Extinction by Paul B. Wignall

Author:Paul B. Wignall
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2019-05-02T00:00:00+00:00

The first extinctions

For the first few billion years of Earth history, during the Precambrian Eon, evolution was on the microbial, single-celled level. This changed around 540 million years ago when animals started to diversify quickly in a time known as the ‘Cambrian explosion’. However, the thirty million years before this saw the appearance of some unique, enigmatic fossils known as the Ediacaran fauna. Their abrupt disappearance at the base of the Cambrian raises the possibility that Earth suffered its first mass extinction at this time.

Ediacarans were broad, flat organisms that came in a range of shapes including discs, elongate ovals, and frond- or feather-like forms built of repeating segments. They are generally preserved as sandstone casts and were likely entirely made of soft tissue. With few exceptions they do not appear to have been able to move and instead either lay flat on the seabed or raised themselves above it while being attached to a stalk and holdfast. The Ediacarans have given palaeontologists plenty to ponder. Some regard them as a unique grade of organism unrelated to anything living, while others assign them to extant groups such as worms and soft corals. If the former view is correct then their disappearance at the end of the Precambrian represents a major extinction. Environmentally, there was certainly a lot happening at this time, especially to levels of ocean ventilation, to suggest a possible cause for such an event. The extinction could also have been driven by the development of actively burrowing organisms that disturbed the sediment on which the Ediacarans sat. However, without any clear idea of what the Ediacarans were, their demise is currently unclear.

With Ediacarans out of the way at the start of the Cambrian, the fossil record rapidly became dominated by the trilobites, an arthropod group, while the first reefs appeared in warm, shallow waters. These were made of sponges, belonging to a group called the archaeocyathids. Sites of exceptionally good fossilization quality such as the Burgess Shale in British Columbia, Canada, and in Kunming, China (the Chengjiang biota), also contain a lot of additional soft-bodied animals that show that Cambrian oceans were teeming with diverse life. However, the trilobites have the best fossil record, thanks to their readily fossilized calcite skeleton, and much of what we know about early extinction events is based on the fortunes of this group. Their early history certainly seems to be a roller-coaster ride of extinctions and recoveries. This resulted in some of the highest extinction rates of the fossil record, and several Cambrian intervals are regarded as mass extinctions. The most severe was around the end of the Early Cambrian, when the archaeocyathid reefs disappeared and, a short time later, two of the dominant trilobite groups—the redlichiids and olenellids—also went. There are further trilobite extinction events in the later Cambrian and they mark the boundaries of biomeres, which were intervals of time characterized by distinct trilobite populations.

The causes of all the Cambrian crises is poorly known, mainly because they have received little study.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.