozoic and Cenozoic marine clays, from which they may get washed out and concentrated on beaches. Fish scales may be quite common; they are usually dark brown, shiny, often 1-2 mm in size, and usually rhombic (♦) in shape. The scales of many ancient fish were thicker than those of familiar fish today.
Amphibians first appeared in the late Devonian, evolving from a group of fish. Some Palaeozoic forms were several metres long, unlike modern frogs, toads and newts. Fossils are very rare.
Reptiles first appeared in the Carboniferous, evolving from a group of amphibians. Fossil reptiles are relatively common, especially in Mesozoic rocks. Mesozoic land-dwelling groups such as dinosaurs and pterosaurs (flying reptiles) are much rarer than Mesosoic marine reptiles, such as ichthyosaurs and plesiosaurs, whose isolated vertebrae can quite often be found in Jurassic mudstones and shales. Many ichthyosaurs and plesiosaurs were several metres in length. Finding just a single bone from such a large, extinct animal can be truly exciting!
Mammals first appeared in the Triassic, evolving from a group of reptiles. Initially, in the Mesozoic, they were small (mostly shrew-sized) and rare. Mammal diversity expanded greatly in the Palaeogene, increasing towards the present day. The most common mammal fossils in Britain are from the Quaternary, such as Ice Age mammoths and woolly rhinos, as well as deer, horse and ox. Mammoths got through six sets of teeth in a full life. Four teeth were fully in operation at any one time, one in each of the four jaw areas (upper and lower, left and right).
Birds first appeared in the Jurassic, evolving from a group of small carnivorous dinosaurs. During the Cenozoic they expanded greatly in diversity. Fossils are very rare.
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Identifying fossil vertebratesApart from trace fossils (e.g. footprints), vertebrates are mostly represented by bones and teeth. These are almost always denser than in life, because the pore spaces have been filled with extra minerals (i.e. permineralised). Fossil bones and teeth tend to be dark in colour, though Quaternary specimens tend to be lighter in colour and less dense. Although sometimes quite common, isolated fragments of bone, especially if lacking a distinctive shape, can be impossible to identify, even for an expert. One may only be able to say, for example, that the fossil is a piece of reptile bone. Fossil bone can be difficult to distinguish from fossil wood. Bone tends to have a rather irregular, spongy texture, lacking the very fine, closely parallel lines or ‘grain’ that fossil wood often possesses.
Teeth are often easier to identify than bones. The teeth of fish (e.g. sharks) and of reptiles tend to be dark and shiny, and often have tiny ridges along their length. Reptile teeth tend to have a rather simple cone-shape, and are usually similar wherever located in the jaw, whereas in a mammal, teeth have different shapes for different functions (incisors, canines, premolars and molars).
Close-up of a typical piece of Quaternary Ice Age bone showing the spongy texture of a broken surface, with pores still mostly unfilled by extra minerals. 2.5 cm.
Close-up of a typical piece of Jurassic bone (an ichthyosaur vertebra), showing the spongy texture of a broken surface, with pores filled by white calcite. 2.5 cm.
A: A typical fossil reptile tooth: cone-shaped, and peg-like. The top of the tooth and the root have broken off. Cretaceous. 3.5 cm.
B: A tooth of the dinosaur Iguanodon. Cretaceous. 4.5 cm.
A single woolly mammoth tooth. Mammuthus. Quaternary. A: The biting surface of this molar from the upper jaw is the gently curving, ridged part at the bottom of the photo; some of the roots can be seen top left. 20 cm across; larger teeth may reach 35 cm. B: Close up-view, 4.5 cm across, of the biting surface, with ridges of hard grey enamel.
A single upper molar tooth of a woolly rhinoceros. Coelodonta. Quaternary. 6 cm across. The front cover shows the left of the lower jaw with five cheek teeth. 35 cm.
Fragment of a woolly mammoth tusk (greatly elongated incisor tooth). Mammuthus. Quaternary. 24 cm.
A B
A B
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Plants
Geological historyAlthough evidence from spores suggests that some plants were growing on land in the Ordovician, it isn’t until the Silurian that we have evidence for what the whole plants looked like. These plants, initially just a few centimetres high, colonised the edges of land via freshwater rivers and lakes. The first trees with woody trunks appeared in the late Devonian. The first really abundant plant fossils in Britain occur in coal-bearing, late Carboniferous rocks. Among the groups flourishing then in equatorial forests and low-lying swamps were giant clubmosses, horsetails and seed ferns.
Conifers were the dominant forest tree throughout most of the Mesozoic Era. Conifers, seed ferns, true ferns, cycads and ginkgos are relatively common in Jurassic sandstones and shales in Britain.
Flowering plants (angiosperms) first appeared in the early Cretaceous. Initially rare, they expanded through the late Cretaceous and Cenozoic Era, and today make up the vast majority of plants in numbers and diversity. Most trees, shrubs, grasses, hedgerow and garden plants, and almost all our food crops, are angiosperms. In Britain, fruits and seeds of angiosperms can be found in deposits such as the London Clay (Palaeogene). Fossil pollen is very useful for revealing climatic fluctuations during the Quaternary Ice Age.
