Animals probably began constructing burrows about 650 to 700 million years ago, prior to the beginning of the geological period known as the Cambrian, which dawned about 570 million years ago (Fig. 6.1). The Cambrian was a remarkable period—it was the time most of the phyla that now make up the animal kingdom first appeared in the fossil record. Even more remarkable, the emergence of these phyla occurred over a relatively short time period, in an "explosive" burst of diversification lasting just a few million years.
There is still a lot of debate over what caused the Cambrian explosion, as it has been called, but it is clear from the fossil record that life at that time was changing in a big way. One of the more significant events was the emergence of so-called macropredators. The word sounds rather lurid, but a macropredator is simply a predator that is bigger than its prey. This distinguishes macropredation from micropredation, what happens when prey are eaten by organisms smaller than themselves. In other words, if you're eaten by a crocodile, you are the victim of a macropredator. If you succumb to an infectious disease, you have been "eaten" by a micropredator.
Burrowing seems to have originated contemporaneously with the emergence of macropredation. We know this from the record of trace fossils, the remnants of burrowing activities. Trace fossils include tunnels and burrows, tracks in mud, impressions of organisms resting in muds, and so forth.1 The trace fossil
Millions of years ago present'
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