Two chapters, the same questions asked of both
Put two Deep Time chapters next to each other — their evidence base, the dating methods behind them, the live debates, and the questions still open.
Dinosaurs
The Mesozoic record and the impact evidence for the end-Cretaceous extinction.
Open full chapter →Ice Ages
Glacial cycles, sea-level change, and the landscapes early humans actually walked.
Open full chapter →Body fossils, trace fossils, bone histology, and the global iridium-bearing boundary layer that marks the end of the era.
Ice cores, deep-sea sediment cores, glacial landforms, and orbital mechanics calculated independently of the geological record.
| Method | What it constrains |
|---|---|
| Biostratigraphy and radiometric dating of volcanic ashAsh beds bracketing fossil-bearing layers | Dates the sediment around a fossil rather than the animal itself, which is why fossil ages are ranges. |
| Bone histologyThin sections of fossil bone | Reveals growth rate and maturity. It supports inferences about metabolism without measuring it directly. |
| Impact geochemistryIridium anomaly, shocked quartz and spherules at the boundary layer | Demonstrates a global impact event coincident with the extinction. Coincidence is established; sole causation is argued. |
| Method | What it constrains |
|---|---|
| Oxygen isotope stratigraphyForaminifera shells in deep-sea sediment cores | Produces a continuous global ice-volume curve. Regional temperature must be inferred separately. |
| Ice core analysisAnnual layers, trapped air bubbles and dust in polar ice | Directly samples past atmosphere back several hundred thousand years. It does not reach earlier glaciations. |
| Orbital calculationComputed eccentricity, obliquity and precession cycles | Predicts the pacing independently of any sediment. Amplitude of response still requires feedbacks. |
How much did Deccan Traps volcanism contribute to the extinction?
Both events are real and near-simultaneous; the weighting between them is genuinely unsettled.
Were non-avian dinosaurs already in decline before the impact?
Answers depend heavily on how sampling bias in the late Cretaceous record is corrected.
Why did cycles shift from roughly 41,000 to roughly 100,000 years?
The mid-Pleistocene transition is not fully explained by orbital forcing alone.
How abrupt were the warming events within glacial periods?
Ice core records show rapid shifts whose triggers and ocean-circulation role remain argued.
- What determined which lineages survived the boundary?
- How widespread was feathering across the major groups?
- How fast was ecosystem recovery in the first million years after impact?
- What sets the amplitude of each cycle?
- How much of the coastline record of early humans is now on drowned shelves?
- How stable are the feedbacks that ended each glacial phase?