Glossary of Useful Words and Terms
Alga (plural = algae): the common name for simple plants which do not have specialised parts such as roots or leaves. They do not have vascular systems, so most are found living in water. Algae can range from single cells (e.g. Euglena) to plants of several metres in length (e.g. kelp).
Ammonoid: A group of extinct cephalopod molluscs with coiled shells. They lived in the seas during the Mesozoic Era. Their shells are divided into chambers. The final chamber of the shell was relatively large and long and held the soft body of the animal, which was probably rather like a squid or octopus with tentacles and a beak. The chambers are separated by walls of shell known as septa, often with highly folded margins where they connect with the outer shell layers (sutures). Each septum was perforated by a tube (the siphuncle) which joins together all of the chambers of the shell from its earliest growth stage (the tiny, egg-like protoconch) to the final septum. This tube has a special surface which allowed the animal to regulate the quantity of gas (mainly nitrogen) and water in each chamber and therefore move up or down in the sea, rather like a submarine. We interpret the characteristics of ammonoids by comparing them with nautiloids which are still alive today.
Amphibians: a group of vertebrate animals that spend part of their time on land and part in the water; so they are considered an intermediate form between fishes and reptiles. Amphibians must return to the water to breed and they have distinct larval and adult forms. Members of this group include frogs, toads, and salamanders.The name is derived from the Greek word amphibios which means "living a double life".
Arthropod: An arthropod is a member of a group of invertebrate animals that includes insects, spiders and crustaceans. An extinct group of fossil arthropods is the trilobites. The arthropods first appear in the fossil record in the early Cambrian, and account for 75% of all animal species ever described. Arthropods are characterized by having exoskeletons, and paired jointed limbs. This word comes from the Greek words arthron (joint) and podos (foot).
Bacteria: Tiny, single-celled, prokaryotic organisms that reproduce by cell division and usually have rigid cell walls. Bacteria are very diverse. They can be shaped like spheres, rods or spirals and can be found in virtually any environment. The earliest fossils found on Earth are bacteria, almost 3.3 billion years old.
Basalt: a dark coloured igneous rock erupted from volcanoes and fissures on the Earth's surface. Flows of basalt cover 70% of the Earth's surface, and large areas of the surfaces of the other terrestrial planets, so it is a very important crustal rock. It is produced by the partial melting of rocks deep inside the Earth's mantle.
Belemnoid: A group of extinct cephalopod molluscs with an elongate, tapering body and an internal shell. They probably looked rather like squid or cuttle fish. The rear part of the bullet-shaped, cylindrical shell is known as the rostrum and has a conical cavity in the wide end into which fits a small conical structure called the phragmacone. This is divided by septa and cut by a siphuncle.
Burgess Shale: a layer of Cambrian rocks in British Columbia, Canada, which contains fossils of a variety of strange soft-bodied animals. The fossils are the centre of a debate amongst palaeontologists about whether they are the ancestors of animals alive today, or whether they are evolutionary oddities with no living descendants.
Cambrian: the period of geological time between 544 and 505 million years ago. The name comes from the Cambrian Mountains in Wales, where the strata of the Cambrian were first defined. See Geological Timescale.
Carboniferous: the period of geological time between 360 and 286 million years ago, named for the thick deposits of coal found in rocks of this age. Forests covered much of the land during this time, the decomposition of which gave rise to the coal. See Geological Timescale.
Cartilage: flexible skeletal tissue found in vertebrates and chordates, made of fibres of a rubbery protein. In most animals the embryo has a skeleton made entirely of cartilage, most of which is replaced by bone as it develops. Some fish, such as sharks and rays, retain a cartilage skeleton throughout life.
Cephalopod: a group of marine molluscs which have a soft body, grasping tentacles and a well developed nervous system. Fossil examples include ammonoids and belemnoids. Living examples include octopus, nautilus and squid.
Chordate: an animal with an internal 'rod' of flexible tissue which supports its body. This rod can be bone or cartilage. Chordates with bony notochords are called vertebrates - for example mammals (including humans), fish, birds and reptiles. Invertebrate chordates include worms.
Class: in taxonomy, a group of orders with shared characteristics. For example, Mammals are a class of vertebrates which are warm-blooded and give birth to live young which they breast-feed. This includes humans, cows, whales and cats.
Coal: a carbon-rich mineral deposit made of the remains of fossil plant life. It is first deposited as peat, but over time is buried, compressed and heated which changes it physically and chemically. There are different grades of coal depending on the amount of water and gases left inside the deposit, and the percentage of carbon present.
