Chapter 11 - Fossils

fossils

preserved trace left by organisms

fossil formation

rapid burial of dead organisms in suitable conditions slow down decomposition, leading to fossil formation

rapid burial

• fossils of human ancestors often found in places where rapid burial can occur

• EG: caves, volcanically active areas

• limeatone found in caves consist of calcium carbonate, so chemical deposits around dead organisms or cave will collapse covering animal bodies

importance of fossil record

• provide physical evidence of past species

• allow scientist to create evolutionary links

• indicates increase in complexity and diversity of species

active sedimentation sites(help in rapid burial)

• drifting sand

• mud deposited by river

• volcanic ash

effect of soil type on fossilization

• we acidic soil

• alkaline soil

wet acidic soil

• minerals in bone dissolved, no fossilization occurs

• no oxygen in peats(wetland), complete preservation of soft tissues and bones may occur

alkaline soil

• produces best bone fossils because minerals in bone not dissolved

• new minerals(lime or iron oxide) are deposited into pores of the bone, replacing organic material

• thus, bone become petrified(turn into rock) while details of structure still preserved

types of fossilization

• fully preserved organisms

• hard parts remaining

• moulds

• casts

• petrification

• trace fossils

fully preserved organism

entire body preserved, including soft tissues

hard parts remaining

soft body parts decay, leaving only bones, teeth, shells

moulds

organism body leave an impression, the cavity it made may form mould

cast

mould cavities filled with minerals

petrification

when minerals enter tissue of dead organism and replace them, turning organism to stone

trace fossil

details preserved in rocks that are an indirect evidence of life

discovery of fossils

• found by chance at ground surface because they got uncovered by erosion

• usually due to slow, careful excavation/digging on likely sites

• when human ancestor fossils found, artefacts usually found in association

artefacts

• objects deliberately made by humans

• EG: stone tools, carvings

problems with fossil record

1) incomplete

2) small proportion of fossils discovered

3) reconstructions are approximations

incomplete

• conditions for fossilization dont always occur

conditions for fossilization

• quick burial of material,

• presence of hard body parts

• absence of decay organisms

• long period of stability where organism is left undisturbed

small proportion of fossils discovered

• fossils buried too deep or inadvertently destroyed

reconstructions are approximations

• usually only a few fragments of bone found

• scientists make reconstructions based on experience, thus only approximate

how are evolutionary relationships represented

• fossil evidence and current observations provide info about organisms anatomy

• allow scientist to compare structural features

• similar structure = similar common ancestor

evolutionary relationships represented by

• phylogenetic trees

• geological time scale

phylogenetic trees

geological time scale

dating of fossils

• determining age of material

• absolute dating and relative dating

absolute dating

determining ACTUAL age of specimen in years

types of absolute dating

• potassium argon dating

• radiocarbon dating

• tree ring dating

potassium argon dating

• based on decay of radioactive potassium-40 to form calcium-40 and argon-40

• decay at a slow constant rate

• as rock ages, proportion of potassium-40 decrease while argon-40 increase

• half life of potassium-40 = 1.3bil years

limitations of potassium argon dating

• not all rocks suitable for this dating

• can only date rocks older than 100 000 - 200 000 years

radiocarbon dating

• based on decay of radioactive carbon-14 to nitrogen-14

• plants absorb CO2 in photosynthesis, incorporating carbon-14 into their tissues

• animals/human eat plant, carbon-14 become part of their tissue

• after death, organisms intake of carbon-14 stop BUT carbon-14 still in tissue

• organism tissue decay at fixed rate

• measure ratio of carbon-14 to carbon-12

• half life of carbon-14=5730 +- 40 yrs

limitations of radiocarbon dating

• material dated must contain organic compound

• cant date back >600 00 years

• now known that amount of carbon-14 in atmosphere varies

tree ring dating

• study of data from tree ring growth

• concentric rings on surface of a cut tree trunk represent one year growth

• can correlate marker rings with timber taken from ancient human structure to find timber age

limitations of tree ring dating

• only can date wood

• timber(building wood) rarely preserved for more than a few thousand years

relative dating

• determiming if specimen is older/younger than another sample

types of relative dating

• stratigraphy

• fluorine dating

stratigraphy

• study of layers of strata

• correlation of rock strata - match layers of rocks from different areas to examine rock itself or the fossils it contain. rocks containing same fossils=same age

• principle of superposition - layer of sedementary rocks ontop is younger and fossils found in top layer younger

index fossils

• widely distributed fossils

• only on earth for limited time

• correlate rock strata if they contain same index fossils

limitations of stratigraphy

• sequence of rocks may be upside down

• fossils may have been buried by animals or eaely humans

fluorine dating

• when bone left in soil and fluoride ions present in soil water replace some ions in bone

• older fossil=contain more fluoride ions

limitations of fluorine dating

• conc of fluoride in water varies from place/time