Geology & Paleontology Lecture Notes Review

Taphonomy and the Fossil Record

• Taphonomy: study of how a fossil is preserved, from death to discovery; includes burial, decay, transport, diagenesis, and how these processes affect available information.

• Taxonomy affects every observation in paleontology; classification choices influence how we interpret fossils.

• Goal: understand the entire preservation pathway to interpret the fossil record accurately; many steps can alter the story.

• All science has error and uncertainty; error is not “wrong” but reflects limitations and noise; recognizing biases improves data use.

• The geologic record is incomplete and biased; adding data (more localities, more specimens) improves but never gives a perfect history.

Bias and Uncertainty in Paleontology

• Uncertainty is inherent; error bars and statistics are essential parts of interpretation.

• Key biases: preservational (what gets fossilized), sampling (where we search), research funding, geographic access, and institutional priorities.

• Bias accumulates at every step from living population to recovered specimen; only a subset becomes a fossil and only a subset is accessible for study.

• Understanding biases helps in data collection and interpretation.

Types of Fossilization

• Bloat-and-float: decomposition produces gas, organism floats; transportation or rapid burial needed for other contexts; can mislead about depositional environment.

• Unaltered preservation: original hard parts (and sometimes soft parts) remain; common in Pleistocene permafrost (e.g., mammoths); some dinosaur tissues (keratin) may be preserved in rare cases.

• Amber (tree resin): inclusions like insects, feathers; often preserves soft parts; molecular preservation is generally limited by time, but amber can capture fine details.

• Permineralization (permineralized): minerals fill pore spaces in bone/wood; fossils become heavier; excellent for details; can capture complete or partial skeletons (e.g., Burgess-type examples).

• Replacement: original material replaced by another mineral; may produce colorful or distinctive fossils.

• Carbonization: compression leaves a carbon film; common for soft tissues and feathers; yields a silhouette/outline rather than original material.

• Impression/mold: original material not present; an impression is preserved in sediment; can give details of skin patterns, feather outlines, etc.

• Unaltered detritus: some soft tissue preserved (fossil feathers, fur, gut contents); preservation may be fragile and contamination-prone.

• Lagerstätten: exceptional preservation with soft tissues and abundant material (e.g., Burgess Shale, Green River); allows articulation and soft-tissue study.

DNA, Soft Tissues, and Preservation Limits

• DNA half-life is short; most DNA survives only ~520 years in ideal conditions, far shorter than geologic timescales.

• For fossils older than a few million years, DNA is not recoverable; proteins/amino acids may persist longer, but not enough to reconstruct genomes.

• Soft tissues can be preserved in some contexts (feathers, fur, gut contents); contamination risk is high in ancient DNA work.

• Amber and permineralization can preserve non-bony material; caution with interpretation of alleged blood cells or vessels—often misinterpreted or contaminated.

• Keratin (e.g., beaks) can be preserved in some dinosaur fossils, but not the entire soft tissue.

Fossilization Details and Examples

• Fighting dinosaurs: exceptional preservation through permineralization can capture interactions (e.g., predator–prey in life position) if burial is rapid.

• In many cases, fields must consider whether the preserved arrangement reflects life position or a post-mmortem rearrangement.

• Impressions of skin or scales can provide distribution and patterning even when original tissue is gone.

Biases in the Fossil Record: Field and Collection Realities

• There is a bias against becoming a fossil at all; most organisms never fossilize.

• Even when fossils exist, discovery and collection depend on access, permits, funding, and logistics.

• Museums often house far more specimens than are on display; many jackets and crates await preparation or study.

• Preparation and curation are processes that filter what becomes part of the scientific record.

• At every step, bias reduces the sample to a subset of the original living population.

Fossilization in Context: Depositional Environments and Rock Types

• Sedimentary environment strongly influences fossil preservation potential.

• Conglomerates: poorly sorted; mix of grain sizes; can trap large pieces but hard to preserve articulation.

• Sandstones: common fossil hosts; can preserve articulated specimens but grain damage may occur for small bones.

• Mudstones: very good for preserving a range of sizes; high potential for soft-tissue preservation; common for diverse fossil assemblages.

• Pit/cave deposits or tar pits can yield dense fossil accumulations in limited areas.

• Lagerstätten: places with exceptional preservation of many taxa, including soft tissues and plant material.

The Big Picture of Time: Stratigraphy and Time Scales

• Stratigraphy: reading rock layers as a geographic and temporal record; helps determine depositional environments and relative ages.

• Principles of stratigraphy:

  • Original horizontality: sediments are deposited horizontally.

  • Superposition: older layers are below younger layers.

  • Cross-cutting relationships: features that cut through strata are younger than the cut strata.

  • Unconformities: erosion or non-deposition creates gaps in the record.

• Biostratigraphy: using fossils within rocks to correlate ages across regions.

• Correlation: overlapping fossils allow linking distinct rock columns into a composite time scale.

• Relative time vs numerical (absolute) time: order vs specific dates; radiometric and magnetostratigraphic methods provide numerical ages.

• Magnetostratigraphy: outer core is liquid iron-nickel; magnetic polarity reversals are preserved in rocks and used to date layers.

• Magnetic North vs geographic north can flip; polarity records help synchronize stratigraphic sequences globally.

Earth’s Interior and Time Framework

• Earth's structure: crust, mantle, core (outer core is liquid; inner core solid).

• Outer core dynamics drive the geomagnetic field and enable magnetostratigraphy.

• Deep time concept: millions to billions of years; humans reason with both relative and numerical time to place events on the geologic clock.

Mass Extinctions and Global Change

• Extinction: permanent disappearance of a species.

• Mass extinction: large numbers of species disappear globally in a geologic interval.

• The Big Five mass extinctions: End-Ordovician, End-Devonian, End-Permian (worst), End-Triassic, End-Cretaceous.

• Causes proposed: sea-level changes, climate shifts, loss of food sources, competition, predation, continental drift, volcanic eruptions, asteroid impacts, diseases.

• Extinctions reshape the fossil record and help delineate geologic time boundaries and turnovers.

From Fossils to the Time Scale: How Ages are Assigned

• Age determination relies on surrounding rocks (stratigraphy) and fossil content (biostratigraphy).

• The geologic time scale (GTS) is built by overlapping fossil zones and radiometric data; periods like Triassic, Jurassic, and Cretaceous are subdivided into early/middle/late stages.

• Names and boundaries of periods/epochs are formalized by stratigraphic commissions; these reflect historical understanding and international consensus.

• The process links local stratigraphy to a global time framework, enabling cross-regional correlations.

Quick Takeaways for Exam Prep

• Taphonomy explains why the fossil record looks the way it does and why data are biased.

• Multiple fossilization modes yield different types of evidence (bones, impressions, soft tissue, amber inclusions).

• DNA does not survive long in geological time; soft tissue and proteins are more likely to be preserved in older fossils.

• Fossil records are filtered by preservation, discovery, collection, and curation; biases must be acknowledged.

• Stratigraphy provides relative ages; magnetostratigraphy and radiometric dating provide numerical ages.

• Mass extinctions mark major turnovers in the fossil record and are linked to global change factors.

• The geologic time scale is built from overlapping fossil evidence and global correlations; periods are subdivided and named with formal definitions.