The origin and earliest evolution of mammals has been a contentious subject among scientists for many years. It’s thought that they started diverging from reptiles in the middle Jurassic, some 170 million years ago, but a few key species with ambiguous features left the matter open for debate. Now, with the discovery of three new and rather adorable extinct species, scientists have suggested that mammals probably originated much earlier than previously thought, at least 208 million years ago. The study has been published in Nature.
As reported in the study, six new fossils were discovered within the last three years in the Liaoning province, northeast China. Liaoning has become famous for its vast collection of diverse fossils that are remarkably well-preserved, including lizards, mammals, dinosaurs and birds. Numerous volcanic eruptions throughout history have created a layer cake of fossil beds that span millions of years, making the site a treasure chest for palaeontologists.
The 160-million-year-old fossils are from three extinct species—Shenshou lui, Xianshou linglong and Xianshou songae—that all belong to a group called Haramiyida. The researchers placed the squirrel-like animals within a new clade called Euharamiyida. The furry critters were small and slender, weighing between 1 and 10 ounces (30-300 grams), and seem to be specialized for life up in the trees with hands and feet suited to gripping branches and climbing rather than running on the ground.
Judging by their strange teeth, which were dotted with numerous cusps, or points, the researchers believe that the animals were probably omnivores that chomped on insects, nuts and fruit.
Haramiyids have been known to scientists for many years, but according to study co-author Jin Meng, researchers have spent decades trying to decide whether they were mammals. Assigning them to the class Mammalia has been tricky because the only other specimens discovered in the past were poorly preserved and mostly consisted of isolated teeth or pieces of jaw bones. However, the new and almost complete haramiyidan specimens have several tell-tale mammal features. For example, they show evidence of a typical mammalian middle ear, the air-filled space behind the eardrum that converts vibrations into signals that are relayed to the brain. Mammals are unique because they have three little bones in the middle of their ears, and it turns out the new fossils are also equipped with this number. Given the available evidence, the researchers concluded that haramiyids were indeed true mammals.
The researchers also found that Euharamiyida is a sister group of Multituberculata, an extinct group of rodent-like animals. Both separated from the lineage that gave rise to modern mammals long ago, meaning they have no living descendants. Based on the age of known haramiyid specimens, the team suggests that mammals first originated in the late Triassic, between 235 and 201 million years ago (Ma) This is much earlier than previous estimates which suggested they started diverging from reptiles in the middle Jurassic, between 176 and 161 Ma. According to Meng, this makes haramiyids one of the oldest groups of mammals, maybe even the oldest.
“What we’re showing here is very convincing that these animals are mammals, and that we need to turn back the clock for mammal divergence,” Meng said in a news release. “But even more importantly, these new fossils present a new suite of characters that might help us tell many more stories about ancient mammals.”