Field guide
2
Introduction
Fossil Field Guide By 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.
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.
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.
3
Introduction
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.
The Jurassic ammonite Psiloceras. 5.5 cm.
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.
4
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.
A pseudofossil. This plant-like pattern was formed by crystals of manganese oxide growing along a crack in limestone. 3.5 cm.
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.
5
Introduction
Responsible fossil collecting Fossils 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.
6
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.
Jurassic strata rich in fossils – but always keep your distance from unstable cliffs like this.
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. References Here 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.
Sponges
pores for taking in water
Millions of yrs ago
0
Quaternary
Sponges are the simplest multicellular animals. They lack definite tissues and organs, e.g. they have no nervous system.
Cenozoic
1.8
Neogene
7
Siphonia
Cretaceous
Cretaceous
65
12 cm
Age Cambrian Period to present day.
Environment
200
Triassic
Mainly marine, on the sea floor; some live in freshwater.
Description Sponges 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.
Carboniferous
Permian
251
Palaeozoic
Interesting fact Sponges 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.
146
Jurassic
Cretaceous
299
359
Devonian
10 cm
Rhizopoterion
416
Silurian
roots
roots
Mesozoic
Ordovician
443
488 Sponges from the chalk preserved in flint. The largest round specimen is nearly 4 cm in diameter.
Cambrian
stem
Palaeogene
23
542
8
Bryozoans
Millions of yrs ago
Quaternary
0
Bryozoans are a separate phylum of colonial animals. They are common fossils, but being rather small and often delicate, are relatively unfamiliar.
Neogene
Cenozoic
1.8
Palaeogene
23
Fenestella
Cretaceous
65
Carboniferous
146
2 cm
A cylindrical, branching colony with large round apertures.
5 mm one zooecium
Triassic
Stomatopora
aperture
Jurassic
Permian
251
4 cm
An encrusting, thread-like colony.
irregular, leaf-like colony
close up 299
Age
1.5 mm across
Metrarabdotos Neogene
Ordovician Period to present day.
Environment Most live in shallow seas, some in freshwater. All aquatic. 359
Devonian
Carboniferous
Hallopora Silurian
apertures 0.1-0.2 mm across
200
Silurian
416
Ordovician
443
488
Description Colonies 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 fact Bryozoans can often be seen on modern beaches, encrusting sea-weed, rocks and shells.
Cambrian
Palaeozoic
3 cm across
The apertures, which occur in two rows along the branches, are not visible at this magnification.
Jurassic
Mesozoic
fan-shaped, net-like colony
542
Graptolites Climacograptus Ordovician
Millions of yrs ago
0
Quaternary
Graptolites are an extinct group of colonial animals. They were hemichordates, a phylum with few species today.
Tetragraptus Ordovician
23
Ordovician
Monograptus
Cretaceous Triassic
Age
251
Permian
Cambrian 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.
Carboniferous
Environment Entirely 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.
The name ‘graptolite’ means ‘writing on stone’ because they resemble pencil markings.
Palaeozoic
Description
Interesting fact
Jurassic 200
3 cm
Many 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.
146
299
359
Devonian
tiny flattened cups (thecae) in which individuals of the colony lived
Cyrtograptus Silurian
2 cm
Mesozoic
first-formed part of colony
65
Silurian
416
Silurian
Dicellograptus
Palaeogene
1.5 cm
443
Ordovician
1.5 cm
2 cm
Neogene
Cenozoic
1.8
488
Cambrian
9
542
10
Worms and Trace fossils
Worms Worm 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 fossils Trace 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 samelooking 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.
A horizontal ‘U’ shaped burrow, called Rhizocorallium, probably made by a crustacean. Jurassic. 20 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.
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.
Brachiopods
11
hole for the pedicle during life
calcite shell shell 4 cm
pedicle (stalk) sea floor
4 cm
Magellania
Epithyris
A modern brachiopod in life position
Jurassic ribs
plane of symmetry
growth lines 2 cm
3 cm
Gibbithyris
Tetrarhynchia
Cretaceous
Jurassic
18 cm
4 cm
Antiquatonia
Gigantoproductus
Carboniferous dark, phosphatic shell (unlike most brachiopod shells which are made of calcite)
Carboniferous 1.5 cm
3 cm
Lingula
Carboniferous
Dolerorthis Silurian
12
Brachiopods
Millions of yrs ago
Quaternary
0
Age
Neogene
Cenozoic
1.8
Palaeogene Cretaceous Jurassic
146
Triassic
200
Permian
251
299
Brachiopods 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 fact They are sometimes called ‘lamp-shells’ after their resemblance to Roman oil lamps.
