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.
Kirk and Williams’ hypothesis about the affinities and history of Mescalerolemur are based on a literal handful of fossil remains. In addition to several lower jaw fragments, the newly-discovered primate is principally known from a pair of right and left upper jaw portions set with several partial and complete teeth. Hardly a fossil bonanza. Yet, as with other fossil mammals, the anatomical details of the teeth and skulls of fossil primates are highly informative, and the various bumps, ridges, crests, and troughs along the Mescalerolemur teeth were key to figuring out that this adapiform was most closely related to its successor in the area, Mahgarita. Kirk and Williams even propose that Mescalerolemur may have been ancestral to Mahgarita, though larger fossil samples of both species will be needed to test this idea.
Yet the most striking features of Mescalerolemur are not those it shares in common with its Devil’s Graveyard relative, but those in which it differs. Not only was Mescalerolemur almost half the size of Mahgarita, Kirk and Williams point out, but the lower jaw of the primate possessed a prominent pocket where the tip of a relatively large canine tooth would have rested. For a small adapiform, Mescalerolemur had big, pointy canine teeth. Mescalerolemur also had large eyes for its size. The small margin of the eye that was preserved indicated big peepers, particularly the fact that the roots of the molar teeth extended up into the floor of the eye socket – a clue that the eye socket was enlarged to such a point that the upper jaw became shallow to give the eye more space.
What is particularly special about Mescalerolemur, though, is that its lower jaws were unfused. The bones possess a rough patch near where the two mandible halves would have met, but, unlike Mahgarita, they did not actually connect to form one solid lower jaw. Given the close relationship of Mescalerolemur to Mahgarita – possibly even an ancestral one! – the unfused jaws of the earlier species confirms that the fused jaw of Mahgarita was a case of convergent evolution. The fused lower jaws of Mahgarita cannot be taken as an indication that adapiforms were closely related to anthropoids, especially since paleontologists now know that the feature is not an ancestral anthropoid trait. In forms closely related to early anthropoids, like Eosimias, as well as early anthropoids such as Biretia, the lower jaws are unfused. The joining of the two jaw halves was a later development among anthropoids, and, as estimated by Matthew Ravosa in a 1999 review paper, this adaptation to feeding on tough plant foods occurred at least ten times among primates as a whole.
The discovery of Mescalerolemur makes sense of why a strange, Old World fossil primate like Mahgarita turned up in Texas. The primate pair were part of an extension of Eurasian adapiforms that inhabited the forests of Eocene Texas after the native North American forms had disappeared. Exactly why this faunal changing-of-the-guard happened is unknown. More than that, though, the anatomy of Mescalerolemur and Mahgarita affirm the importance of rooting out instances of convergence in primate evolution. The lemur-like adapiforms, in particular, appear to have independently evolved traits seen in later anthropoid primates, and these features have previously led researchers astray. Now paleontologists know better.
(The problem of convergence even mars efforts to parse the relationships of the earliest putative humans – at least three species are in competition for the title of “earliest hominin” – and the possibility that some of these candidates independently evolved traits seen in confirmed humans makes it difficult to separate close kin from more distant cousins. As paleontologists get ever closer to the point of divergence between two lineages, especially, distinguishing which fossil belongs in which group becomes ever more difficult.)
Studying the primate fossil record is akin to trying to understand the plot of a book with only a handful of letters from a smattering of words sampled from a few pages. Yet each new fossil find adds a few more letters, sentences, and paragraphs to the story of primate evolution. Each new species that is discovered adds a new dimension to the prehistoric tale, and some, like Mescalerolemur, warn us to watch out for red herrings.
Top Image: A ring-tailed lemur in the Bronx Zoo’s Madagascar exhibit. Lemurs are among the closest living relatives of Mescalerolemur and Mahgarita. Photo by author.
Kirk, E., & Williams, B. (2011). New adapiform primate of Old World affinities from the Devil’s Graveyard Formation of Texas Journal of Human Evolution DOI: 10.1016/j.jhevol.2011.02.014
Rasmussen, D. (1990). The phylogenetic position ofMahgarita stevensi: protoanthropoid or lemuroid? International Journal of Primatology, 11 (5), 439-469 DOI: 10.1007/BF02196131
Ravosa, M. (1999). Anthropoid Origins and the Modern Symphysis Folia Primatologica, 70 (2), 65-78 DOI: 10.1159/000021678
Wilson, J., & Szalay, F. (1976). New Adapid Primate of European Affinities from Texas Folia Primatologica, 25 (4), 294-312 DOI: 10.1159/000155722