Chapter 1-7: Geologic Time & Osteology

Numerical Time, Time Scales, and Uncertainty

• Relative time vs numerical (absolute) time: you can put events in order (A → B → C) or assign numerical dates; this session uses numerical time and you have to put numbers on it.
• Error and bias: these are regular parts of scientific measurement; they do not mean something is wrong, they indicate uncertainty around a hypothesis.
• Uncertainty in dating: often shown as a value with a ± interval after it (e.g., an age of X ± ΔY years).
• Use rock relationships and general geologic laws to order events: sequences of deposition, cross-cutting relationships, and intrusions constrain what had to happen after what.
• Erosional surfaces: unconformities create missing time; subsequent deposition and more erosion continue to complicate the record.
• Read the logic progressively: when one event passes through another, the earlier thing had to exist before the later thing that cuts through it.
• Biostratigraphy (fossils) helps correlate across locations: preserved fossils (plants, animals) in different spots can indicate similar ages.
• In field correlations beyond a single location, fossils are used to establish relative time across sites. This leads to a composite stratigraphic column used to construct a time scale.

Stratigraphy, Correlation, and the Composite Time Scale

• Relative dating tools:
  • Deposition sequences in layers.
  • Cross-cutting relationships and intrusions to constrain order.
  • Erosional surfaces and missing time (unconformities).
  • Overlap and reorganization when correlating multiple locations.
• Bio-stratigraphy: using fossils (plants, animals, especially preserved fossils) to determine relative order across sites; e.g., blues/greens/red fossil assemblages that match between location A and B.
  • If assemblages are similar, orderings are considered similar; if new units are preserved in one location, adjustments are made to the layer positions.
• Composite stratigraphic column: integrates multiple local strata into one overarching time scale.

Geologic Time Scale: Eras, Periods, and Epochs

• The geologic time scale is built from relative and numerical data to place units in time.
• A small section shows a larger unit called an era: the Mesozoic Era (oldest to youngest order is bottom to top, like a cake).
• Within the Mesozoic, there are three periods: Triassic, Jurassic, Cretaceous (oldest at bottom; youngest at top).
• Epochs lie within periods (finer subdivisions).
• Nomenclature notes:
  • Mesozoic means "middle life."
  • Cretaceous roughly relates to chalk (chalky sediments) indicating warm, marine deposition environments.
• Numerical dating adds precision to the time scale:
  • Radiometric dating uses the decay of radioactive isotopes to assign absolute ages to rocks.
  • Zircon crystals are especially useful because they are chemically resistant and commonly contain uranium, which decays to lead over time.

Radiometric (Numerical) Dating and Key Isotopes

• Radiometric dating concept: decay of unstable isotopes provides a clock to date crystallization or formation of rocks.
• Common concepts and terms:
  • Zircon crystals are the preferred mineral in many dating studies due to their resilience and ability to incorporate uranium.
  • Uranium-lead dating (U-Pb) is widely used for old rocks.
  • Potassium-argon dating (K-Ar) is another common method for dating volcanic rocks.
  • Carbon-14 dating (C-14) is effective for relatively recent materials; its half-life is short enough that it becomes negligible for very old samples.
• Radioactive decay mechanics (conceptual):
  • Each radioactive parent nuclide decays to a stable daughter nuclide over time.
  • The decay follows exponential decay, which can be described by the equations:
    N(t) = N0 \, \left(\dfrac{1}{2}\right)^{\dfrac{t}{t{1/2}}}
    \lambda = \dfrac{\ln 2}{t_{1/2}}
  • Here, $N(t)$ is the amount of parent remaining at time $t$, $N0$ is the initial amount, and $t{1/2}$ is the half-life.
• Half-life notes:
  • For Carbon-14, $t_{1/2} \approx 5.7 \times 10^3$ years, so C-14 dating is effective for relatively young materials (e.g., Archaeology/Anthropology-scale timescales) but not for deep-time fossils.
  • Uranium and its decay products have long half-lives, making them suitable for dating very old rocks and fossils.
• Zircon crystals and dating considerations:
  • Zircon is a common mineral used in U-Pb dating because it can incorporate uranium but usually excludes lead at the time of crystallization, effectively “locking in” a numerical age.
  • Detrital vs. in-situ zircon matters: whether the dated zircon formed in place or was transported in from elsewhere affects interpretation; there are corrections and considerations for detrital grains.
  • Radiometric ages are most robust when there are clear, well-formed zircons with sharp edges and minimal weathering, indicating little post-crystallization disturbance.
• Context and integration with biostratigraphy:
  • The numerical ages from radiometric dating are often used to anchor relative dates from biostratigraphy (fossil assemblages) to build a composite time scale.
  • When dating rocks around an fossil, you can constrain that fossil’s age within a range; not every layer can be dated, but enough to bracket most fossils to key time intervals.
• Important historical note:
  • Marie Curie was a pioneer in radioactivity research; her work with radioactive elements was groundbreaking and earned her Nobel Prizes, but she exposed herself to high radiation risks and developed cancer as a consequence.

