An international team of paleontologists has discovered a well-preserved skeleton of a new tiny, tree-dwelling primate named Archicebus achilles that lived in what is now central China during Eocene about 55 million years ago.
The find, described in the journal Nature, is the oldest known fossil primate skeleton. “This is the oldest primate skeleton of this quality and completeness ever discovered and one of the most primitive primate fossils ever documented. The origin of primates sets the first milestone for all primate lineages, including that of humanity,” explained co-author Dr Dan Gebo, an anthropologist with the Northern Illinois University.
“Although scientists have found primate teeth, jaws, occasionally skulls or a few limb bones from this time period, none of this evidence is as complete as this new skeleton from China. With completeness comes more information and better evidence for the adaptive and evolutionary themes concerning primate evolution. It takes guessing out of the game.”
Archicebus achilles was recovered from sedimentary rock strata that were deposited in an ancient lake roughly 55 million years ago, a time of global greenhouse conditions, when much of the world was shrouded in tropical rainforests and palm trees grew as far north as Alaska.
Like most other fossils recovered from ancient lake strata, the skeleton of Archicebus was found by splitting apart the thin layers of rock containing the fossil. As a result, the skeleton is now preserved in two complementary pieces, each of which contains elements of the actual skeleton as well as impressions of bones from the other side.
The skeleton of Archicebus is about 7 million years older than the oldest fossil primate skeletons known previously. It belongs to an entirely separate branch of the primate evolutionary tree from those specimens, lying much closer to the lineage leading to modern monkeys, apes, and humans.
“Archicebus marks the first time that we have a reasonably complete picture of a primate close to the divergence between tarsiers and anthropoids,” explained study lead author Dr Xijun Ni from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and the American Museum of Natural History.
Senior author Dr Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh added: “Archicebus differs radically from any other primate, living or fossil, known to science. It looks like an odd hybrid with the feet of a small monkey, the arms, legs and teeth of a very primitive primate, and a primitive skull bearing surprisingly small eyes. It will force us to rewrite how the anthropoid lineage evolved.”
The most unusual aspect of Archicebus is its foot anatomy. There’s an odd combination of foot features. We see typical robust grasping big toes, long toes and nailed digits of primitive arboreal primates, but we also have rather monkey-looking heel bones and monkey-like long metatarsals, often viewed as advanced features that you would not normally find in a primitive early Eocene fossil primate,” Dr Gebo said.
“We have interpreted this new combination of features as evidence that this fossil is quite primitive and its unique anatomical combination is a link between the tarsier and monkey-ape branches of dry-nosed primates.”
“This new view suggests that the advanced foot features of anthropoids (monkeys and apes) are in fact primitive for the entire lineage of dry-nosed primates,” Dr Gebo concluded.
Bibliographic information: Xijun Ni et al. 2013. The oldest known primate skeleton and early haplorhine evolution. Nature 498, 60–64; doi: 10.1038/nature12200