NDA (II) 2023 General Ability Test · Question 79 of 146

Correct — A, Precambrian. Of the four names offered it is by far the earliest — the Precambrian runs from the formation of the Earth about 4.6 billion years ago to the beginning of the Cambrian about 539 million years ago, which is roughly 88 per cent of all geological time. The other three are the eras of the Phanerozoic that come after it, in the order Palaeozoic, then Mesozoic, then Cenozoic. Rocks assigned to the Precambrian are therefore the oldest in this dating column, which is what the question is asking for.

• (b)Mesozoic — The Mesozoic runs from about 252 to 66 million years ago — the middle of the three Phanerozoic eras and the age of the dinosaurs. It is younger than both the Precambrian and the Palaeozoic.
• (c)Cenozoic — The Cenozoic runs from about 66 million years ago to the present and is the current era, so it is the youngest of the four, not the oldest.
• (d)Paleozoic — The Palaeozoic runs from about 539 to 252 million years ago. It is the oldest of the three Phanerozoic eras, which makes it the strongest distractor, but it begins where the Precambrian ends and so is far younger.

Geologists order rocks by placing them in a column of named time divisions built from fossil evidence and, since the twentieth century, from radiometric dating. The largest divisions are eons, then eras, then periods, then epochs. Everything before the first abundant shelly fossils of the Cambrian is bundled together as the Precambrian, and everything after it belongs to the Phanerozoic, split into Palaeozoic, Mesozoic and Cenozoic.

Two honest points are worth carrying away. First, in the formal international time chart the Precambrian is not strictly an era at all — it is an informal span covering three eons, the Hadean, the Archean and the Proterozoic. The exam uses the word ‘era’ loosely, and that looseness does not change the answer, because whatever you call it the Precambrian is still the oldest division on offer. Second, the prefix trap is real, because ‘palaeo’ means old, so Palaeozoic reads like the oldest name in the list. It is only the oldest of the three eras of visible life. The scale is what settles it — the Precambrian on its own is about eight times longer than everything that has followed it.

• The Precambrian spans from the formation of the Earth about 4.6 billion years ago to the start of the Cambrian about 538.8 million years ago.
• It covers three eons — Hadean (about 4567 to 4031 million years ago), Archean (4031 to 2500) and Proterozoic (2500 to 538.8) — and about 88 per cent of geological time.
• The Phanerozoic eras in order are Palaeozoic (about 539 to 252 million years ago), Mesozoic (252 to 66) and Cenozoic (66 million years ago to the present).
• We live in the Cenozoic, the third and current era of the Phanerozoic eon.

• Choosing Palaeozoic because the prefix means old.
• Treating the Precambrian as a single era when it is a span covering three eons.

As an oldest-or-youngest item like this one, as a chronological ordering of the four names, or through the rock systems of India.

• practice — not a real PYQ

  Which one of the following gives the divisions of geological time in the correct order from oldest to youngest?

  • (a)Precambrian, Palaeozoic, Mesozoic, Cenozoic
  • (b)Palaeozoic, Precambrian, Mesozoic, Cenozoic
  • (c)Precambrian, Mesozoic, Palaeozoic, Cenozoic
  • (d)Cenozoic, Mesozoic, Palaeozoic, Precambrian

  Answer(a) Precambrian, Palaeozoic, Mesozoic, Cenozoic — the Cambrian boundary at about 539 million years ago separates the first from the rest.

• practice — not a real PYQ

  The Mesozoic era is popularly described as the age of

  • (a)mammals
  • (b)dinosaurs
  • (c)fishes
  • (d)flowering grasses

  Answer(b) dinosaurs — the Mesozoic runs from about 252 to 66 million years ago and ends with their extinction.