A scrappy family of mammals with unusual, mismatched features moved underground and, like living in a perpetual bomb shelter, managed to survive the mass extinction event 65 million years ago that wiped out the world's non-avian dinosaurs.
We know this thanks to new research on the fossil mammal Necrolestes patagonensis, whose name translates to "grave robber," referring to its burrowing and underground lifestyle. The animal, described in the latest Proceedings of the National Academy of Sciences, had an upturned snout, a sturdy body structure, and short, wide legs.
It lived 16 million years ago, long after the dinosaur demise. But it was found to be related to another fossil mammal, Cronopio, which belonged to the Meridiolestida, a little-known group of extinct mammals from the Late Cretaceous and early Paleocene (100–60 million years ago) of South America.
Cronopio and Necrolestes share a number of features in common, including the fact that they are the only known mammals to have single-rooted molars. Most mammals have double-rooted molars.
The animals were so odd and puzzling, at least to modern eyes, that they mystified scientists for years.
"Necrolestes is one of those animals in the textbooks that would appear with a picture and a footnote, and the footnote would say 'we don't know what it is,'" co-author John Wible of the Carnegie Museum of Natural History said in a press release.
For a long time it was thought that "grave robber" was a marsupial. Further analysis, however, found that Necrolestes actually belonged in a completely unexpected branch of the evolutionary tree believed to have died out 45 million years earlier than the time of Necrolestes.
This is an example of the Lazarus effect, in which a group of organisms is found to have survived far longer than originally thought. ("Lazarus" comes from the Bible story about how Jesus raised a man from the dead.)
"It's the supreme Lazarus effect," said Wible. "How in the world did this animal survive so long without anyone knowing about it?"
A good example of the Lazarus effect is the ginkgo tree, thought to be extinct until it was rediscovered growing in China in the 17th century.
The researchers believe that Necrolestes's supreme burrowing adaptations are exactly what enabled it to survive for 45 million years longer than its relatives.
"There's no other mammal in the Tertiary of South America that even approaches its ability to dig, tunnel, and live in the ground," explained Wible. "It must have been on the edges, in an ecological niche that allowed it to survive."
(Image: Reconstruction by Jorge Gonzalez, Copyright Guillermo W. Rugier)