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.