About 65 million years ago the Cretaceous era came to a dramatic end when a huge asteroid slammed into the Earth and likely jump started the mass extinction that wiped out the dinosaurs. In the wake of such devastation, plucky mammals in their underground burrows survived and eventually rose to the prominence they enjoy today.
At least, that's the narrative scientists constructed after decades of research. But a new study, published in the Journal of Evolutionary Biology, shows that the passing of the crown from non-avian dinosaurs to mammals wasn't a sure thing. Mammals narrowly escaped their own wholesale extinction by the skin of their teeth.
"The extinction was a lot more severe than we previously thought and the recovery was more rapid," Nick Longrich, paleontologist at the University of Bath and a co-author of the new research, tells Smithsonian.com.
During his years of studying fossils, Longrich noticed that specimen after specimen was yet another long-extinct species that once crawled, skittered or scuttled across the surface of Ancient Earth. In short, there were a lot more dead ends on the tree of life than he expected.
So Longrich and his colleagues set out to quantify how the mammals fared during the end-Cretaceous extinction. They looked at the fossil record from a few million years just before and after the asteroid impact and counted as many species as they could. Most fossils from that time period—so all the fossils they studied—hail from North America because the ancient conditions there were just right to preserve dead creatures, and much of that rock now conveniently sits near the surface.
The study documents over 8,000 specimens from 23 locations across 145 species. From that lot, the researchers only found 7 percent that survived the extinction. "The picture is more dire than we thought," Longrich says. The researchers also note that the victims of the mass extinction were species that occupied small ranges. Common, widespread species were more likely to survive.
Previous estimates have determined that about 75 percent of mammal species were killed off in the end-Cretaceous extinction, leaving nearly a quarter to populate the newly dinosaur-less world. That interpretation, however, falls apart under closer inspection of the fossil record.
"Extinction wipes out rare things and the common things tend to survive," explains Longrich. That means that the fossil record is peppered with specimens of species that survived and only lightly seasoned with a few members of rarer species that may have succumbed to extinction. To account for all these rare, sparely distributed species, scientists must examine a massive number of samples. A more cursory look at the fossil record could result in an overestimate of mammal survival.
Though the situation may seem dire—"the glass is 93 percent empty," says Longrich—the new findings also show that mammals were very resilient. Within 300,000 years after the mass extinction, just a blink of an eye in evolutionary time, mammal diversity around the world exploded.
"The mammals quickly started specializing, getting large and doing interesting things," Longrich says. The rapid recovery after extinction included an explosion in species diversity that rivals the rapidly diversifying of Galapagos finches or cichild fish in African lakes—two common examples of fast splintering species.
The results of the study also suggest that mammals were somehow uniquely posed for success, Longrich explains. This astounding recovery occurred in all the continents of the world despite the fact that water separated these landmasses at the time. In essence, each continent was a separate experiment and all came up with the same results: mammal domination. Lizards, turtles and crocodiles didn't make such a quick recovery.
So why did mammals thrive? "It might have been the warm blood, intelligence or maybe because they had more complex teeth," Longrich says. More study is needed to say precisely what gave them the advantage—it could have been a number of things.
Even so, these most recent results suggest that other mass extinction events may deserve a closer look. Just as the history books are written by the victors, the fossil record is dominated by survivors. But in both fields, recognizing the bias helps provide a clearer picture of the history of life on Earth.