Two years ago, a bombastic media event proclaimed that paleontologists had finally found “The Link” to our early primate ancestry. Dubbed Darwinius masillae – and affectionately nicknamed “Ida” – the lemur-like creature was represented by the most exquisite primate fossil ever found. Yet this petrified media darling was not all she seemed. Contrary to all the hype, Darwinius belonged to a lineage of primates on the lemur and loris side of the family tree. (A finding confirmed through the description of another fossil primate a few months later called Afradapis.) Ida was about as distantly related to our own predecessors as it was possible to be while still being a primate.
This was not the first time a lemur-like primate was dethroned as one of our potential ancestors. From the time skeletons of Notharctus were discovered in Wyoming at the beginning of the 20th century until the late 1980s, lemuroid fossil primates were thought to be ancestral to the first anthropoids (the diverse group of primates that includes monkeys, apes, and their closest extinct allies). One key species in this supposed connection was a fossil primate from Texas known as Mahgarita stevensi. Described in 1976 by John Andrew Wilson and Frederick Szalay, this primate was part of an entirely extinct group called adapiforms. But Mahgarita was special. Not only did this roughly 41 million year old species show some resemblance to earlier adapiforms found in Europe, but its short face, fused lower jaws, and other minute features of the skull and teeth seemed to place it close to the ancestry of anthropoid primates. Though not awarded as much fanfare as Darwinius, Mahgarita was once given a similar position near a critical juncture in our evolutionary history.
Mahgarita did not enjoy its privileged primate position for very long. New discoveries of anthropoid forerunners in Asia – such as Eosimias, described in 1994 – hinted that the North American adapiforms had nothing to do with the origin of monkeys, and some of the features thought to link species like Mahgarita with eo-anthropoids turned out to be instances of evolutionary convergence. In other words, evolutionary quirks had spurred adapiform primates to independently acquire traits – such as short faces and fused lower jaws – that were present in later anthropoid primates. With more material for comparison, paleontologists were able to confirm that fossil tarsiers and tarsier-like primates called omomyids were far more relevant to the ancestry of anthropoids (and thus our own history). The adapiforms, after enjoying pride of place for decades, turned out to be an archaic group of primates far more closely related to the lemurs they resembled. The last time we shared a common ancestor with Darwinius, Mahgarita, and their kind was over 50 million years ago.
A newly-described fossil primate detailed by E. Christopher Kirk and Blythe Williams in the Journal of Human Evolution confirms that Mahgarita was only one of our distant, collateral cousins. They have named it Mescalerolemur horneri, and it is yet another adapiform primate found just below the layers that have yielded specimens of Mahgarita in the 44-41 million year old strata of the Devil’s Graveyard Formation in Texas. As it turns out, the two species were closely related, and together they record a curious facet of prehistory from when lemuroid and tarsier-like primates clambered through the lush forests of Eocene North America.
Shortly prior to the time of Mescalerolemur, the forests of western North America were populated by many different species of adapiform primates which, in turn, belonged to a peculiar subgroup called notharctines. They were the descendants of adapiforms that had migrated from Eurasia – possibly belonging to the genus Cantius – over 50 million years ago, but by about 44 million years ago they had almost entirely disappeared. Only species belonging to Notharctus and Hesperolemur remained by this time. What makes Mescalerolemur so strange, then, is that it was not very closely related to the species which evolved in North America. Despite being found in Texas, Mescalerolemur was part of a second wave of Old World adapiforms that showed up after the North American forms disappeared in the prehistoric southwest.