57-million-year-old digging mammal

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The pangolin, or 'scaly anteater', is the only mammal to have large keratin scales covering its skin. It belongs to the order Pholidota, along with a number of extinct species. Credit: Sandip kumar; Wikimedia

Palaeontologists have taken a closer look at the fossilised remains of a rare, 57-million-year-old mammal to discover that this dogged digger was more closely related to the modern-day pangolin, or ‘scaly anteater’, than we thought.
The creature is Ernanodon antelios, an extinct placental species of mammal from Asia that grew to around the size of a badger, with powerful limbs and large, specialised claws for scratch-digging its meals and shelter out of the dry earth. It lived during the Paleocene epoch, a geological era that spanned 65.5 to 56 million years ago, just after the mass extinction of the non-avian dinosaurs at the end of the Cretaceous. As first epoch of the Cenozoic era, often called the ‘age of the mammals’, the Paleocene saw the first appearance of many modern orders of mammals such as horses and rhinos.
Very little is known about E. antelios, because all researchers have had to work with for the past 30 years has been an ambiguous, warped set of fossilised bones discovered in Guangdong Province of China in 1973, while, until recently, a better preserved specimen has been sitting in storage since it was unearthed in 1979.

Original Chinese field record on the discovery of the first skeleton of Ernanodon antelios in 1973. Credit: Suyin Ting

The latter specimen, a near-complete skeleton discovered by researchers from the Paleontological Institute of the USSR Academy of Sciences in a layer of Mongolian rocks, preserves most of the arms, legs and backbone of the animal, including an array of bones that were not preserved in the 1973 specimen. This week, palaeontologists Alexander Agadjanian and Peter Kondrashov from the Borissiak Paleontological Institute of the Russian Academy of Sciences published a paper in the Journal of Vertebrate Paleontology, describing for the first time this specimen in detail, and putting to rest a debate that has raged for over three decades regarding E. antelios’ evolutionary relationships with modern mammals.
Following a series of papers questioning the authenticity of the holotype, including a 2003 study that claimed it represented two completely different taxa and elements that might have originated from different localities, it was concluded that, fortunately, the specimen was just extremely deformed. Which didn’t make the classification of the animal it once belonged to any easier, so for years, there was disagreement on whether E. antelios was more closely related to the order called Xenarthra, which includes anteaters, tree sloths and armadillos, or the Pholidota, an Old World order to which only seven living species of pangolin now belong, or the Palaeanodonta, an entirely extinct order of mammals. The telltale signs for Kondrashov and Agadjanian, were the animal’s legs, teeth and claws. “Every detail of the structure of the humerus of Ernanodon corresponds well to that of a digging mammal,” the researchers reported, adding that its hind limb muscular was well developed to support the body as it dug, and the teeth were minimal, suggesting a diet of small insects. The claws would have been impressive, the authors describing the ungual phalanges, or claws, on the hands as “extremely large” and robust, about 3.5cm long.

Skeletons of Chinese pangolin (Manis pentadactyla) (top) and Ernanodon antelios (bottom) from China. Specimens are not to scale. Credit: Peter Kondrashov

A detailed comparison of the animal’s morphology made it clear that rather than being a close relative to armadillos and anteaters, E. antelios shared more in common with the mysterious Palaeanodonts, with the Pholidota – the pangolins – sitting alongside as a sister group. So this strange Asian Paleocene mammal turns out to be a very early side branch of the pangolin family tree. “Few other fossil mammals presented as many controversies in the scientific world as Ernanodon did,” Kondrashov said, “and we are glad the new skeleton helped us resolve them.”