359
A
Devonian
Carboniferous
Entirely marine. They live on the sea floor.
Description 65
Mesozoic
Cambrian Period to present day. Brachiopods were much more abundant and diverse during the Palaeozoic and Mesozoic Eras than they are today.
Environment 23
B
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Brachiopods (pronounced ‘bracky-o-pods’) are a separate phylum.
542
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.
Gastropods
13 3 cm
apex (first-formed part of shell)
aperture through which head and foot emerged
central rod (columella)
4 cm
Viviparus Cretaceous
spirally coiled tube into which body withdrew
Fusinus
Palaeogene
long canal for siphon along which clean water was drawn to the gills
5 cm cross-section through the shell of an undetermined genus, showing internal structure
spines growth lines
Palaeoxestina Palaeogene
5.5 cm
aperture canal for siphon
2 cm
Cornulina Palaeogene
Symmetrocapulus Jurassic
3.5 cm
limpet-like form, indicating specialisation for clinging to rocks with large, sucker-like foot
7 cm
4 cm
Natica
Neogene
unusually, the aperture is on the left in this species (N. contraria)
Neptunea Neogene
14
Gastropods
Millions of yrs ago
Quaternary
0
Neogene 23
Age
65
Environment
146
Triassic Permian
251
299
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 fact 359
Devonian
Carboniferous
Description Familiar 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.
200
Occasionally 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. A
B
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Cambrian Period to present day. Gastropods first became really abundant in the Cenozoic Era, exceeding other molluscs in numbers and diversity, as they do today. Most gastropods are marine, in shallow seas, but many inhabit rivers, lakes or ponds; others live on dry land.
Jurassic
Mesozoic
Cretaceous
Palaeogene
Cenozoic
1.8
Gastropods 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.
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.
542
Bivalves
15
an oyster with typical irregular form, lacking symmetry
plane of symmetry
9 cm
5.5 cm
Pleuromya Jurassic
ribs
growth lines (successive edges of shell during growth)
growth lines
8 cm
Lopha
zig-zag margin
Jurassic interlocking ‘teeth’ and sockets to guide valves back into a tight fit as shell closed
internal view
Venericor Palaeogene hinge line
muscle scars marking site of muscles that closed shell umbo (first-formed part of shell)
ribs
8.5 cm
Pseudopecten Jurassic
9 cm
Inoceramus
conspicuous growth lines
Cretaceous small bumps (tubercles)
Myophorella Jurassic 7 cm
16
Bivalves
Millions of yrs ago
Quaternary
0
Age
Neogene
Cenozoic
1.8
Palaeogene Cretaceous
65
146
Jurassic
Mesozoic
Triassic
200
Entirely 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.
Description Most 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 fact
Permian
251
299
One 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. A
B
359
Devonian
Carboniferous
Cambrian Period to present day. More common in the Mesozoic and Cenozoic than the Palaeozoic.
Environment 23
C
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Examples of these molluscs include cockles, mussels, scallops and oysters.
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.
542
17
Belemnites phragmocone guard
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. radiating calcite crystals
faint concentric growth lines
Chambered phragmocone. This often falls out or dissolves away to leave a cone-shaped hole.
Cylindroteuthis Jurassic Cross-section The radiating calcite crystals distinguish belemnites from burrows, bones, wood and other structures that lack them.
18 cm
3 cm
Neohibolites
9 cm
Cretaceous
Acrocoelites Jurassic
8 cm
Gonioteuthis Cretaceous
18
Belemnites
Millions of yrs ago
Quaternary
0
Age
Neogene
Cenozoic
1.8
Palaeogene
23
146
Jurassic
Mesozoic
Cretaceous
65
Triassic
200
Permian
251
Jurassic Period to the end of the Cretaceous Period.
Environment Entirely marine. Belemnites were carnivores that swam in the open sea.
