For 121 years, paleontologists had been vexed by the enigma of Necrolestes patagonensis.
Just what was this strange South American mammal, whose fossils were discovered in 1891 in Patagonia and whose Latin name translates into "grave robber," referring to its burrowing and underground lifestyle?
Where did Necrolestes, with its upturned snout and large limbs for digging, fit in the mammal evolutionary tree?
Was it related to modern mammal groups such as placentals (live-bearing mammals such as humans) or marsupials (pouched mammals such as opossums)?
Using scientific perseverance, a recent fossil discovery and comparative anatomical analysis, an international team of researchers -- including Carnegie Museum of Natural History scientist John Wible -- found the answer: Necrolestes was neither a marsupial nor a placental mammal but instead was the last remaining member of an extinct mammal lineage.
"Necrolestes was an enigma. There are pictures of Necrolestes in textbooks of paleontology, but there was always a question mark next to it. Now the question mark is gone," Mr. Wible said Monday.
Because of the discovery, the endpoint for the fossil's evolutionary lineage is moved forward 45 million years than what had been originally believed. That means that the mammal survived the extinction event that marked the end of the Age of Dinosaurs. Researchers believe that Necrolestes' supreme burrowing ability enabled it to survive for 45 million years longer than its relatives.
The scientific paper resolving the mystery of Necrolestes appeared Monday in Proceedings of the National Academy of Sciences USA.
Mr. Wible said the discovery is significant because "we want to try to put together the events that shape the modern world. We're now trying to get a better picture of what happened in South America."
He added that the discovery "probably poses more questions than it answers. All of a sudden there is a 45-million-year gap we didn't know about before."
Solving the mystery was "a total detective story," he said.
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. Its simple, triangular teeth served it well in feeding on subterranean invertebrates, but until recently they provided no help in classifying it.
But that all changed in 2011 with the discovery in South America of an extinct mammal named Cronopio.
Discovered by Mr. Wible's co-author Guillermo Rougier from the University of Louisville, Cronopio belongs to the Meridiolestida, a little-known group of extinct mammals found in the late Cretaceous and early Paleocene periods (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 a single-rooted molar -- most mammals have double-rooted molars. That 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.
Until the discovery of Cronopio, Mr. Wible said, "we didn't have the right search image. Now we know what its relatives look like and can put it into the context of the times and have a better understanding of what it is.
"This shows the value of going back and looking at material people have looked at before. New discoveries lead to new ideas. Cronopio led us to a new idea of what Necrolestes was."
And that, Mr. Wible said in nonscientific terms, "is kind of cool."
Michael A. Fuoco: email@example.com or 412-263-1968.