MPSC 2021 Prelims Paper-I · Question 23 of 100

Correct — option (4), which reads (c), (b), (e), (a), (d). The question asks for the order from the oldest period to the newest, so each item has to be placed on the geological time scale and then read forward in time. The Cambrian at (c) is by far the oldest of the five: it opens the Palaeozoic era and the Phanerozoic eon at about 539 million years ago and runs to about 487 million years ago, and it is the period of the great diversification of hard-shelled marine animals that gives it its fame. The Carboniferous at (b) comes next, from about 359 to about 299 million years ago, the fifth period of the Palaeozoic and the age of the vast coal-forming swamp forests that give the period its name. The Permian at (e) follows immediately, about 299 to 252 million years ago, and it is the last period of the Palaeozoic; it ends at 251.9 million years ago in the Permian-Triassic extinction, the largest extinction event of the Phanerozoic. The Triassic at (a) is next, from 251.902 to 201.4 million years ago — the first and shortest period of the Mesozoic era, opening in the aftermath of that extinction. The Quaternary at (d) is the youngest of all, running from 2.58 million years ago to the present day as the last period of the Cenozoic. Read forward, that gives Cambrian, Carboniferous, Permian, Triassic, Quaternary, which is (c), (b), (e), (a), (d) — the row printed at option (4). Two things make the question easier than it looks. The first is that only one row begins with the Cambrian; the second is that only one row ends with the Quaternary, and it is the same row. Either observation alone settles the question.

• (1)(b), (a), (c), (d), (e) — This row opens with the Carboniferous and puts the Cambrian third, which reverses the two ends of the Palaeozoic. The Cambrian begins the Palaeozoic at about 539 million years ago and the Carboniferous is its fifth period, starting about 359 million years ago, so 180 million years separate them and nothing can put the Carboniferous first. The row then closes with the Permian, placing it after the Quaternary — that is, making the last period of the Palaeozoic younger than the period we are living in. Since the question asks for oldest to newest, the last item in a correct row must be the youngest of the five, and only the Quaternary, which began 2.58 million years ago, can hold that place. Checking the first and last cells of an ordering row before reading the middle disposes of this option immediately.
• (2)(b), (c), (a), (e), (d) — This row also begins with the Carboniferous, which cannot be right in an oldest-to-newest sequence that contains the Cambrian, the period that opens the whole Palaeozoic era 180 million years earlier. Its interior is nearer the truth than the other wrong rows — it does keep the Cambrian before the Triassic and the Permian before the Quaternary — and that is what makes it dangerous, because a candidate who checks a pair or two in the middle will find them plausible and accept the row. The test that never fails on an ordering question is positional rather than relational: identify the extreme member of the sequence, here the Cambrian as the oldest, and strike out every row that does not open with it. That leaves one row standing.
• (3)(d), (a), (b), (e), (c) — This row is the sequence run almost backwards. It opens with the Quaternary, which began 2.58 million years ago and is the youngest period on the whole scale, and it closes with the Cambrian, which is the oldest of the five. In other words it answers the ascending question when the stem asked for the descent from oldest to newest — the same reversal trap that this paper sets again a few questions later, where the stem turns on the words 'descending order'. The ordering word in the stem is the question, and it should be read and underlined before the rows are looked at. Once the direction is fixed, a row that opens with the youngest item can be struck out without reading any of its remaining cells.

The geological time scale divides the earth's past into eons, eras, periods and epochs, with boundaries fixed by the International Commission on Stratigraphy at points in the rock record, most of them marked by a change in the fossils preserved. The Phanerozoic eon, the eon of visible life, begins with the Cambrian at about 539 million years ago and is split into three eras. The Palaeozoic runs from the Cambrian through the Ordovician, Silurian, Devonian, Carboniferous and Permian, ending 251.9 million years ago; it takes in the Cambrian explosion of marine life, the colonisation of the land by plants and then by animals, and the coal swamps of the Carboniferous. The Mesozoic follows with the Triassic, Jurassic and Cretaceous, the era of the dinosaurs, and ends 66 million years ago with the extinction that removed them. The Cenozoic runs from then to now, with the Palaeogene, Neogene and Quaternary periods; the Quaternary, from 2.58 million years ago, holds the Pleistocene ice ages and the Holocene in which agriculture and civilisation appear. Two mass extinctions define the era boundaries here — the Permian-Triassic event at 251.9 million years ago, the largest of the Phanerozoic, and the Cretaceous-Palaeogene event at 66 million years ago.

