Tracking Notharctus, Wyoming’s Prehistoric “Lemur”

Despite all the overhyped nonsense which surrounded the debut of the 47-million-year-old primate Darwinius masillae (“Ida” to her fans) last year, I have to admit that the first-described specimen was a gorgeous fossil. It was a paleontologist’s dream – a complete, articulated skeleton with traces of hair and even intact gut contents. Never before had a primate fossil been found in such an exquisite state of preservation, but Darwinius was not the only one of its kind to be known from a complete skeleton.

The study of fossil primates goes all the way back to the beginnings of paleontology, but most 19th century naturalists who described primate fossils did not immediately realize that they were doing so. The primates they had discovered were so fragmentary – often represented by only a tooth or section of jaw – that it was easy to misidentify the scraps as belonging to other kinds of animals. This is precisely what the Philadelphia polymath Joseph Leidy did in 1870 when he established the name Notharctus on the basis of a few pieces of jaw he thought came from a “pachyderm” (a generalized category of herbivorous animals which is no longer in use). Nevertheless, in an 1873 follow-up Leidy expressed his uncertainty about the fossils, saying that “In many respects the lower jaw of Notharctus resembles that of some of the existing American monkeys quite as much as it does that of any of the living pachyderms.”

The discovery of other fossil primates from the American west eventually confirmed that Leidy’s doubts about Notharctus had been well-founded – it was a primate, after all – but so little of it was known that it was unclear how it was related to other primates. It would not be until the summer field seasons of 1903 and 1904 that the American Museum of Natural History’s paleontologist Walter Granger would find more complete material from the approximately 50-million-year-old rock of Wyoming’s fossil-rich Bridger Basin. Among the haul were several incomplete skeletons, a handful of partial skulls, and one complete skull. Taken all together, these bones presented scientists with their first complete look at a fossil primate.

As reconstructed by the anatomist W.K. Gregory in his landmark 1920 monograph on Granger’s fossils, Notharctus would have looked like a lemur in life. There were a few differences – the snout of Notharctus was a little shorter, and its brain was smaller – but the overall resemblance was undeniable, especially in regard to its arms and legs. “Every limb bone of Notharctus,” Gregory wrote, “is fundamentally similar in all its parts and processes to the corresponding elements in modern lemurs,” and in his estimation Notharctus moved around by climbing and leaping through the forests which covered the American west during the hothouse climate of the Eocene.

Notharctus looked like a lemur and probably behaved like a lemur, but Gregory followed Leidy in thinking that it had a relatively close relationship to New World monkeys. Whether it was a direct ancestor or not, Gregory could not say, but at the very least Notharctus seemed to represent the form of animal from which the New World monkeys could have been derived. Other naturalists had proposed similar scenarios. Citing the lack of some diagnostic lemur traits – such as a forward-facing tooth comb formed from lower incisor teeth – and the presence of some characteristics seen in living monkeys, in 1904 Jacob Wortman argued that Notharctus was near the ancestry of anthropoid primates (the group which includes both monkeys and apes). This would make it one of our early ancestors, too, and for much of the 20th century Notharctus was placed near the root of our primate family tree.

But, as paleontologists came to realize as fragmentary remains of fossil primates trickled in, Notharctus was not as closely related to monkeys – or to us – as had been proposed. Like the more recently-discovered Darwinius, Notharctus belonged to an extinct group of primates called adapiforms, and the few monkey-like traits it possessed were either archaic traits shared by many primates or were instances of convergence.

As is currently understood, Notharctus sits nested among a bush of prehistoric primate lineages on the lemur/loris side of a major split in the primate evolutionary tree. Tarsiers and anthropoids (collectively, haplorrhines) made up our side, and adapiforms were part of the radiation of strepsirrhines (containing modern lemurs, lorises, and galagos) on the other side, entirely barring Notharctus from being one of our ancestors. That Notharctus was placed so close to us by some 19th and 20th century paleontologists may have been a result of its completeness compared to the scarcity of other fossil primates; when your main guide is the swath of extant primate diversity, it makes sense to slot exceptional fossil forms in according to what is seen among living animals. It is only when the fossil record is better sampled that extinct species can be placed in their proper evolutionary context.

References:

Gregory, W.K. (1920). On the structure and relations of Notharctus, an American Eocene primate. Memoirs of the AMNH, 3 (2), 49-243