The near-complete fossil of a tiny creature unearthed in China in 2002 has bolstered the idea that the anthropoid group of primates — whose modern-day members include monkeys, apes and humans — had appeared by at least 55 million years ago. The fossil primate does not belong to that lineage, however: it is thought to be the earliest-discovered ancestor of small tree-dwelling primates called tarsiers, showing that even at this early time, the tarsier and anthropoid groups had split apart.
The slender-limbed, long-tailed primate, described today in Nature, was about the size of today’s pygmy mouse lemur and would have weighed between 20 and 30 grams, the researchers estimate. The mammal sports an odd blend of features, with its skull, teeth and limb bones having proportions resembling those of tarsiers, but its heel and foot bones more like anthropoids. “This mosaic of features hasn’t been seen before in any living or fossil primate,” says study author Christopher Beard, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
By analyzing almost 1,200 morphological aspects of the fossil and comparing them to those of 156 other extant and extinct mammals, the team put the ancient primate near the base of the tarsier family tree. Dubbed Archicebus achilles, the creature’s genus name roughly translates as 'original long-tailed monkey', whereas the species name is a wry nod to the creature’s anthropoid-like heel bone.
The number of anatomical characteristics scrutinized by Beard and his colleagues is much higher than is typically covered by a single study, largely thanks to high-resolution scans of the well-preserved fossil and the team’s detailed, decade-long analysis. “This is really fantastic, impressive work,” says Zhe-Xi Luo, a vertebrate paleontologist at the University of Chicago in Illinois. “Every detail that can be extracted from these fossils has been extracted.”
The primate's remains were recovered from a layer of shale formed from sediments deposited in a lake in what is now eastern China between 54.8 million and 55.8 million years ago, says paper co-author Xijun Ni, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Fragmentary fossils of primates had already been unearthed from around this era, says Beard, but those usually consisted of just teeth and bits of jawbones; the oldest well-preserved primate skeletons until the new find came from around 48 million years ago.