The beginnings and endings of the subdivisions of geologic time are usually well recorded in the rocks. Many of them are major changes in the life of the time, as indicated by fossils. And many of those changes are mass extinction events.
Extinctions occur probably almost continuously, but there’s clear evidence in the fossil record for relatively short time spans when the rate of extinctions ramped up dramatically, killing many more species than usual. These mass extinctions punctuate the geologic record. You’re probably familiar with the mass extinction at the end of the Cretaceous period, 65 million years ago, when most of the dinosaurs died, and the even greater event at the end of the Permian period when about 96% of all marine species vanished.
Many of the marine animals that we described in the Cambrian explosion of life died. Brachiopods and trilobites especially saw a serious reduction in the number of species, and this is clearly recorded in the fossil record.
What caused it? Until quite recently we haven’t been able to point to smoking guns, explicit causes for mass extinctions. You’re undoubtedly familiar with the idea of an asteroid impact causing the end Cretaceous extinction. Other extinctions are not so clear cut.
There is evidence for increased glaciation at about the start of Ordovician time, and that’s been cited as a possible cause or factor in the mass extinction. Besides colder temperatures, glaciation lowers sea level by locking water up in ice, so there would have been fewer of the popular shallow water niches for trilobites and such to live in. Cooler water is also less able to hold oxygen, so oxygen depletion is also cited as a possible factor in the end-Cambrian extinction.
Bottom line: we have some reasonable well thought-out ideas for causes of the Cambrian mass extinctions. But we really don’t know. UPDATE: New dating evidence ties one of the middle to late Cambrian mass extinctions, the one at 510-511 million years ago, to volcanic eruptions in Australia. Here's the link.
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Today, February 28, 1743, is the birth date of René-Just Haüy, at St-Just in Picardy, France. Haüy was a mineralogist, often called the Father of Crystallography. He studied the regular way minerals break apart, a property called the mineral’s cleavage, and applied mathematical approaches to crystal forms, anticipating the much later understanding of molecular crystal structure. He was imprisoned during the French Revolution, but survived, and under Napoleon became a professor of mineralogy. He died in 1822.
—Richard I. Gibson
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