Adalatherium hui: Paleontologists Find 66-Million-Year-Old Fossil of Bizarre Mammal
Named Adalatherium hui, the ancient creature lived about 66 million years ago (Cretaceous period) among dinosaurs, massive crocodiles, and snakes.
It belongs to Gondwanatheria, a lesser known group of mammals that are only known from the ancient supercontinent of Gondwana and previously were represented by only a single skull and isolated jaws and teeth.
Gondwanatherian fossils were first found in Argentina in the 1980s, but have since also been found in Africa, India, the Antarctic Peninsula, and Madagascar.
“Gondwanatherians were first thought to be related to modern-day sloths, anteaters, and armadillos but now 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,” said lead author Dr. David Krause, a vertebrate paleontologist in the Department of Earth Sciences at the Denver Museum of Nature and Science and the Department of Anatomical Sciences at Stony Brook University.
The well-preserved skeleton of Adalatherium hui — the most complete for any mammal from the entire Mesozoic of the southern hemisphere — was recovered from the Maevarano Formation, northwestern Madagascar.
Around the size of a domestic cat at about 3.1 kg, the ancient mammal was unusual in that it was very large for its day; most mammals that lived alongside dinosaurs were much smaller, mouse-sized on average.
“Knowing what we know about the skeletal anatomy of all living and extinct mammals, it is difficult to imagine that a mammal like Adalatherium hui could have evolved; it bends and even breaks a lot of rules,” Dr. Krause said.
“In fact, although a life-like reconstruction might lead one to think that Adalatherium hui was a run-of-the-mill badger, its normality is literally only skin deep.”
“Adalatherium hui is the oddest of oddballs,” said co-author Dr. Simone Hoffmann, a paleontologist in the New York Institute of Technology.
“Trying to figure out how it moved is nearly impossible because, for instance, its front end is telling us a different story than its back end.”
Adalatherium hui had primitive features in its snout region that hadn’t been seen for a hundred million years in the lineage leading to modern mammals.
“Its nasal cavity exhibits an amazing mosaic of features, some of which are very standard for a mammal, but some that I’ve never seen in anything before,” added co-author Dr. James Rossie, a paleontologist in the Department of Anthropology at Stony Brook University.
Adalatherium hui had more foramina — holes that served as passageways for nerves and blood vessels supplying a very sensitive snout that was covered with whiskers — on its face than any known mammal.
And there is one very large hole on the top of its snout for which there is just no parallel in any known mammal, living or extinct.
The teeth of Adalatherium hui are vastly different in construction than any known mammal. Its backbone had more vertebrae than any Mesozoic mammal and one of its leg bones was strangely curved.
According to the team, the plate tectonic history of Gondwana provides independent evidence for why this mammal is so bizarre.
“Adalatherium hui was found in rocks dated to near the end of the Cretaceous period, at 66 million years ago. Madagascar, with the Indian subcontinent attached to the east, separated from Africa over a hundred million years before and finally became isolated as an island in the Indian Ocean when the Indian subcontinent detached at approximately 88 million years ago and drifted northward,” Dr. Krause said.
“That left the lineage that ultimately resulted in Adalatherium hui to evolve, isolated from mainland populations, for over 20 million years — ample time to develop its many ludicrous features.”
The discovery of Adalatherium hui is reported in a paper in the journal Nature.
D.W. Krause et al. Skeleton of a Cretaceous mammal from Madagascar reflects long-term insularity. Nature, published online April 29, 2020; doi: 10.1038/s41586-020-2234-8