Lecture 20 – Fossils and Taphonomy

Q: What is a fossil?

Any evidence of life preserved in the rock record, including leaves, carbon residues, coprolites, and bones.

Q: What are body fossils?

Preserved body parts of organisms, such as bones, teeth, and skin.

Q: What are trace fossils?

Evidence of an organism’s behaviour or activities preserved in rock, such as footprints, trails, and burrows.

Q: What steps are needed for an organism to enter the fossil record?

The organism dies, its remains are buried by sediment, lithification occurs, then uplift and erosion bring the fossil to the surface where it can be discovered.

Q: What is “original remains” preservation?

Fossilization with little or no chemical alteration of the remains. It is exceptionally rare, restricted to the recent rock record, and can preserve soft tissues, organs, and DNA (e.g., baby mammoth in permafrost).

Q: What is encasement?

Fossilization by covering an organism in a substance (tree sap/amber, mud, or tar) that chemically isolates it, minimizing alteration. DNA cannot be extracted from amber because it is a rock — only air is trapped inside.

Q: What makes the La Brea Tar Pits notable for taphonomy?

Animals became trapped in tar; soft tissues broke down but bones were “pickled” and preserved. Predators are overrepresented because they were attracted to trapped prey, skewing the normal predator-to-prey ratio of ~1:10.

Q: What is replacement?

Growth of a secondary mineral (commonly calcite, silica, or pyrite) at the expense of original biological material. It occurs via pore-water movement in sediments and can preserve amazing anatomical detail even though none of the original material remains.

Q: What is permineralization?

Water flows through the pore spaces of remains and deposits minerals (usually silica or calcite) in those spaces, reinforcing the original structure rather than replacing it.

Q: Can permineralization and replacement occur in the same fossil?

Yes, both can affect the same fossil, though usually not simultaneously — if they happened at the same time, original material would dissolve as pore space was being filled.

Q: What is recrystallization?

A change in mineral structure without a change in chemical composition (e.g., aragonite converting to calcite, both CaCO₃). It can cause loss of internal information and anatomical detail.

Q: What is carbonization?

Preservation of an organism (most commonly plants) as a thin carbon film — essentially a natural charcoal drawing. Carbonized plant fossils are not found in the same layers as vertebrate fossils like dinosaurs.

Q: What is mold-and-cast preservation?

The original material dissolves, leaving a void (mold) that is later filled by new material (cast). The external shape of the organism is preserved, but no original material or internal anatomy remains.

Q: What is the Devil’s Corkscrew (Daemonelix)?

Infilled terrestrial beaver burrows preserved as trace fossils. Gnaw marks on the edges match the teeth of the beavers that made them. Their presence indicates the water table was deep and the area was not susceptible to flooding.

Q: What is taphonomy?

The study of all physical, biological, chemical, geological, and anthropological processes between an organism’s death and its discovery as a fossil, so that biases from each process can be accounted for.

Q: What are the preservational control factors?

Rate, quantity, and composition of remains entering the record; environmental conditions before burial; time to burial (nature recycles quickly via scavenging and erosion); post-depositional sedimentary conditions; and the ultimate fate of the lithified sediments (e.g., subduction destroys them).

Q: Why are hard parts like teeth and bones most commonly preserved?

Teeth are the hardest body parts, followed by bones, making them most resistant to decomposition and destruction before burial.

Q: What is anatomical fidelity?

How complete and intact a fossil is relative to the living organism. Most fossils are fragmentary because scavenging disperses remains and decomposition breaks them down almost immediately after death. A 98%-complete dinosaur fossil is absurdly rare.

Q: What is spatial fidelity?

Whether the location where a fossil is found reflects where the organism actually lived. The “bloat and float” phenomenon illustrates this — a dead animal bloats, floats to a different location (e.g., the middle of a lake), sinks, and is preserved far from where it lived.

Q: What is temporal fidelity?

Whether fossils found together actually coexisted in time. For example, ant hills concentrate small pebbles and rodent teeth of similar size from surrounding areas, mixing materials of different ages into one deposit.

Q: What is compositional fidelity?

Whether a fossil assemblage accurately represents the original living community. Certain groups (e.g., insects) are almost always underrepresented, making any reconstruction of a past ecosystem incomplete.

Q: When do mass extinctions occur?

When the extinction rate increases and overwhelms the origination rate, causing a significant decrease in biodiversity. Interpreting mass extinctions requires acknowledging taphonomic and sampling biases.