Palaeontology

Fossils, Index Taxa & Mass Extinctions

Index fossils, major fossil groups and the five great extinctions — the biostratigraphic clock of Earth history.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Fossils preserve past life and underpin biostratigraphy. An ideal index (guide) fossil is geographically widespread but stratigraphically short-lived, giving fine time resolution. Key groups: trilobites (Paleozoic arthropods), ammonoids (Mesozoic cephalopod molluscs) and foraminifera (abundant calcareous microfossils used in petroleum biostratigraphy). Earth's history is punctuated by mass extinctions — the end-Permian (~252 Ma, largest) and the end-Cretaceous K–Pg (~66 Ma, iridium anomaly, end of non-avian dinosaurs).

The Core Formula Matrix

Stratigraphic range: FAD (first appearance) − LAD (last appearance).

Extinction %:

(1 - survivors / total) \times 100

.

Decline (half-life form):

N = N_{0} (1/2)^{t / T}

.

Index-fossil rule: wide area + short duration ⇒ best correlation.

The ‘IIT Traps’

⚠
Index ≠ long-lived. Long-ranging fossils give poor time resolution.
⚠
Trilobites = Paleozoic. They vanish at the end-Permian, not at K–Pg.
⚠
K–Pg killed dinosaurs, not trilobites. Match the extinction to the right taxa.

📚 Standard references

Invertebrate Palaeontology and Evolution — E.N.K. Clarkson
Principles of Paleontology — Foote & Miller

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

An ideal index fossil should have

Q2MEDIUM1 Mark · MCQ

Ammonoids, key Mesozoic index fossils, belong to the phylum

Q3MEDIUM2 Marks · NAT

80

genera before a mass extinction,

20

survived. The extinction percentage is _____ %.