Description Belemnites 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 fact 359
Devonian
Carboniferous
299
In mediaeval times, belemnites were thought to be petrified thunderbolts (lightning strikes). The word ‘belemnite’ comes from the Greek for dart or javelin.
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Belemnites are an extinct group of cephalopods (molluscs) that in many ways were probably rather like squid.
542
Pachyteuthis. Jurassic. 11 cm. Note the hole at the end on the right where the phragmocone has fallen out or dissolved away.
Ammonites and Goniatites
19
internal moulds (infillings) of two successive chambers. The chambers often get filled with sediment or calcite crystals.
broken edge of shell
ribs
Ammonites
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.
first-formed part of shell 6 cm
Oxynoticeras
highly complex suture pattern typical of ammonites. The sutures mark the chamber partitions, or septa.
Jurassic
keel (ridge) with grooves either side strong ribs, varying in length
Hildoceras
6.5 cm
6.5 cm
Cardioceras
Jurassic
sickle-shaped ribs
8 cm
Harpoceras Jurassic
Goniatites
Jurassic
keel (ridge) later-formed part of shell loosely coiled
7.5 cm
Some ammonites, early-formed especially in part of shell the Cretaceous, spirally coiled became uncoiled as usual or coiled into irregular, curious Scaphites shapes. Cretaceous body chamber missing
Carboniferous 4 cm
shell mostly broken away, revealing internal chambers filled with sediment
simple zigzag suture
Goniatites This group of cephalopods lived only in Palaeozoic seas. Their chambered shells had sutures with a complexity between that of nautiloids and ammonites.
20
Ammonites and Goniatites
Millions of yrs ago
Quaternary
0
Age
Neogene
Cenozoic
1.8
Palaeogene Cretaceous
65
146
Jurassic
Mesozoic
Triassic
200
Permian
251
299
359
Entirely 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.
Description The 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 fact Ammonites 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.
Devonian
Carboniferous
Ammonites: Triassic to the end of the Cretaceous. Goniatites: Devonian Period to Permian Period.
Environment 23
Silurian
416
Ordovician
443
A 488
Cambrian
Palaeozoic
Ammonites and their older relatives, the goniatites, are extinct groups of cephalopods (molluscs). Living cephalopods include squid, cuttlefish, octopus and nautilus.
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.
542
Nautiloids
shell dissolved away, showing chambers filled with sediment
Millions of yrs ago
0
Quaternary
Nautiloids are a group of cephalopods (molluscs). body chamber for living animal
6 cm
Jurassic
Michelinoceras
straight sutures
23
Palaeogene
Cenoceras
Neogene
1.8
simple, gently curving sutures
Cenozoic
broken edge of shell
Silurian
Cretaceous
65
Age
Triassic
200
Description In 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.
Permian
251
Some straight nautiloids were over 5 metres long. siphuncle (tube connecting chambers)
septa
body chamber (at head end)
Palaeozoic
Interesting fact
Carboniferous
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.
299
359
Devonian
Entirely marine. Many extinct nautiloids lived in shallow seas, but the few living species of nautilus inhabit deep water. Tentacles are used to catch prey.
146
416
Silurian
Environment
Mesozoic
Cambrian Period to present day.
Jurassic
9 cm (incomplete)
Ordovician
443
Section through part of a straight nautiloid. Ordovician. 12 cm.
488
Cambrian
21
542
22
Corals
Rugose Tabulate
Scleractinian
Scleractinian corals
colonies 9 cm
5 cm
Isastraea
Fungiastraea
Jurassic
Jurassic
top of aragonite skeleton on which sat the anemone-like soft parts when the coral was alive
septa (radial partitions)
solitary individual
3 cm
4 cm
Montlivaltia
Parasmilia
Jurassic
Cretaceous
colony composed of a few large individuals
Thecosmilia Jurassic 7 cm
23
Corals
Rugose Tabulate
wrinkled surface
Scleractinian
Rugose corals
solitary individuals
Tryplasma Silurian
Dibunophyllum Carboniferous
5 cm
10 cm septa (radial partitions) colonies
top view
tubes 3 mm across
5 cm
Acervularia
Siphonodendron
Silurian
Carboniferous reduced septa
Tabulate corals
close-up
Heliolites
6 cm
Favosites
6 cm
Silurian
Silurian
septa (radial partitions) missing or reduced 4 cm 2 cm
Syringopora Carboniferous
Halysites Silurian
24
Corals
Millions of yrs ago
Quaternary
0
Neogene
Cenozoic
1.8
Palaeogene Cretaceous Jurassic
146
Ordovician Period to present day.
