Fossils, preservation and index fossils: the modes of fossil preservation (unaltered hard parts, recrystallisation, replacement, moulds and casts, carbonisation, trace fossils); the conditions that favour preservation; and the characteristics that make a good index (zone) fossil for biostratigraphic correlation.

What this dot point is asking

Eduqas wants you to describe the modes of fossil preservation (unaltered hard parts, recrystallisation, replacement, moulds and casts, carbonisation and trace fossils), the conditions that favour preservation, and the characteristics of a good index (zone) fossil used to correlate rocks of the same age. Fossils are the link between the rock record and geological time, so this topic feeds directly into correlation, the time scale and palaeoenvironments.

The answer

What a fossil is, and the two main types

A fossil is any preserved evidence of past life in the rock record. There are two broad types:

• Body fossils: the remains of the organism itself (shells, bones, teeth, leaves), preserved in one of several ways.
• Trace fossils (ichnofossils): evidence of an organism's activity rather than its body, such as burrows, tracks, trails, borings and coprolites (fossil faeces). Trace fossils are useful because they stay where the organism lived, so they record the environment directly.

Modes of preservation

Hard parts can be preserved in several ways, in roughly decreasing fidelity:

• Unaltered hard parts: the original shell or bone survives little changed (common in young sediments).
• Recrystallisation: the original mineral changes to a more stable form (aragonite to calcite) while keeping the overall shape; fine detail is lost.
• Replacement: the original material is dissolved and replaced by a different mineral, commonly silica or pyrite, preserving the form in a new substance.
• Permineralisation: mineral-rich water fills the pore spaces (as in petrified wood), hardening the fossil.
• Moulds and casts: the shell dissolves to leave a mould (an impression of its surface); if the cavity is later filled, the infill is a cast.
• Carbonisation: volatile elements are driven off under pressure, leaving a thin carbon film (common for leaves, fish and graptolites).

Soft parts are only rarely preserved, in exceptional anoxic deposits (Lagerstatten).

Conditions that favour preservation

Fossilisation is the exception, not the rule, because most remains are destroyed. Preservation is favoured by:

• Rapid burial, which protects the remains from scavengers, decay, currents and weathering.
• The presence of hard parts (shells, bones, wood), which survive far better than soft tissue.
• Anaerobic, low-energy conditions (quiet, oxygen-poor water), which slow bacterial decay.
• The absence of later destruction by metamorphism, deep weathering or recrystallisation that would erase the fossil.

This bias means the fossil record over-represents shelly marine organisms in quiet-water sediments and under-represents soft-bodied and terrestrial life.

Index (zone) fossils and correlation

An index (zone) fossil is used to identify and correlate rocks of the same age in different places. A good one has:

• a wide geographical distribution (so it appears in many regions),
• a short stratigraphic (vertical) range (so it pins a narrow age),
• abundance and ready preservation (so it is reliably found), and
• a distinctive, easily identified form.

The two essential properties are widespread but short-lived: that combination ties widely separated rocks to the same brief interval, which is what biostratigraphic correlation needs. Classic index fossils include many ammonites (Mesozoic), graptolites (Lower Palaeozoic) and trilobites (Palaeozoic).

Examples in context

Example 1. Ammonites as Mesozoic zone fossils. Ammonites evolved rapidly and spread through the world's oceans, so successive species mark fine time zones in Jurassic and Cretaceous rocks, allowing precise correlation across Britain and beyond.

Example 2. Pyritised and silicified fossils. Replacement by pyrite (in anoxic muds) or silica (in some limestones) preserves shells in a new mineral, sometimes in exquisite detail, long after the original aragonite has gone.

Try this

Q1. State the difference between a body fossil and a trace fossil, with an example of each. [2 marks]

• Cue. A body fossil is the organism's remains (for example a shell); a trace fossil is evidence of its activity (for example a burrow or footprint).

Q2. Name two conditions that favour fossil preservation. [2 marks]

• Cue. Any two of: rapid burial; the presence of hard parts; anaerobic, low-energy (quiet) conditions; the absence of later metamorphism or weathering.

Q3. State the two most important characteristics of a good index fossil. [2 marks]

• Cue. A wide geographical distribution and a short stratigraphic (time) range, so it is widespread but short-lived.