IELTS Reading 106: The Day the Oceans Died

Adapted from Okinawa Institute of Science and Technology / ScienceDaily (Jan 2026). IELTS Reading Section 3 difficulty, approximately 1,050 words. Source: https://www.sciencedaily.com/releases/2026/01/260110211202.htm

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Reading Passage

A. Roughly 445 million years ago, the planet underwent a catastrophe so severe that it erased about 85 percent of all marine species. In a geologically instant span, glaciers crept across the southern supercontinent Gondwana, locking up ocean water and draining the shallow seas in which nearly all life then lived. The event, known as the Late Ordovician Mass Extinction, is one of the "Big Five" die-offs that punctuate the history of life, and it is the oldest of them. The world it destroyed was warm, green and ice-free at the poles: a greenhouse planet where vast, sunlit continental shelves hosted forests of towering sponges, scurrying trilobites, human-sized sea scorpions and nautiloids with shells five metres long. On land, only simple liverwort-like plants and many-legged arthropods had yet ventured ashore. Among this alien cast, the early ancestors of jawed vertebrates were rare and unremarkable — a bit part in a story they would one day come to dominate. It is hard, looking at such a world, to imagine how thoroughly it was about to be remade. The creatures that defined the Ordovician — the trilobites, the brachiopods, the giant nautiloids — were not merely inconvenienced by what followed; whole orders and families vanished from the rock record overnight, leaving ecosystems with no obvious replacement for their roles.

B. The fossil record shows that the disaster arrived in two punishing waves. First, the climate lurched from greenhouse to icehouse. Ice sheets spread over Gondwana, and as water was drawn into them, sea level dropped, stranding and de-watering the shelf habitats on which most species depended. Tropical waters cooled sharply. Then, after several million years, just as ecosystems seemed to be struggling back, the climate reversed again. The ice caps melted, flooding the shelves with warmer water that was laden with sulphur and starved of oxygen. Species that had adapted to the cold were now overwhelmed by poisonous, oxygen-poor seas. Scientists continue to debate the ultimate trigger — volcanic activity, nutrient runoff and orbital shifts have all been proposed — but everyone agrees on the result: a sharp dividing line between the Ordovician world that had existed, and the Silurian world that followed.

C. What has fascinated palaeontologists recently is not the dying itself but who survived it. A study published in 2026, drawing on a database assembled from two centuries of late-Ordovician and early-Silurian fossil collecting, tracked where the survivors went and what they became. During the repeated crises, surviving vertebrates were largely confined to refugia: isolated pockets of safe water, separated by deep-ocean barriers that few species could cross. Within these pockets, one group appears to have held an unexpected advantage — the jawed fishes, the gnathostomes. Their diversity rose steadily in the aftermath, even as the trilobites, conodonts and giant nautiloids of the old world faded. The researchers could show, for the first time in quantitative detail, that the extinction pulses directly triggered a surge in new species millions of years later. Before then, such claims had rested on isolated fossils and educated guesswork; the sheer size of the database allowed the team to track individual genera across the crisis rather than marvel at it from a distance.

D. Geography, the analysis revealed, was destiny. By mapping species distributions before and after the event, the team identified the refugia that fuelled later diversification. One striking example comes from what is now South China, where fossils include the earliest complete remains of jawed fishes closely related to modern sharks. These animals stayed tucked away in stable refuges for millions of years before evolving the ability to cross open oceans and spread. The pattern resembles a familiar evolutionary tale: confined to limited habitats, the pioneers found many vacant roles left behind by the extinct jawless species. With so many niches empty, they diversified rapidly into whatever food sources and lifestyles the emptied ecosystems offered — much as Darwin's finches later diversified on the Galápagos. The study therefore supports the view that jaws did not evolve first to open a brand-new way of life, but rather that early fish filled existing niches and only then diversified. It is a quietly different ordering of cause and effect than the textbooks have long assumed, and one that owes nothing to imagination and everything to two centuries of patient collecting.

E. It would be wrong to imagine the extinction simply wiped the slate clean. Even as jawed fishes remained bottled up in their refuges, jawless vertebrates continued to dominate the open oceans for another 40 million years, diversifying into reef fishes with their own mouth designs. Why the jawed forms eventually outcompeted them, once they finally ventured out, is still not fully understood. Rather than a fresh start, the researchers describe an ecological reset: early vertebrates moved into roles once held by conodonts and sea scorpions, rebuilding the same familiar ecosystem shapes with new species. Such "diversity-reset cycles", in which evolution converges on the same functional designs after environmental shocks, recur throughout the ancient record. The broader lesson reaches the present day: the marine life we know traces back to these ancient survivors in their hidden refuges, not to the glamorous creatures that ruled the Ordovician seas. Catastrophe, it turns out, does not merely destroy — it quietly decides who inherits the future. The same pattern has been detected after later upheavals, when a sharp environmental shock clears the field and the surviving groups reoccupy the emptied roles in much the same shape as before. Recognising that the modern fish, sharks and ultimately humans descend from unassuming refugees in a South China refuge is, in the authors' words, to draw a line between the fossil record, the geography of ancient seas and the long, improbable chain of ancestry that every living vertebrate now represents.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The two waves of an ancient climate whiplash ii. Who survived — and why the jawed fishes won iii. How geography and empty niches drove diversification iv. Why the slate was not wiped clean v. The warm Ordovician world before the disaster vi. Volcanoes: the only proven trigger of extinction vii. Modern fish farming in South China

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. What was the immediate effect of the first wave of extinction? A. Volcanoes covered the land in lava. B. Glaciers spread and sea level fell, draining shallow habitats. C. Oxygen levels in the oceans rose sharply. D. Jawed fishes disappeared completely.

6. What happened during the second wave, as the ice melted? A. Seas became warm, sulphur-rich and low in oxygen. B. Sea scorpions returned in large numbers. C. Coral reefs grew rapidly. D. Temperatures dropped even further.

7. Why were refugia important to jawed fishes? A. They were isolated safe pockets where the fishes survived and later diversified. B. They contained no food, forcing the fishes to evolve. C. They were destroyed by glaciers. D. They were open oceans full of predators.

8. What does the study suggest about the evolution of jaws? A. Jaws evolved first to create an entirely new lifestyle. B. Early fish filled existing niches and only then diversified. C. Jaws were useless in cold water. D. Jawless fishes always had jaws.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Roughly 85 percent of all marine species vanished in the extinction.
10. The ultimate cause of the extinction is agreed by all scientists to be volcanic activity.
11. Jawless vertebrates continued to dominate open oceans for about 40 million years after the event.
12. The fossils found in South China include early jawed fishes related to modern sharks.
13. Human beings already existed on land during the Ordovician period.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

Rather than erasing life entirely, the extinction acted as an ecological (14) __________, after which evolution repeatedly converged on the same functional designs — a pattern the researchers call a diversity-reset (15) __________.

Answers & Explanations

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