MPSC asks about the geological time scale because it is the backbone of physical geography: coal comes from the Carboniferous, the Deccan traps from the end of the Cretaceous, the Himalaya from the Cenozoic, and the ice ages and the evolution of humans from the Quaternary. An ordering question of this kind tests whether the sequence is held as a sequence rather than as a bag of names. Notice how the paper builds it. It prints five items rather than the usual four, and it scrambles them so that the printed labels (a) to (e) run Triassic, Carboniferous, Cambrian, Quaternary, Permian — no two adjacent labels are adjacent in time. That is what makes the label-only rows, which show no period names at all, hard to check by eye. The reliable method is to write the five ages against the five labels in the margin before looking at any row, then order them, then find the matching row. It takes under a minute and it converts a memory question into an arithmetic one.

• The Cambrian is the first period of the Palaeozoic era and of the Phanerozoic eon, running from about 539 to about 487 million years ago; its base was revised from 541 to 538.8 million years ago.
• The Carboniferous, about 359 to 299 million years ago, is the fifth Palaeozoic period and the age of the coal-forming swamp forests from which the world's major coal seams derive.
• The Permian, about 299 to 252 million years ago, closes the Palaeozoic; the era ends at 251.9 million years ago in the Permian-Triassic extinction, the largest extinction event of the Phanerozoic.
• The Triassic, 251.902 to 201.4 million years ago, is the first and shortest period of the Mesozoic era and follows immediately on that extinction; it precedes the Jurassic.
• The Quaternary, from 2.58 million years ago to the present, is the youngest period of the Cenozoic and comprises the Pleistocene, with its glacial and interglacial cycles, and the Holocene.

• Reading the ordering word carelessly — oldest to newest and newest to oldest produce different rows, and both are printed among the options
• Assuming the printed labels run in chronological order; here they are deliberately scrambled, so (a) is a Mesozoic period while (c) is the oldest item in the list
• Confusing the Carboniferous, which is Palaeozoic and gave the world its coal, with the Cretaceous, which is Mesozoic and ends with the dinosaurs; the two names look alike and are often swapped
• Checking a pair or two in the middle of an ordering row rather than testing its first and last cells, which is where a wrong row is fastest to catch

The geological time scale reaches MPSC papers as ordering questions of exactly this shape, as single-line items asking which era or period a named event belongs to, and as matching questions pairing a period with a rock system or a life form. The Commission also uses it inside Indian geology questions — the age of the Deccan traps, of the Gondwana coal measures, of the Siwaliks. Prepare it as one sheet with the eras and periods in order, their boundary ages rounded to the nearest million years, one defining event for each, and the two era-ending extinctions marked. Practise reciting the sequence forwards and backwards, since an ordering question can be set in either direction and the direction is the whole question.

No directly related past PYQ was found.

• practice — not a real PYQ

  Arrange the following geological periods from the oldest to the newest: 1. Permian 2. Devonian 3. Jurassic 4. Cretaceous

  • (a)2, 1, 3, 4
  • (b)1, 2, 3, 4
  • (c)2, 1, 4, 3
  • (d)3, 4, 1, 2

  Answer(a) 2, 1, 3, 4 — Devonian (about 419 to 359 million years ago), then Permian (about 299 to 252), then Jurassic and finally Cretaceous, which closes the Mesozoic 66 million years ago. The Devonian and Permian are both Palaeozoic and the Devonian is the earlier of the two; the Jurassic and Cretaceous are Mesozoic and follow the Triassic in that order.

• practice — not a real PYQ

  The Permian-Triassic extinction event, the largest extinction of the Phanerozoic eon, marks the boundary between which two geological eras?

  • (a)the Proterozoic and the Palaeozoic
  • (b)the Palaeozoic and the Mesozoic
  • (c)the Mesozoic and the Cenozoic
  • (d)the Palaeogene and the Neogene

  Answer(b) the Palaeozoic and the Mesozoic — the Permian is the last period of the Palaeozoic and the Triassic the first of the Mesozoic, and the boundary between them is dated to 251.9 million years ago. The Mesozoic-Cenozoic boundary at 66 million years ago is marked by the separate Cretaceous-Palaeogene extinction that ended the dinosaurs, while the Palaeogene and Neogene are periods within the Cenozoic, not eras.