Evidence from Fossils

Explain means cause and effect, and the second clause asks for a limit. A question that asks what a method cannot do is testing whether you understand the method rather than just its name.

5 marks. State the principle and why it holds, apply it to a sequence, state what that establishes, then state clearly what it does not, then say what supplies the missing part.

The principle of superposition states that in an undisturbed sequence of sedimentary rock, each layer is younger than the layer beneath it and older than the layer above. It holds because sediment is deposited from above, layer upon layer, so a layer cannot be laid down beneath one that already exists.

Because a fossil is formed within the layer in which the organism was buried, the fossil shares the age relationships of its layer. If species A is found only in a lower layer, species B in a middle layer and species C in an upper layer, then A lived before B, which lived before C. The vertical order of the rock therefore gives the chronological order of the organisms, and a change in form from A to C through B is evidence of change in life forms over time.

The conclusion depends on the sequence being undisturbed. Folding, faulting or overturning can invert layers, intrusions can introduce younger rock into older, and erosion can remove layers entirely, leaving an unconformity where time is missing from the record with nothing to mark it. Geologists therefore check for these before reading a sequence as a straight chronology.

Relative dating establishes order only, not age. It can show that A is older than B, but it cannot say by how much, nor give either an age in years. Two layers a centimetre apart may represent a year or a hundred thousand years, because deposition rate varies with conditions and is not recorded in the layer itself. It follows that relative dating alone cannot measure the rate of evolutionary change, which is precisely the quantity the gradualism and punctuated equilibrium models disagree about.

Absolute dating supplies what relative dating cannot, by measuring the decay of radioactive isotopes in the rock or in associated material to give an age in years with a stated uncertainty. The two methods are used together: superposition establishes the order and shows the sequence is coherent, while absolute dates anchor that order to a timescale, so that rates of change can be estimated rather than merely inferred.

Analyse asks you to identify components and their relationships. The question also draws a sharp distinction, between incomplete and biased, and the whole answer depends on respecting it.

5 marks. Define the distinction, list the conditions fossilisation requires, show that each condition favours particular organisms, then draw the consequence for inference.

An incomplete sample contains less than everything but in the same proportions as the original, so conclusions drawn from it still apply to the whole. A biased sample over-represents some parts and under-represents others, so conclusions drawn from it may not apply to the whole even if the sample is large. The fossil record is both, and the bias is the more serious problem.

Fossilisation is rare and requires a particular combination: rapid burial in sediment before the remains are scavenged or decay, conditions that slow decomposition such as low oxygen, and usually the presence of hard tissue such as bone, shell or wood that resists breakdown long enough to be mineralised. The remains must then survive geological time without being destroyed by heat, pressure or erosion, and finally must be exposed and found.

Each condition selects non-randomly. The hard-tissue requirement means organisms with bones or shells are heavily over-represented, while soft-bodied organisms such as worms, jellyfish and most microorganisms are almost absent regardless of how abundant they were. The rapid-burial requirement favours organisms living in or near environments where sediment accumulates, so marine and floodplain species are over-represented and species from upland, forest or arid environments, where remains are exposed and eroded, are under-represented. The survival and exposure requirements add further bias towards younger rocks, since older rock has had longer to be destroyed, and towards regions that are geologically accessible and well studied.

The consequence is that patterns in the record cannot be read directly as patterns in past life. An apparent absence of a group may mean it was absent, or that it was soft-bodied, or that it lived where fossils do not form. An apparent sudden appearance may be a real origin or the point at which a lineage acquired hard parts or entered a depositional environment. The record understates past diversity, and it understates it unevenly.

The bias is a reason for care rather than for dismissing the evidence. It can be partly corrected by comparing like with like, such as comparing marine shelly faunas across time rather than comparing a marine sequence with a terrestrial one, by concentrating on sequences with exceptional preservation where soft tissue is present, and by testing fossil-based conclusions against independent evidence from comparative anatomy and molecular data. A known bias with a known direction can be reasoned about, which is very different from an unknown one.