Module 5 - Section 3 段落填空 - Part 1 - Exercise 10

原文

The Burgess Shale fossils were created at a time when the future Canadian land mass was situated near the Earth's equator. The creatures were preserved when an entire marine ecosystem was buried in mud that eventually hardened and became exposed hundreds of millions of years later in an outcrop of the Rocky Mountains. American paleontologist Charles Walcott, following reports of fabulous fossil finds by construction workers on a Canadian railway who were digging in the mountains in the late 19th century, is said to have tripped over a block of shale in 1909 that revealed the area's remarkable supply of specimens. It has long been believed that the curious fauna that lived there vanished in a series of extinctions because the fossil record ends abruptly. But that no longer appears to be the case.

The Burgess Shale began to form soon after a period known as the Cambrian explosion, when most major groups of complex animals arose over a surprisingly short period. Before 560 million years ago, most living things were either individual cells or simple colonies of cells. Then, and for reasons that remain a mystery, life massively diversified and became evermore complex as the rate of evolution increased. An unusual feature of the Burgess Shale is that it is one of the earliest fossil beds to contain impressions of soft body parts alongside the remains of bones and shells, which is highly unusual.

One such example is Opabinia,a creature that grew to about 8cm (3 inches), had five eyes, a body that was a series of lobes,a tail in the shape of a fan, and that ate using eye proboscis. The proboscis had a set of grasping claws on the end, with which it grabbed food and stuffed it into its mouth. Nectocaris, meanwhile, could be mistaken for a leech, but with fins and tentacles. Weirdest of all was Hallucigenia, described by paleontologist Simon Conway Morris, when he re-examined Walcott's specimens in 1979. With its multiplicity of spines and tentacles, little about Hallucigenia made sense but scientists hypothesized that the spines were legs that helped it move and the tentacles were for feeding. Like and abstract painting, its orientation is a mystery at first, making it difficult to workout which way up it went, which hole food went into, and which hole food came out of.

Now another misconception has been quashed. Writing in Nature recently, Peter Van Roy of Yale University in the United States and his colleagues suggest that the sudden absence of such crazy soft-bodied fossils does not indicate a mass extinction, but merely an end to the unusual local circumstances that caused the creatures to be preserved. In an area of the Atlas Mountains of Morocco, Van Roy's team of researchers has found another diverse (and sometimes bizarre) assemblage of soft-bodied organisms from a period after the Burgess Shale was formed. One discovery includes something that maybe a stalked barnacle. This suggests that the evolution of such complex life went on uninterrupted. For its part, the Burgess Shale continues to produce an astonishing array of indefinable creatures faster than paleontologists can examine them. The world still has plenty to learn about this wonderful life.