Extinction or Opportunity

Part of the Darwin exhibition.

Fossil ammonite

At least five times in the past 500 million years, rapid environmental change drove much of life on Earth to extinction. But such mass extinctions offered surviving species an opportunity, as regions that were once inhabited became vacant and environments changed. Surviving populations adapted to newly vacated habitats and available resources, each group following its own evolutionary path and diversifying into new, distinct species.

Ammonites, a type of marine cephalopod, are no exception. At the end of the Devonian period 360 million years ago, most species of ammonites died out. But after this mass extinction, the few surviving species dispersed, adapted to new habitats and diverged. Several more times over the next 300 million years, many other ammonite species became extinct, and each time new ones quickly filled their places. Finally, about 65 million years ago, when nearly half of all living species became extinct, all species of ammonites died out, leaving only their intricate shells as proof of their existence.

Chambered nautilus modelA close living relative of the ammonite, the chambered nautilus (Nautilus pompilius) lives at great depths and is an active swimmer.

Punctuated Equilibria

Evolution is an ongoing process. But fossil records show that species often remain unchanged for millions of years. Evolutionary change then comes in brief bursts of activity. Niles Eldredge of the American Museum of Natural History, together with Stephen Jay Gould, labeled this phenomenon punctuated equilibria.

Stable environments provide few opportunities for new species to emerge. But when environments change, or when species colonize new environments, evolution can occur very rapidly.

A Living Ammonite?

Ammonites were marine mollusks that hunted small fish and crustaceans. Their closest living relative is the chambered nautilus, which has a similar spiral-shaped shell.