Dinosaurs
233 – 66 million years ago
The Mesozoic record and the impact evidence for the end-Cretaceous extinction.
Every claim in this period carries its evidence with it.
Body fossils, trace fossils, bone histology, and the global iridium-bearing boundary layer that marks the end of the era.
- 01
Dinosaurs dominated terrestrial vertebrate ecosystems for well over a hundred million years across three geological periods.
- 02
Birds are theropod dinosaurs — supported by skeletal anatomy, feather preservation and developmental biology.
- 03
A large asteroid impact at the Cretaceous–Palaeogene boundary is the leading cause of the mass extinction, with the Chicxulub crater as the impact site.
Each row states the method, what is actually measured, and — the part usually skipped — what the result does not settle.
| Method | Material | What it constrains |
|---|---|---|
| Biostratigraphy and radiometric dating of volcanic ash | Ash beds bracketing fossil-bearing layers | Dates the sediment around a fossil rather than the animal itself, which is why fossil ages are ranges. |
| Bone histology | Thin sections of fossil bone | Reveals growth rate and maturity. It supports inferences about metabolism without measuring it directly. |
| Impact geochemistry | Iridium 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. |
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.
“Dinosaurs and early humans overlapped.”
Tens of millions of years separate the last non-avian dinosaurs from the earliest hominins. Nothing in the record narrows that gap.
“All dinosaurs were reptilian and scaly.”
Feathers and filamentous integument are preserved across multiple lineages, particularly among theropods.
- 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?
Chicxulub impact and K–Pg boundary studies
Science / PNAS
Dinosaur palaeobiology monographs
Indiana University Press / academic press