PreservationThe most common type of plant preservation is where the tissues have been reduced to carbon (usually black or very dark brown). In shales, many plants occur rather flattened (due to compaction), whereas in sandstones their 3-dimensional form is more often maintained. Sometimes plant tissues may be replaced by quartz or other minerals precipitated from solutions seeping through the sediment: this often preserves fine cellular details, as, surprisingly, does charcoal from natural fires. Pollen and spores are highly resistant to decay and often very abundant as fossils, but a microscope is needed to see them after they have been chemically released from the rock.
The presence of fossil land plants does not necessarily indicate rocks formed in a terrestrial setting: they may also occur in marine deposits, especially near estuaries, and fragments of trunks and branches may drift far out to sea before becoming waterlogged. On stagnant sea floors, such bits of wood are often replaced and permineralised with iron pyrites (fool’s gold).
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Plants
30 m high
LepidodendronCarboniferous
CalamitesCarboniferous
NeuropterisCarboniferous
NilssoniaJurassic
ConiopterisJurassic
GinkgoJurassic
scar where grass-like leaf was attached
clubmoss tree
trunk and branches
view 5 cm across
root system (separately called Stigmaria)
20 cm across
rosettes of leaves of Calamites (separately called Annularia)
horsetail
5 cm across
infilling of hollow stem
seed fern
5 cm longpinnule
6 cm across
pinna (leaflet of compound leaf)
pinna (leaflet of compound leaf)
4 cm long
fern
pinnule
cycad
incomplete single leaf
5 cm long
pinna
7 cm
root
single leaf
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Plants36
Identifying fossil plantsThis is a specialised skill, especially as whole plants are very rare. Isolated, different parts of the same plant (stem, leaves, roots, seeds, pollen, etc) have usually been given different names, which may be retained even when associations become known. Fragments of fossil wood are relatively common, but can be difficult to distinguish from fossil bone. Fossil wood usually has a more regular ‘grain’ than fossil bone, which has a rather irregular, spongy texture (with pore spaces, as in wood, normally filled by minerals). When seen under a microscope, angiosperm wood tends to have two cell types of distinctively different sizes – larger vessels and smaller diameter fibres. In conifers and their relatives, wood cells are basically all of the same type, and generally small. Annual growth rings may be seen in some fossil wood.
Fragments of fossil wood, all Jurassic. A: Internal view of a broken piece. 4 cm. B: Piece of wood in limestone. Cracks in the wood have been filled with calcite. 9 cm. C: Internal view of a broken piece of wood, showing two directions of grain at right angles to each other. 2.5 cm.
The Carboniferous seed fern Mariopteris, preserved in a nodule of iron carbonate. Part of a compound leaf. 9 cm.
A typical occurrence of Carboniferous plants, with small fragments of various species preserved as thin carbon films in shale. 5 cm.
Cut and polished piece of silicified fossil wood, with tissues replaced by quartz. 5.5 cm.
A B
C
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More than two million people have already chosen to study with The Open University - our wide range of courses and qualifications allows you to create a flexible programme of study to meet your needs and inspire you.
You can start with a single course in an area of interest or in a topic that’s relevant to your job, or your future career. Or you may want to begin with a diploma, or have a degree in mind.
Choose from courses in: science, information technology, environment, humanities (such as the arts, history, geography, music), business and management studies, law, mathematics, health and social care, social sciences & languages.
Beginning to studyIf you have no experience of higher education, deciding to become a university student can be a big step. Our programme of short introductory courses has been specially designed to give you a taste of university study before committing yourself to longer courses. Short courses, such as Fossils and the history of life, are also suitable for people who already have study experience, and are interested in a particular topic.
Fossils and the history of life (s193)
This short course explains how organisms become fossilised, helps you to identify common fossils and show you where they fit into the story of evolution.
other open University introductory science short courses include:
Life in the oceans: exploring our blue planet (s180)
If you want to learn more about life in the oceans, then you might like to take this short course based around the spectacular Blue Planet BBC TV series.
Volcanoes, earthquakes and tsunamis (s186)
This course provides an introduction to the science behind volcanoes, earthquakes and tsunamis, including why they occur, how they are triggered and the hazards they pose.
archaeology: the science of investigation (sa188)
You’ll develop an appreciation of the processes involved in the discovery, investigation and interpretation of a wide variety of artefacts and archaeological sites.
Find out moreTo learn more about our courses and qualifications, and to find out what it’s like to be an OU student,
visit our website at www.open.ac.uk
call our Student Registration and Enquiry Services on 0845 300 60 90
e-mail [email protected] or
write to The Open University, PO Box 197, Walton Hall, Milton Keynes, MK7 6BJ
The Open University has a wide range of learning materials for sale, including self study workbooks, videos and software. For more information visit the website www.ouw.co.uk
Published in 2008 by The Open University, Walton Hall, Milton Keynes, MK7 6AA, to accompany Fossil Detectives, first broadcast on BBC4, 2008
Executive Producer: Fiona PitcherSeries Producer: Kerensa JenningsAcademic Consultant & author of this field guide: Dr Peter SheldonExecutive Producer for The Open University: Catherine McCarthyBroadcast Learning Exec for The Open University: Dr Janet SumnerBroadcast Project Manager: Andrea MillsGraphic Designer & Illustrator: Glen Darby
Copyright © The Open University 2008
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the copyright holders.
Enquiries regarding extracts or the re-use of any information in this publication should be addressed to the Viewer and Listener Information Officer at The Open University, or email
Edited, designed and typeset by The Open University.
All photographs copyright © Peter Sheldon.
Printed and bound in the United Kingdom by CKN Print Ltd.
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