Environment Description Corals 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:
Triassic
200
Rugose corals (solitary individuals or colonies). Age: Ordovician Period to Permian Period. Tabulate corals (always colonies). Age: Ordovician Period to Permian Period.
Permian
251
299
359
Devonian
Carboniferous
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.
All corals are, and have been, entirely marine. They usually live on the sea floor. 65
Mesozoic
Scleractinian
Age 23
Scleractinian corals (solitary individuals or colonies). Age: Triassic Period to present day.
Interesting fact Although 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. A
B
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Rugose Tabulate
542
A: Lithostrotion. Carboniferous. 5 cm. A colonial rugose coral. B: Kodonophyllum. Silurian. 2.5 cm. A solitary rugose coral.
Sea urchins (echinoids)
25
petal-like rays of plates bearing tiny pores through which tube feet projected
top view
side view
5 cm
Micraster Cretaceous
mouth
pores for tube feet
top view
interlocking calcite plates (edges often hard to see)
underside
5 cm anus
Echinocorys Cretaceous
attachment points for tiny spines (fallen off)
side view ray of porebearing plates
top view
anus
Hemicidaris
3 cm
Jurassic
3 cm
Nucleolites
ball joint for base of large spines (fallen off)
side view (mouth is central on underside) 3 cm
Jurassic
spines often break across flat, reflective surfaces
top view
3 cm
base of spine with socket for ball joint
anus examples of large detached spines
Sea urchins (echinoids)
26 Millions of yrs ago
Quaternary
0
Neogene
Cenozoic
1.8
Palaeogene
23
Cretaceous 146
Jurassic
Mesozoic
Triassic
200
Permian
251
299
Environment Entirely 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.
Description Sea 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 fact 359
Devonian
Carboniferous
Age Ordovician Period to present day. Sea urchins are much more common in Mesozoic and Cenozoic rocks than in Palaeozoic ones.
65
Silurian
416
Some 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. A
B
Ordovician
443
488
Cambrian
Palaeozoic
Sea urchins or echinoids (pronounded ‘eck-in-oids’) and crinoids are the most common fossil echinoderms (‘eckeye-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.
A: Echinocorys. Cretaceous. 5 cm. Typical preservation in flint. B: Tylocidaris. Cretaceous. 6.5 cm. View of underside. The large spines were defensive.
542
27
Crinoids anal tube
branched arms for gathering food particles
branched, flexible arms which opened up for feeding
cup with central mouth at top flexible stem with many calcite plates (stem ossicles are called columnals)
Each plate of a crinoid is called an ossicle.
cup 2.5 cm
reconstruction on sea floor root-like holdfast
cup (or calyx) broken stem
Sagenocrinites Silurian
Dictenocrinus
broken bases of arms (seldom preserved)
Ordovician
calcite plates of cup
incomplete arm (others broken off)
cup 3.5 cm
globular cup with large, thin plates
cup 3.5 cm
Marsupites Cretaceous
broken stem
no stem
Apiocrinites Jurassic
central canal for soft tissue
plates typically 3 – 15 mm across stem plates (columnals) are usually round or star-shaped with 5 points
28
Crinoids
Millions of yrs ago
Quaternary
0
Age
Neogene
Cenozoic
1.8
Cambrian Period to present day. Crinoids were much more abundant and diverse during the Palaeozoic and Mesozoic Eras than they are today.
Environment
Palaeogene
23
Entirely marine. Most fossil ones lived in shallow seas.
Description
Cretaceous
65
146
Jurassic
Mesozoic
Crinoids are echinoderms that are sometimes called ‘sea lilies’ as some look superficially like plants.
Triassic
200
Permian
251
Most 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.
Interesting fact Some rocks consist almost entirely of isolated crinoid plates and stem fragments.
B
A
C
Devonian
359
Silurian
416
Ordovician
443
A: Marsupiocrinites. Silurian. Cup 3 cm across. 488
Cambrian
Palaeozoic
Carboniferous
299
542
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.
