How did it manage such a feat? It lived underground, moved around a lot, and grew up really fast, and didn't get very old. While Lystrosaurus before the extinction event lived for about 13 years, after the event, paleontologists found that they lived only two to three years. They also shrank from the size of hippos to the size of dogs. But they were still spreading. This new paper concludes that that's because they starting reproducing earlier and more often than normal in the face of extinction, managing to boost their numbers as other animals slowly died out, unable to keep up.