'Grave robber' survived end of Dinosaur era by burrowing underground
An international team of researchers has resolved the evolutionary relationships of Necrolestes patagonensis, whose name translates into "grave robber," referring to its burrowing and underground lifestyle.
This much-debated fossil mammal from South America has been a paleontological riddle for more than 100 years. Scientific perseverance, 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.
This finding unexpectedly 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.
This is an example of the Lazarus effect, in which a group of organisms is found to have survived far longer than originally thought.
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,'" said co-author John Wible, Carnegie Museum of Natural History mammalogist and member of the discovery team that also includes researchers from Australia and Argentina.
Based on 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.
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 Guillermo Rougier from the University of Louisville, Kentucky, in South America, Cronopio belongs to the Meridiolestida, a little-known group of extinct mammals found in the Late Cretaceous and early Paleocene (100 and #65533;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.
The mass extinction that ended the Age of Dinosaurs wiped out thousands of species. Included in the devastation were the Meridiolestida, the mammal group to which Cronopio and Necrolestes belong, 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 and #65533;1.8 million years ago). Necrolestes is therefore the only remaining member of a supposedly extinct group.
The researchers believe that Necrolestes's supreme burrowing adaptations are exactly what enabled it to survive for 45 million years longer than its relatives.
The scientific paper resolving the mystery of Necrolestes has been published in Proceedings of the National Academy of Sciences USA.