Trilobites
29
Note: Most trilobite fossils are bits and pieces of exoskeleton shed during moulting; complete specimens like these are rare.
glabella compound eye
headshield
line of weakness along which headshield split during moulting
13 trunk segments 8 trunk segments
trunk
large tailpiece
5 cm
tailpiece
10 cm left lobe
axis
right lobe
Basilicus
Calymene
Ordovician
Silurian
rows of pits with unknown function (not eyes)
3 cm
smooth exoskeleton
4 cm
Trinucleus
Illaenus
Ordovician
Ordovician
eye
8 cm
very small tailpiece
20 cm
Dalmanites
Paradoxides Cambrian
Silurian lenses in large compound eye
rolled up individual viewed from the side
headshield with similar shape to tailpiece
glabella only 2 trunk segments
6 cm
Phacops Silurian
6 mm
Agnostus Cambrian
30
Trilobites
Millions of yrs ago
Quaternary
0
Neogene
Cenozoic
1.8
Age
Palaeogene
23
Cretaceous 146
Triassic
200
Permian
251
299
359
Devonian
Carboniferous
Entirely marine. Most trilobites lived on or near the floor of shallow seas; some swam higher up in the ocean.
Description
Jurassic
Mesozoic
Cambrian Period to Permian Period, i.e. trilobites lived only in the Palaeozoic Era.
Environment 65
Like 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 fact Out 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. A
B
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
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.
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.
542
31
Vertebrates
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.
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.
B
A
C An ichthyosaur. These marine reptiles lived only in the Mesozoic Era. They averaged 2-4 metres in length; some reached 16 metres.
D
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.
One of many bony plates (called scutes) that were embedded in the skin of an alligator. Diplocynodon. Palaeogene. 4.5 cm.
32
Vertebrates
Millions of yrs ago
Quaternary
0
Neogene
Cenozoic
1.8
Palaeogene
23
146
Jurassic
Mesozoic
Cretaceous
65
Triassic
200
Permian
251
Devonian
359
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Carboniferous
299
542
Geological history The 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 landdwelling 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.
33
Vertebrates
Identifying fossil vertebrates Apart 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.
B
A
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
B
B: A tooth of the dinosaur Iguanodon. Cretaceous. 4.5 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. Fragment of a woolly mammoth tusk (greatly elongated incisor tooth). Mammuthus. Quaternary. 24 cm.
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.
34
Plants
Millions of yrs ago
Quaternary
0
Neogene
Cenozoic
1.8
Palaeogene
23
146
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.
Cretaceous
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.
Jurassic
Mesozoic
Triassic Permian
251
Preservation 299
359
Devonian
Carboniferous
Although 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.
65
200
Silurian
416
Ordovician
443
488
Cambrian
Palaeozoic
Geological history
542
The 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).
Plants
35
scar where grass-like leaf was attached
trunk and branches
view 5 cm across
30 m high root
clubmoss tree
Lepidodendron Carboniferous
pinna (leaflet of compound leaf)
20 cm across
root system (separately called Stigmaria)
5 cm long pinnule
infilling of hollow stem
5 cm across
horsetail
Calamites
Carboniferous
6 cm across rosettes of leaves of Calamites (separately called Annularia)
seed fern
Neuropteris Carboniferous
pinna (leaflet of compound leaf)
incomplete single leaf
single leaf pinna
pinnule 7 cm
Ginkgo Jurassic
5 cm long
4 cm long
cycad
fern
Nilssonia
Coniopteris
Jurassic
Jurassic
36
Plants
Identifying fossil plants This 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.
A typical occurrence of Carboniferous plants, with small fragments of various species preserved as thin carbon films in shale. 5 cm. The Carboniferous seed fern Mariopteris, preserved in a nodule of iron carbonate. Part of a compound leaf. 9 cm.
A
Cut and polished piece of silicified fossil wood, with tissues replaced by quartz. 5.5 cm.
B
C
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.
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[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 Pitcher Series Producer: Kerensa Jennings Academic Consultant & author of this field guide: Dr Peter Sheldon Executive Producer for The Open University: Catherine McCarthy Broadcast Learning Exec for The Open University: Dr Janet Sumner Broadcast Project Manager: Andrea Mills Graphic 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
[email protected] 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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