The greatest documented mass extinction in the history of life occurred at the end of the Paleozoic Era about 252 million years ago (mya). What happened then to the existing ecological complexity? At this time, 56% of marine genera and as much as 90% of species were decimated. The fossil record is most complete for skeletonized marine benthic (bottom-dwelling) animals. Geographically widespread, marine higher taxa with drastically different lifestyles were reduced to few surviving species or were extinguished entirely. Paleontologists have known of mass extinctions for almost 150 years, from counts of fossil taxa in successive geologic strata. Gradual shifts among dominant higher taxa and guilds (different ways of life) are also known, at least at a coarse global scale, from these counts and from analogies with living forms. However, tracking how the ecological world became complex has proven to be a daunting task.