Conifer: a plant belonging to a group of seed-bearing trees and shrubs, which have needle or scale-like leaves and resinous sap. Fertile parts of the plant are cones. The group first appeared in the Carboniferous.
Coral: important reef-building organisms known from the late Palaeozoic onwards. Fossil corals are good indicators of warm shallow marine environments. Corals can be solitary or part of a colony. A corallite is the skeleton formed by an individual coral polyp.
Core: 1. The iron-rich central part of the Earth. The core is divided into two zones (the inner core and outer core) because of the different way in which they transmit seismic waves. The core is responsible for the Earth’s magnetic field, and accounts for 32% of the mass of the planet. 2. a long sample of sediment or rock extracted by drilling down into the Earth. This is used to help to analyse strata which are not exposed at the surface.
Correlation: the method by which rocks units or strata are compared and time-relationships between them are established. This can be done by examining the rock type and succession, the fossil content, or by chemical analysis.
Crust: the thin, outermost, solid layer of the Earth. It varies in thickness from 5 km beneath the oceans (oceanic crust) to 60 km beneath mountain chains (Continental crust). It is broken into a number of plates.
Cycad: a seed-bearing plant similar in appearance to modern palm trees. They first appeared in the Permian, and were common in the Mesozoic, but are rare today, having been displaced by flowering, fruit-bearing plants which appeared in the Cretaceous.
Dendroid: a branching type of graptolite. Most types lived attached to the sea-bed, and were upright and bushy in appearance.
Devonian: the period of geological time between 410 and 360 million years ago. The name comes from the strata in Devon, S.W. England, which define this part of geological history. The Devonian is also known as the Old Red Sandstone (often referred to as O.R.S.) because the characteristic colour of the rocks is red. The red colour is because of a high iron content in the rock, and tells us that the climate was hot and dry when they were deposited. See Geological Timescale.
Ecosystem: a term used to describe a natural unit that consists of living and non-living parts which interact to form a stable system. The ecosystem idea can be applied at different scales in the same way e.g. a pond or an ocean.
Erosion: the part of the process of denudation which includes the physical breakdown, chemical solution and transportation of material from the surface of the Earth. This includes the removal of the surface layer of rock or soil by agents of erosion such as ice, wind and water.
Eukaryote: an organism with cells with a true nucleus. This includes fungi, plants and animals. Compare with prokaryote.
Exoskeleton: a hard body covering found on the outside of many invertebrates. The term is most often applied to the body covering on arthropods. This is made of a protein compound which is secreted by a layer of cells underneath it. Because it is not stretchy, the creature has to moult regularly to accommodate growth.
Extinct, extinction: the disappearance of a type or groups of organism from the Earth. Mass extinctions are the disappearance of large numbers of species in one event, and are believed to be the result of catastrophic events which cause a break-down in the ecosystem.
Fauna: the animal life of a region or period of geological time.
Fern: a type of plant with large, divided leaves. First found in the Devonian. During the Palaeozoic era tree-ferns, some up to several metres in height, dominated the forest vegetation. Today ferns grow mostly as smaller plants and tree ferns are rare.
Foodchain: the transfer of energy through an ecosystem starting with primary producers (plants) which use the energy of the sun to produce sugars, through a series of organisms that eat and are in turn eaten. An example might be: lettuce > slug > blackbird > hawk. A food web is a more realistic model of energy flow, which demonstrates how food chains combine.
Gastropod: a class of mollusc which typically have a single, often coiled shell, and a head and unsegmented soft body. They are found as fossils in rocks from the early Cambrian, and now live successfully on the land, in the sea, and in fresh water. The name means “stomach foot”. Examples are snails, whelks and slugs.
Genus, genera: a group of species with similar characteristics that are closely related. An example are the lions, tigers, panthers and cats.
Granite: a light-coloured intrusive igneous rock made up of the minerals quartz and feldspar with biotite and / or muscovite mica. The crystals are big enough to be visible without magnification. Granite is formed from the melting of the continental crust rocks.
Graptolite: a stick-like group of colonial, marine animals which lived from the middle Cambrian to the Lower Carboniferous. They are very important for defining the stratigraphy of Ordovician and Silurian rocks in the U.K.
Grasses: a group of flowering plants which now dominate the world's vegetation. They first appeared during the Tertiary.
Helix, helical: something spiral in form, such as a gastropod shell.
Horsetail: a plant which first appeared in the Devonian. Horsetails have jointed stems with a ring of long, pointed leaves and branches at each joint. Equisetum is the only type of horsetail alive today. Fossil horsetails grew up to 30 m in height, for example Calamites, which had a very thin stem but grew extremely long and probably crept like bind weed.
