During the Devonian period, around 400 million years ago, vertebrate species were getting bigger and bigger, with descendants usually larger than their parental species. But after the Hangenberg event—a mass extinction that took place 359 million years ago, marking the transition from the Devonian to the Mississippian period—this trend reversed and vertebrates kept getting smaller. This shrinking lasted for at least the next 36 million years.

These size reductions do not seem to correlate with changes in temperature or atmospheric oxygen levels. The researchers that identified the trend do not think that small size itself was selected for. Rather, they suggest that small size piggybacks on other traits that were selected for but are harder to spot in the fossil record, like high reproductive rates and short generation times. Smaller, fast breeding vertebrates—aquatic at this point in time—expanded and diversified. Larger, slower breeding vertebrates didn't.

There have been five mass extinctions of life on Earth in the last half a billion years. In more recent extinction events, like those that occurred in the Late Pleistocene (126,000-12,000 years ago), large mammals were also preferentially extinguished. Sure, big species recovered. But as the Hangenberg event teaches us, the trends that are disrupted by an extinction event do not necessarily pick up where they left off.

Science, 2015. DOI: 10.1126/science.aac7373 (About DOIs).

Science, 2015. DOI: 10.1126/science.aad6283 (About DOIs).