Plate Tectonics: Structure, Boundaries, and Motion

• Interior structure:
  • Earth’s crust plus upper mantle form the lithosphere, which sits above a more ductile asthenosphere.
  • Plates are moved by convection currents in the mantle.
• Plate boundary types:
  • Divergent boundaries: plates move apart; form rift zones and mid-ocean ridges (e.g., Mid-Atlantic Ridge). Divergence can create new ocean basins; Iceland sits atop the Mid-Atlantic Ridge now.
  • Convergent boundaries: plates move toward one another; subduction zones (ocean plate under a continental plate) or continental collisions (mountain building).
  • Example of subduction: oceanic plate under North American plate contributing to broad mountain-building and volcanic arcs (as seen along the western coast).
  • Example of collision: India colliding with Asia forming the Himalayas.
  • Transform boundaries: plates slide past one another horizontally; associated with earthquakes (e.g., San Andreas Fault in California).
• Driving mechanism:
  • All plate motion is driven by mantle convection, with hot material rising and cooler material sinking to form convection cells.
• Oceanic crust aging and recycling:
  • Oceanic crust ages as it moves away from spreading centers but becomes recycled at subduction zones.
  • Old oceanic crust is not preserved indefinitely; by the Early Jurassic, much of it has been recycled.
• Evidence for plate tectonics:
  • Fossil distributions across continents (e.g., Africa–South America). Similar fossils on widely separated landmasses suggest historical connections.
  • Fit of continental margins (e.g., Africa fitting into South America) supports historical connection.
  • Paleomagnetism: rocks preserve records of Earth’s past magnetic field orientation, used to reconstruct past plate positions and polarity reversals; the pattern is often visualized as polarity lines in rocks.
  • Oceanic crust ages and symmetry of magnetic patterns around mid-ocean ridges provide a timeline for seafloor spreading.
  • Modern geodetic measurements (e.g., GPS) can track plate motions with millimeter-per-year precision.
• Fossil and plate movement evidence across deep time:
  • Fossil distributions corroborate plate placements; changes in positions of continents inferred through paleobiogeography.
  • The fossil record shows continents previously connected or closer together than today, supporting drift and plate motion history.

Paleomagnetism and Time Markers in Rocks

• Magnetic field record in rocks helps locate past positions and polarity reversals, forming a polarity time scale.
• This paleomagnetic data helps link rock ages to a time framework even when radiometric dating isn’t possible for every layer.

Osteology, Homology, and Comparative Anatomy

• Osteology: the study of bones and the skeleton, crucial for understanding anatomy and evolutionary relationships.
• Why bone-focused studies matter:
  • Most fossils are fragments; osteology provides a framework to infer the whole skeleton and its function by comparing with living animals.
  • Living anatomy is used to interpret fossil bones and confirm homologies.
• Axial vs. Appendicular skeleton:
  • Axial skeleton: bones along the central axis (skull, spine, rib cage).
  • Appendicular skeleton: bones of the limbs and girdles (shoulder girdle, pelvic girdle, limbs).
• Homology vs analogy:
  • Homologous structures: similar bones in different animals with a common evolutionary origin and similar arrangement (e.g., the upper arm bone humerus appears in humans, frogs, lizards, etc.).
  • Analogous structures: different bones that perform similar functions due to convergent evolution (e.g., wings of bats, birds, and pterosaurs). Although all contribute to flight, the underlying bone structure differs (bat digits, bird wing feathers, pterosaur wing finger).
• Functional inference from bones:
  • The arrangement and size of bones, plus muscle attachment marks, reveal how the skeleton moved, supported mass, protected organs, and how it chewed or fed.
• Comparative examples across taxa:
  • A shared “shoulder system” with a common underlying plan can be seen in horse, human, dog, and cat, though the specifics vary.
  • The same bones arranged in a recognizable pattern reveal deep homologies rather than mere resemblance.
• Key terminology and consistency:
  • Anatomical direction terms help communicate precise location and orientation, especially given bilaterally symmetric organisms.
  • Directional pairs include anterior/posterior, cranial/caudal (synonyms for anterior/posterior), dorsal/ventral (top/bottom), medial/lateral, proximal/distal.
• Jaws and teeth:
  • Teeth terms are numerous due to their complexity, especially in mammals where teeth are often the most fossilized remains.
  • While many jaw terms exist, you will not be asked to memorize every term; focus on core concepts and use slides as reference.
• Practical paleontology approach:
  • Compare fossil fragments to a well-understood living reference to place fragments in a skeletal context.
  • Use homology and analogy concepts to interpret evolutionary relationships and functional morphology.

Anatomical Landmarks and Orientation (Important Directional Terms)

• Anterior vs. posterior (cranial/caudal): anterior toward the nose/head; posterior toward the tail.
• Cranial vs. caudal: cranial = anterior; caudal = posterior; can be used interchangeably in fossils.
• Dorsal vs. ventral (top/bottom): dorsal = top; ventral = bottom; apply to skull and body orientation.
• Medial vs. lateral: medial toward the midline; lateral toward the sides; left and right sides exist in bilaterally symmetric organisms.
• Proximal vs. distal: proximal toward the trunk/point of attachment; distal away from the trunk toward the limb tip.
• Special caveats:
  • For animals with different postures (e.g., humans vs. four-legged animals), terms can shift with orientation (e.g., the anterior of a lizard vs. its ventral relative to standing posture).
  • Many terms are especially relevant for limbs and the tail; not all parts use proximal/distal.

Jaws, Teeth, and Morphology

• Teeth are highly diagnostic for diet and ecology; many terms exist to describe cusps, ridges, and wear facets.
• Mammal fossil records are often dominated by isolated teeth; thus, detailed dental terminology is extensively used.
• While you don’t need to memorize every dental term, you should understand that teeth provide crucial clues to diet and phylogeny and that teeth terminology exists to facilitate precise communication.

Final Notes and Preparation Guidance

• You will have slides and videos after Thursday; review those to reinforce all terms and definitions.
• Expect a term-heavy day with many specialized vocabulary items; focus on understanding core concepts and how they connect across geology and osteology.
• Key takeaways: integration of relative and numerical dating, the geologic time scale, plate tectonics and its evidence, and how osteology underpins interpretation of fossilized life.
• If you get tired during the lecture, take short breaks and revisit sections that discuss the relationships among dating methods, plate tectonics, and skeletal anatomy.