AS Fossils and Time

Question 1

Fossil

Answer 1

used to describe any trace of past life. Fossils may be parts of organisms, such as teeth or shells or whole organisms such as body fossils. Fossils may also be traces of organisms, such as footprints and burrows.

Question 2

Organism

Answer 2

an individual life form, such as an animal, plant, fungus or bacteria. Organisms may be a single cell or multicellular (many cells). Organisms may be preserved as fossils.

Question 3

Body fossil

Answer 3

the hard parts of an organism, such as the skeleton or shell.

Question 4

Replacement

Answer 4

atom by atom substitution of one mineral for another.

Question 5

Dissolution

Answer 5

the process whereby minerals that make up the fossils are dissolved away and removed in solution by groundwater.

Question 6

Taphonomy

Answer 6

the study of the process of fossilisation from the death of the organism to discovery of the fossil. It is an overlap area between biology and sedimentary geology.

Question 7

Mould

Answer 7

the impression of the outside or inside of a fossil.

Question 8

Cast

Answer 8

an in-filled fossil void, usually with another mineral.

Question 9

Extant

Answer 9

organisms that are still alive today.

Question 10

Extinct

Answer 10

organisms that are no longer alive today.

Question 11

Death assemblage

Answer 11

a collection of organisms found in a different place and position than they occupied in life, such as a collection of disarticulated shells.

Question 12

Disarticulated

Answer 12

organisms found as fragments, such as separate shells or parts of trilobites.

Question 13

Life assemblage

Answer 13

a collection of organisms found within sediments in the same position as they would have been when they were alive, such as a bivalve in a burrow.

Question 14

Geopetal structures

Answer 14

allow us to see the way up of a rock, for example a coral or bivalve in life position.

Question 15

Derived fossil

Answer 15

weathered out of one rock and re-deposited in another. Different fossils may give conflicting dates.

Question 16

Ornament

Answer 16

expressed on the surface of the fossil, such as ribs, tubercles, spines and growth lines.

Question 17

Robustness

Answer 17

the ability of the fossil to resist abrasion. Robust forms are more likely to be preserved whole or with slight damage only.

Question 18

Articulated

Answer 18

organisms found whole or connected, as in life.

Question 19

Littoral zone

Answer 19

the high-energy area between high and low tide.

Question 20

Anoxic

Answer 20

or anaerobic conditions lack oxygen and are unsuitable for life.

Question 21

Trace fossils

Answer 21

formed by benthonic infaunal and benthonic epifaunal organisms. These can be aquatic or terrestrial, although the preservation potential for terrestrial traces is poor.

Question 22

Tracks

Answer 22

footprints of an organism made when it moved along the sediment.

Question 23

Trails

Answer 23

impressions of animals which were travelling. This could have formed due to part or all of the animal dragging along the surface of the substrate, for example a trilobite tail.

Question 24

Resting traces

Answer 24

a type of trail as the whole body of the animal had stopped moving (rests).

Question 25

Terrestrial

Answer 25

refers to anything formed on land.

Question 26

Bioturbation

Answer 26

refers to burrowing or working the sediment in a way that disrupts the bedding. This is caused by the activity of living organisms.

Question 27

Substrate

Answer 27

the name given to the sediment or rock on the sea floor.

Question 28

Catastrophism

Answer 28

the theory that changes in the Earth’s crust during geological history have resulted chiefly from sudden violent and unusual short-lived events.

Question 29

Gradualism

Answer 29

assumes that change comes about gradually or that variation is gradual in nature. It is a model applied especially in evolution (Phyletic gradualism) where one species is transformed into another by slow changes.

Question 30

Uniformitarianism

Answer 30

maintains that slow, incremental changes such as erosion, created all the Earth’s geological features. Uniformitarianism holds that the present is the key to the past: that the geological processes observable now were acting in the same way in the past.

Question 31

Absolute dating

Answer 31

gives specific dates for rock units or events given in millions of years before (Ma).

Question 32

Relative dating

Answer 32

putting units or events into sequential order, by saying one event is older or younger than another.

Question 33

Half-life

Answer 33

the time taken for half the unstable parent atoms to decay and form stable daughter products.

Question 34

Closure temperature

Answer 34

the temperature at which a system has cooled, so there is no diffusion of isotopes in or out of the system.

Question 35

Closed system

Answer 35

when a mineral neither gains nor loses atoms. The higher the temperature, the more likely exchanges of atoms or ions will be.

Question 36

Phanerozoic eon

Answer 36

the eon of ‘visible life’. The explosion of hard-shelled and thus species that could be fossilised due to the increase in oxygen levels.

Question 37

Period

Answer 37

a unit of time, a division of an era, e.g. the Carboniferous.

Question 38

Xenoliths

Answer 38

pieces of rock within an igneous rock which have not been derived from the original magma but have been introduced from elsewhere. These are usually from the surrounding country rock.

Question 39

Lateral variation

Answer 39

means there are changes in thickness or lithology in beds laid down at the same time.

Question 40

Marker horizon

Answer 40

shows a bed or bedding plane with a change of lithology easily distinguished which covers a wide geographical extent.

Question 41

Varve deposits

Answer 41

alternating light and dark layers of sediment in glacial lakes, with each pair representing one year. The summer deposits will vary in thickness according to the amount of meltwater produced and can be correlated.

Question 42

Diachronous beds

Answer 42

have the same lithology but vary in age along their extent.

Question 43

Zone fossils

Answer 43

(index fossils) those with the ideal properties to delimit a biozone which is often named after the fossil.

Question 44

Stratigraphic range

Answer 44

the time between the first and last appearance of the fossil.

Question 45

Assemblage

Answer 45

the combination of different fossils identified in a rock unit.

Question 46

Biozones

Answer 46

intervals of strata that are defined by their characteristic fossils.