125 Fossils

Key Idea: Fossils are the remains of long-dead organisms that have escaped decay and have, after many years, become part of the Earth’s crust. A fossil may be the preserved remains of the organism itself, the impression of it in the sediment (a mould), or marks made by it during its lifetime (trace fossils). For fossilisation to occur, rapid burial of the organism is required (usually in waterborne sediment). This is followed by chemical alteration, whereby minerals are added or removed. Fossilisation requires the normal processes of decay to be permanently arrested. This can occur if the organism's remains are isolated from the air or water and decomposing microbes are prevented from breaking them down. Fossils provide a record of the appearance and extinction of organisms, from species to whole taxonomic groups. Once this record is calibrated against a time scale (by using a broad range of dating techniques), it is possible to build up a picture of the evolutionary changes that have taken place.

Fossilisation best occurs when an organism dies in a place where sediment can be laid down relatively quickly. This is often an aquatic environment, e.g. an estuary, but it can be caused by rapid burial, e.g. by a landslide or volcanic ash. After death, the flesh may rot or be scavenged, but hard materials, usually bones and teeth, are able to remain long enough for burial.

Soft material such as the cartilaginous skeletons of sharks don't fossilise well. Often the only remains are their teeth (above).

3D After burial, the bones are subjected to pressure. Minerals in the surrounding sediments move into the bones and replace the minerals in them.

1. Describe how a fossil forms:

2. Explain why the rapid burial of an organism is important in the formation of fossils:

3. Explain why the fossil record is biased towards marine organisms with hard parts:

Erosion of the sediments exposes the fossils on the surface.

f An index fossil is a fossil that is characteristic of a particular span of geologic time or environment. Index fossils help scientists with relative dating (placing rock layers in a relative order to each other), define boundaries in the geologic time scale, and correlate strata from different regions. f Trilobites make excellent index fossils because they are easily recognisable, abundant in the fossil record, and different families are characteristic of different geographic distributions and different time periods. f Trilobites (meaning three lobes) are a fossil group of extinct marine arthropods.

They first appeared in the fossil record near the beginning of the Cambrian (520 mya) and disappeared in the mass extinction at the end of the Permian (250 mya). f They were a very diverse group and underwent several radiations during the

Cambrian, inhabiting a wide range of marine environments and exhibiting diverse life strategies. The wide diversity in their appearance reflects this. f The trilobite fossil record provides evidence of several evolutionary trends in the different lineages. These included streamlined shape in swimming forms, broadening of the head in filter feeders, improvement in the ability to curl up into a defensive ball, decreased size, and the evolution of spines as defences (below).

Fossilised trilobite

3D

Loss of surface detail could have helped with burrowing. A decrease in size allowed exploitation of new microhabitats. Spines provided defence from attack and stabilisation on loose surfaces. An enlargement of the head region was probably an adaptation to filter feeding. 3D

Elongated and streamlined bodies aided swimming in pelagic (open ocean) forms.

4. (a) Explain the importance of index fossils in determining relative time lines:

(b) Why do trilobites make good index fossils?

5. What fossil evidence do we have that trilobites were a diverse group adapted to many different niches?