The scientific paper resolving the mystery of Necrolestes appears this week in Proceedings of the National Academy of Sciences.
Necrolestes translates into “grave robber” and refers to the animal’s burrowing and underground lifestyle. The strange 16-million-year-old fossil indicated that the animal had an upturned snout and large limbs for digging. Since its discovery in Patagonia in 1891, scientists have been trying to place Necrolestes into the correct evolutionary tree.
Scientific perseverance, another fossil discovery made by Rougier in 2011, and comparative anatomical analysis helped the team identify it as a mammal. This finding unexpectedly moves forward the endpoint for Necrolestes’s evolutionary lineage by 45 million years, showing that this family of mammals survived the extinction of the dinosaurs.
A paleontological riddle
“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,’” said Carnegie Museum of Natural History scientist John Wible, a member of the research team and co-author of the paper with Rougier.
Despite being excellently preserved, the mysterious fossils defied classification. They moved from institution to institution and researcher to researcher, the classification 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 the animal was a marsupial.
This classification intrigued Rougier. As a specialist in South American mammals, he was not convinced that the marsupial identification was accurate, and he embarked on his own attempt to solve the puzzle.
“This project was a little daunting, because we had to contradict 100 years of interpretation,” Rougier said.
During the process of preparing the fossil for further study, Rougier uncovered characteristics of the skull anatomy that had previously gone unnoted. 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 historically had been linked. Rather, it actually belonged in a completely unexpected branch of the evolutionary tree that was thought to have died out 45 million years before the time of the fossil under investigation.
Part of the Necrolestes riddle had always been that its seemingly mismatched anatomical features never seemed to fit a 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 it was particularly specialized for digging — perhaps more so than any other known burrowing mammal. 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 its classification because they are so simplified and show no unambiguous similarities to those of other mammals. Enter Cronopio.
The mystery solved
Rougier’s 2011 discovery of an extinct mammal named Cronopio dentiacutus unlocked the mystery of Necrolestes.
Also discovered in South America, Cronopio belongs to a little-known group of extinct mammals found in the Late Cretaceous and early Paleocene eras (100 – 60 million years ago) of South America called Meridiolestida. 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,” said Wible 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. The Meridiolestida, the mammal group to which Cronopio and Necrolestes belong, were among them — or so scientists thought.
Before the conclusive identification of Necrolestes, only one member of the Meridiolestida was known to have survived the extinction event. 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,” Rougier said. “How in the world did this animal survive so long without anyone knowing about it?”
In the Lazarus effect, a species previously thought to be extinct is rediscovered — sometimes living, sometimes found elsewhere in the fossil record. The Lazarus effect is well represented by 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,” Wible said. “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.
“In a way, while not related, it’s somewhat similar to how the platypus lives today,” Rougier said. “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.”
Necrolestes’s survival for 45 million years longer than expected challenges more than a century of scientific thought on the effects of the Late Cretaceous extinction event in South America, and shows how scientific thought is constantly changing based on new evidence. For example, because the paleontological landscape is much better understood in North America and Eurasia, extinction models on those continents were assumed to apply to all continents.
“We can’t do that anymore. This story is more complex, a very distinct picture. We’re just getting there with South America,” Rougier said.
Wible, Rougier and their team — which includes scientists from Australia and Argentina — said they are looking forward to filling in the 45-million-year gap between Necrolestes and its nearest known relatives, applying that knowledge to other related species that crossed the Cretaceous-Tertiary extinction boundary — a seemingly South American phenomenon.