An international team of researchers has resolved the evolutionary relationships of Necrolestes patagonensis, the so-called “grave robber” because of its burrowing and underground lifestyle. This much-debated fossil mammal from South America has been a paleontological riddle for 121 years but a recent fossil discovery and comparative anatomical analysis helped researchers to correctly place the strange 16-million-year-old Necrolestes, with its upturned snout and large limbs for digging, in the mammal evolutionary tree.
The new result is an example of the Lazarus effect, in which a group of organisms is found to have survived far longer than originally thought, because it moves forward the endpoint for the fossil’s evolutionary lineage by 45 million years, showing that this family of mammals survived the extinction event that marked the end of the Age of Dinosaurs. A famous example of the Lazarus effect is the ginkgo tree, thought to be extinct until it was rediscovered growing in China in the 17th century.
Since its discovery in Patagonia in 1891, Necrolestes has been an enigma. “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,’” says co-author John Wible, Carnegie Museum of Natural History mammalogist and co-author of the new paper.
To-date, the mysterious fossils have moved from institution to institution and researcher to researcher, and the classification of Necrolestes kept changing with each new move. As recently as a few years ago, Necrolestes still could not be definitively classified in a mammal group. A CAT scan of the ear region in 2008 led to another research team’s hypothesis that Necrolestes was a marsupial. This classification intrigued another co-author on the paper, Guillermo Rougier from the University of Louisville, who was not convinced that the marsupial identification was accurate. So he embarked on his own attempt to make a classification. “This project was a little daunting, because we had to contradict 100 years of interpretation,” admits Rougier.
During the process of preparing the fossil for further study, Rougier uncovered characteristics of the skull anatomy that had previously gone unnoticed. Based on these newly revealed features, the research team came to the groundbreaking realization that Necrolestes belonged to neither the marsupial nor placental lineages to which it had historically been linked. Rather, Necrolestes actually belonged in a completely unexpected branch of the evolutionary tree which was thought to have died out 45 million years earlier than the time of Necrolestes.
Part of the riddle of Necrolestes has always been its seemingly mismatched anatomical features, which never seemed to fit any single classification. Based on its decidedly upturned snout, sturdy body structure, and short, wide leg bones, researchers had always agreed that it must be fossorial—a burrowing, digging mammal.
Burrowing mammals have a wide humerus (upper arm bone) that is specialized for digging and tunneling. The humerus of Necrolestes is wider than any other fossorial mammal’s, indicating that Necrolestes was particularly specialized for digging—perhaps more so than any other known burrowing mammal—but this trait didn’t make classification any easier. The simple triangular teeth of Necrolestes served it well in feeding on subterranean invertebrates. However, until recently, its teeth have proved of little help in classifying Necrolestes, because they are so simplified and show no unambiguous similarities to those of other mammals.
In 2011, a newly discovered extinct mammal named Cronopio was the key that unlocked the mystery of the burrowing enigma. Discovered by co-author Rougier in South America, Cronopio belongs to the Meridiolestida, a little-known group of extinct mammals found in the Late Cretaceous and early Paleocene (100–60 million years ago) of South America.
Not only were Cronopio and Necrolestes found to have remarkable similarities, they are the only known mammals to have single-rooted molars—most mammals have double-rooted molars. This conclusively showed that Necrolestes was neither a marsupial nor a placental mammal, and was in fact the last remaining member of the Meridiolestida lineage, thought to have gone extinct 45 million years earlier.
“If we didn’t know those fossils,” Wible said of Cronopio, “we might have come to the same conclusion that everybody else had—that the relationships of Necrolestes were unknowable.”
The mass extinction that ended the Age of Dinosaurs wiped out thousands of species, including the Meridiolestida mammal group to which Cronopio and Necrolestes belong, and cutting short their evolutionary lineage — or so scientists thought. Before the conclusive identification of Necrolestes, only one member of the Meridiolestida was known to have survived the extinction event, and that species died out soon after, early in the Tertiary Period (65–1.8 million years ago). Necrolestes is therefore the only remaining member of a supposedly extinct group. “It’s the supreme Lazarus effect,” comments Wible. “How in the world did this animal survive so long without anyone knowing about it?”
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,” explains Wible. “It must have been on the edges, in an ecological niche that allowed it to survive.”
The researchers point out that other extinct digging species are known by many specimens, while Necrolestes is only known from a few fossils from a narrow geographic area. This means it was not abundant in its time, which fits with the model of a life form existing in a marginal environment. Rougier comments, “In a way, while not related, it’s somewhat similar to how the platypus lives today. There aren’t many of them, they are found only in Australia, and they live in a specific niche among modern mammals—just as Necrolestes is an isolated lineage only found in South America, with very few individuals living among large numbers of marsupials.”
Published in Proceedings of the National Academy of Sciences