雅思阅读 196: A Window in Stone — How the Burgess Shale Kept Soft Bodies（石中之窗：布尔吉斯页岩如何保存柔软躯体）

改编自 Royal Society / PNAS / Royal Ontario Museum（2015-2025年）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://www.rom.on.ca/magazine/seabed-summit

Reading Passage

A. The fossil record, as a rule, is biased. Bone, shell and tooth mineralise readily; flesh, gut and eye do not. When an animal dies on an ordinary sea floor, scavengers arrive, bacteria begin to rot the soft tissues, and within weeks everything that was not already hard is gone, scattered or trampled before it can be buried. What is left for the geologist of a hundred million years later is a scattered set of skeletons and shells, from which the rest of the animal must be guessed. For almost the whole of geological time, this is the default outcome, and it shapes our view of the past as decisively as any rock does. We tend, without noticing, to reconstruct ancient ecosystems from the creatures that happened to have shells. Reconstructing a long-vanished creature from its shell is rather like reconstructing a city from its ruins — you can infer a great deal about its walls and streets, but the interiors, the soft parts and the ways the organism actually lived are lost. It is this normal silence that makes a handful of exceptional deposits so precious, because in them the rule is broken. When such a deposit is found, it is as though a curtain has been torn aside to reveal a scene that decay had almost entirely erased.

B. The most celebrated of these exceptions lies high in the Canadian Rockies. The Burgess Shale, laid down about 508 million years ago in the middle of the Cambrian, preserves animals that had no shells at all as flat, dark imprints on fine-grained rock. Here a scientist can trace the outlines of guts, the lobes of brains and the stalks of eyes on creatures such as the five-eyed, trunk-snouted Opabinia, alongside the more familiar trilobites that also carry hard parts. Some of the animals are so strange that early researchers could not decide which side of their bodies faced up, and others seemed to defy placement in any living group. The deposits were discovered by the American geologist Charles Walcott early in the last century, and they have been quarried ever since, with new localities still yielding species no one had seen. What survives is not the original flesh — that has long since decayed — but a thin film of carbon that records where the flesh once was, pressed flat between layers of mud. It is a shadow, but a remarkably detailed one.

C. How did such shadows come to be frozen in stone? The setting matters. The animals lived near a steep underwater cliff known as the Cathedral Escarpment, on the quiet floor of a tropical sea, and from time to time slurries of fine silt swept down its slopes, smothering whole communities in a sudden burial. Death was violent but preservational: the mud sealed the bodies away from scavengers that would otherwise have torn them apart, and from oxygen that would otherwise have fed decay. The bottom waters in which they settled were also unusually poor in oxygen, so the bacteria that normally dismantle a corpse worked only feebly. The fineness of the silt meant that even delicate anatomical outlines, gills and limbs included, could be captured before they rotted. In effect, each mudslide was a sudden, anaerobic burial shroud, and a succession of them over thousands of years built up a stacked archive. A single storm event could smother an entire living community at once, so that animals that had never met in life were preserved side by side.

D. Geochemical work has added a deeper explanation for why such preservation clustered in the Cambrian rather than everywhere and always. The seawater of that period contained far less dissolved sulphate than the oceans of today. That scarcity mattered because decay is largely carried out by bacteria that use sulphate to break down tissue; with little sulphate available, those bacteria were partly held in check. At the same time, the water was unusually alkaline, which promoted early carbonate cement that quickly sealed the sediment around the carcasses before disturbance could reach them. Elemental mapping of the fossils shows that clay minerals later replicated the decaying tissues in fine detail, the mineral chemistry shifting to match the chemistry of each organ as it rotted. Exceptional preservation, in other words, was not pure luck but the product of a rare combination — rapid burial, stagnant bottom water, unusual ocean chemistry and a still, fine-grained mud. Remove any one ingredient, and a normal, shell-only fossil record returns; the Burgess Shale is precious precisely because so many rare things fell into place at once, and the chances of such a rare convergence recurring anywhere on Earth today are low.

E. The scientific prize is the chance to look at a moment soon after the "Cambrian explosion", when nearly all modern animal body plans appear in the fossil record within a few million years. Ordinary deposits from that interval would show only shells; the Burgess Shale shows whole ecologies, revealing which animals swam, which burrowed, and how the earliest food webs were wired. Similar exceptional deposits have since been found elsewhere — the Chengjiang fauna in China and the more remote Sirius Passet in Greenland — each confirming that the Burgess was not a one-off oddity but a preservational window onto an entire, vanished world. Together they show that the animals of the Cambrian were far more diverse and experimentally varied than shells alone would ever suggest, with body plans that later vanished entirely from the tree of life. They remind us that the fossil record usually flatters the hard parts, and that the soft, strange beginnings of animal life are visible only where the normal rules of decay were, briefly, suspended.

Questions 1-4

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

List of Headings i. What the deposits actually preserve ii. Why fossils are usually biased iii. The burial mechanism behind the preservation iv. Ocean chemistry as the deeper explanation v. The discovery of the Cathedral Escarpment vi. Why these sites illuminate the Cambrian explosion vii. How to date Cambrian rocks

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. Why is the ordinary fossil record described as "biased"? A. It contains too many plant fossils. B. It preserves mainly hard parts and loses soft tissues. C. It is mostly made of fake specimens. D. It records only recent animals.

6. What do Burgess Shale fossils actually consist of? A. Fully preserved original flesh and organs. B. A thin carbon film recording where soft tissues were. C. Solid copies made of gold. D. Casts in volcanic lava.

7. Why did the mudslides help preservation? A. They exposed bodies to oxygen and scavengers. B. They buried bodies suddenly, excluding scavengers and oxygen. C. They dissolved all the soft tissues completely. D. They melted the surrounding rock.

8. According to the geochemical explanation, low seawater sulphate helped preservation because A. it encouraged scavengers to leave the area. B. it limited the bacteria that use sulphate to decay tissue. C. it made the water much warmer. D. it caused shells to dissolve.

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. The Burgess Shale dates from roughly 508 million years ago.
10. The bottom waters where the animals settled were unusually rich in oxygen.
11. Cambrian seawater contained far less dissolved sulphate than today's oceans.
12. The Chengjiang fauna is located in Greenland.
13. Walcott was employed by a Canadian museum when he discovered the fossils.

Questions 14-15

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

Burgess Shale animals were smothered by sudden mudslides near the Cathedral (14) __________, and their soft tissues survive today as a thin film of (15) __________ pressed between layers of mud.

答案与解析

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