A large mammal that survived life among dinosaurs and massive crocodiles on an incredibly diverse island is bizarre enough by itself to stump researchers. Add in that the 66-million-year-old animal’s teeth have never been seen before, its skeletal anatomy breaks all known rules, it has more holes on its face than any known mammal, and its front end tells a different story than its back end and you’ve got a recipe for “weird” that would delight any paleontologist.
More than a decade ago, David Krause, curator at the Denver Museum of Nature & Science, and a team of paleontologists discovered a remarkably rare complete 3-D fossil on Madagascar. They spent years working to determine where it falls in the history of mammalian evolution and what it tells us about geography and changes in global fauna over time.
The opossum-sized mammal that resembles a badger, now named Adalatherium, belongs to an extinct group of mammals called gondwanatherians, who are named after the ancient southern supercontinent of Gondwana where they called home. Prior to the discovery of Adalatherium, gondwanatherians were only known from isolated teeth and jaw fragments, with the exception of a cranium from Madagascar described by Krause and his team in 2014. Gondwanatherians were initially thought to be related to modern-day sloths, anteaters and armadillos but Krause says this discovery changes the previous perspective.
"Now, [they] are known to have been part of a grand evolutionary experiment, doing their own thing, an experiment that failed and was snuffed out in the Eocene, about 45 million years ago," Krause said.
Madagascar is known for unique animal species that developed in isolation over millions of years. The island broke off from India and over a period of 100 million years moved toward Africa, yet never quite arrived. According to the research, Madagascar separated from India 88 million years ago, leaving 22 million years before this Adalatherium was fossilized for the species to evolve distinctly from the larger mainland continents.
“[That] is ample time to develop its many ludicrous features," said Krause.
Bizarre, unique features
Part of the reason it took the team so long to figure out where Adalatherium fits in the evolutionary puzzle is due to its teeth.
“Teeth in mammals reflect their ancestry, diet and environment. In the case of Adalatherium, the morphology is so peculiar that it is hard to use the characters we normally use to establish family relationships,” said Guillermo Rougier, an anatomical professor at the University of Louisville and part of the international Krause research team. “Mammalian systematics and evolution rest heavily on dental morphology, so when you do not have teeth—or they are so strange that you do not know what to do with them—we have a problem.”
According to the researchers’ paper, published in Nature, Adalatherium had primitive features in its snout region (like a septomaxilla bone) that haven’t been seen for 100 million years in the lineage leading to modern mammals. Its backbone had more vertebrae than any Mesozoic mammal, and one of its leg bones was strangely curved. The species also had more holes on its face than any known mammal, holes that served as passageways for nerves and blood vessels supplying a very sensitive snout that was covered with whiskers. There was also a very large hole on the top of its snout that researchers say there is just no parallel for in any known mammal—living or extinct.
"Knowing what we know about the skeletal anatomy of all living and extinct mammals, it is difficult to imagine that a mammal like Adalatherium could have evolved,” Krause said. “It bends and even breaks a lot of rules."
The research team is still uncovering clues but thinks that, although Adalatherium might have been a powerful digging animal, it was also capable of running and potentially even had other forms of locomotion.
Although the discovery of Adalatherium is a breakthrough, especially for the fossil record in the southern hemisphere, there is still a lot to uncover.
“Adalatherium is just one piece, but an important piece, in a very large puzzle on early mammalian evolution in the southern hemisphere,” said Krause. “Unfortunately, most of the pieces are still missing.”
Photo: The fossil of Adalatherium. Credit: Denver Museum of Nature & Science/Andrey Atuchin.