Back during the late Cretaceous period there lived a stocky, mole-like mammal that spent its days burrowing tunnels underground. Called Necrolestes patagonensis, it managed to survive the Age of the Dinosaurs and live for another 45 million years before finally dying out. And as scientists have recently learned, Necrolestes was neither placental or marsupial, instead belonging to a long lost line of mammals.
Because of its burrowing behavior, paleontologists have nicknamed it the "grave robber." It had turned-up nose and short, wide legs. It also had simple, triangular teeth which allowed it to feed on subterranean invertebrates. And to its credit, Necrolestes's underground habitat was very likely the key to not just surviving life among the dinosaurs, but also the catastrophic event that wiped them out completely. Living in tunnels, it would seem, was not without it benefits. And, fascinatingly, Necrolestes came from a line of mammals that no longer exists.
Back in 2011, Guillermo W. Rougier discovered the fossil of another extinct mammal called Cronopio, a Meridiolestidal animal that lived in the Late Cretaceous and early Paleocene (100–60 million years ago) of South America.
Further analysis revealed striking similarities to Necrolestes, including single-rooted molars. Most mammals have double-rooted molars, providing proof to the researchers that Necrolestes was neither a marsupial nor a placental mammal, and was instead the last remaining member of the Meridiolestida lineage — a species that went extinct 45 million years earlier.
And indeed, the paleontologists believe that Necrolestes's excellent burrowing adaptations were what allowed it to survive for 45 million years longer than Cronopio.
"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 researcher John Wible through the official release. "It must have been on the edges, in an ecological niche that allowed it to survive."
The researchers speculate that Necrolestes never lived in abundance, likely subsisting in small populations and in tough, marginal environments.
"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," he said.
The entire study can be found at Proceedings of the National Academy of Sciences USA.
Image: Carnegie Museum of Natural History.