Igneous: igneous rocks are new rocks, formed when hot magma rises up from inside the Earth and solidifies. Intrusive igneous rocks do not reach the surface, but form bodies of rock within the crust. Granite is an intrusive igneous rock. Extrusive igneous rocks are formed when the magma breaks through the crust and erupts on to the surface of the Earth. Once magma has reached the surface of the Earth it becomes known as lava - these rocks, such as basalt, can form volcanoes.
Jurassic: the period of geological time running from 213 to 145 million years ago. The name comes from the Jura Mountains on the French-Swiss border, where the most complete section of these rocks is found. See Geological Timescale.
Kingdom: the top unit in taxonomy. Originally there were just two kingdoms; Animals and Plants. Now there are thought to be at least five kingdoms: animals, plants, fungi (mushrooms and lichen), protists (true algae) and monera (bacteria).
Lava: the term for magma which has reached the Earth's surface and lost its dissolved gas content.
Mantle: the internal layer of the Earth between the crust and the core. It is usually solid, but can also be molten or partially molten. The mantle is composed of iron- and magnesium-rich silica minerals.
Marine reptiles: a group of mostly large, carnivorous animals many of which lived in the oceans during the Mesozoic period. Marine crocodiles, turtles and sea-snakes are the only members of this group alive today, but during the Jurassic there were many more types, including ichthyosaurs and pliosaurs.
Medusoid, (plural: medusae): free-swimming, jellyfish-like young members of the Cnidaria (the group of invertebrates which includes corals, sea anemones and jellyfish). Corals and sea anemones produce medusae as larvae, which settle and live as polyps as they mature, often in colonies. Jellyfish remain as medusae througout their life.
Metamorphism: The process by which rocks are changed by heating and / or squashing. As rocks are buried over millions of years, they are exposed to heat and pressure from the overlying rocks and from movements within the Earth's crust. This causes changes in their properties. The heat can make the minerals which make up the rock recrystallise, often in distinct layers or bands. Metamorphism can occur where two of the rigid crustal plates making the Earth's surface collide, or where a magma rising through the Earth's crust bakes the rocks surrounding it.
Mineral: minerals are the building blocks of rocks. They are naturally occurring substances, which often have a crystalline form. They can be single elements (such as gold or diamond) or compounds (such as quartz or pyrite).
Mollusc: a phylum of invertebrates which include modern creatures such as snails, slugs, cockles, and squids. Molluscs have a muscular 'foot' for digging, movement, or swimming. Many have a hard protective shell. Some forms such as most slugs and octopuses have lost their shells as they have developed other ways of protecting themselves. Molluscs are common fossils found in rocks from the Cambrian period onwards, and are especially common in the Mesozoic and Cenozoic. (See Geological Timescale).
Mosses: small, simple land plants which first appeared in the Devonian, although they are rarely found as fossils. Mosses rely on water for reproduction.
Moult: to shed a hard exoskeleton to allow body growth, or to lose thick body fur during warmer times of the year.
Nautilus, nautiloids: a type of cephalopod mollusc with a coiled shell, tentacles and nervous system. Nautiloids first appeared in the Devonian, and live in deep tropical oceans today. Nautiloids are thought to be a descendant of the extinct ammonites and are sometimes referred to as 'living fossils' because they have helped us to understand how ammonites lived.
Opposing muscles: Muscles which work against one another to allow side to side or up and down movement, e.g. the biceps and triceps allow humans to bend or straighten their lower arms by relaxing and contracting muscles in opposition to one another. Fish have opposing blocks of muscles around their spines that allow their bodies to flex from side to side, propelling them through the water.
Ordovician: the period of geological time between 505 and 440 million years ago. The name comes from the Ordovices, a Celtic tribe which lived in Wales in the region where these rock units were first defined. See Geological Timescale.
Partial Melting: incomplete melting of a 'parent rock' to produce a molten rock (like magma) with a different chemical composition. This happens because different minerals melt at different temperatures.
Period: a unit of geological time, for example the Devonian Period.
Petrified, petrification: the process by which organic matter is fossilised by the infiltration of water and the deposition of dissolved minerals. The Petrified Forest in Arizona, USA is a famous example. Here many trees have been completely converted to stone (lithified), but all the features of the wood have been perfectly preserved.
Phylum, phyla (pl.): the second biggest grouping in taxonomy. Phyla are groups of classes with shared characteristics. For example, the Chordate phylum is made up of classes of animals which have spines or notochords. The equivalent to phylum for plants is a division - for example, flowering plants are defined into the antrophyta division of the plant kingdom.
Plate: 1. One of about fifteen rigid sections of the Earth's surface; a section of the Earth's surface bordered by seismic activity (earthquakes and volcanoes). 2. Pieces of calcium carbonate forming the outer shell of some marine creatures, for example echinoids.
Polyp: soft-bodied adult form of a member of the Cnidaria (corals, jelly fish and sea anemones) which has a cylindrical body which is anchored down at one end. At the other end is a mouth, usually surrounded by grasping tentacles used for feeding. The young are often free-swimming medusoids. Jellyfish are free-swimming and remain in the medusoid form throughout their lives.
Recrystallise, recrystallisation: the growth of new types of minerals in a rock due to the effects of heat and / or pressure.
Reptiles: a class of air-breathing vertebrates that include the alligators and crocodiles, lizards, snakes, turtles, and extinct forms such as dinosaurs and pterosaurs. Reptiles are characterized by a bony skeleton and a body usually covered with scales or bony plates.
Rock: "A consolidated or unconsolidated aggregate of minerals or organic materials". (Oxford Dictionary of Earth Sciences). Rocks can be made of a single type of mineral, or more than one mineral. For example, limestone is made (mostly) of calcite and quartzite is made just of quartz. Granite is made up of three main minerals: quartz, feldspar and mica.
Rock Cycle: a collection of processes which shape the surface of the Earth. Rocks are constantly recycled by burial, melting, uplift and erosion. These processes are driven by energy from inside the Earth and from the Sun (which drives processes in the atmosphere and leads to the weathering of rock).
Rostrum: in belemnites, the rear part of the bullet-shaped, cylindrical internal shell, also called the guard. Its purpose was possibly to act as a counter-balance to the weight of the animal's head and tentacles.
Sediment: the material produced by the effects of weathering rocks close to or at the surface of the Earth.
Sedimentary rocks: recycled rocks made of fragments most often of other rocks and minerals, or organic remains, such as shells or shell debris. The organic parts of any sedimentary rock are referred to as fossils.
Shale: a thinly laminated sedimentary rock made of tiny clay-sized sedimentary particles.
Species: the basic unit of taxonomy. A species is defined as a group of individuals that are genetically related and can interbreed to produce fertile young of the same kind.
Strata: distinctive beds or layers of rocks.
Suture lines: the line marking the join between the septa and the inside wall of the shell of a cephalopod mollusc. Suture lines are visible when the shell has been preserved as a fossil mould. The lines can be very simple curves, or folded into complex crenulations. Ammonite evolution has been defined around the increasing complexity of the pattern of suture lines.
Taxonomy: the science of the classification of living things, grouped by similarity: species are grouped into genera, genera into families, families into orders, orders into classes, classes into phyla, and phyla with similar characteristics at the top level of the classification system - the kingdom. Plants are classified in a similar way, although the phylum unit is called a division.
Tertiary: the period of geological time between 65 and 1.6 million years ago.
Tetrapods: 1. any four-legged animals, including mammals, birds, reptiles and amphibians. 2. descriptive name given to the first four-limbed creatures to emerge on to the land during the Devonian, for example Acanthostega.
Trilobite: member of an extinct group of marine arthropods characterised by three body sections and bearing a superficial resemblance to a woodlouse. First appeared in the Cambrian and became extinct in the Permian.
Tubercles: small knobbles on the surface of echinoid and trilobite bodies. On echinoids they are attachment sites for spines which are adapted for protection and movement. On trilobites they are thought to have had a sensory purpose.
Valves: paired, hard shells, for example in bivalves.
Volcano: layers of lava or ash erupted on to the surface of the Earth forming a cone-shaped structure. Volcanoes are found in areas of the Earth which are geologically unstable, such as near plate boundaries, or 'hot-spots' where magma wells-up from deep inside the Earth.
Weathering: the breakdown of rocks and minerals at and just below the Earth's surface caused by a number of physical and chemical processes. Physical or mechanical weathering is caused by atmospherically controlled processes such as temperature or humidity changes. Many of these processes are due to crystal growth - for example ice growth and expansion in freeze-thaw action, or the growth of salt crystals in salt weathering. Physical weathering does not involve chemical changes to rock. Chemical weathering is caused by decomposition of rocks making them more sensitive to erosion. This includes the effects of acid rain on limestones, or the decomposition of granite to clay. Physical and chemical weathering processes work together to disintegrate rocks.