Research: First Steps in Primate Evolution

by Stephen G. B. Chester

Stephen Chester
Stephen Chester

Editor’s note: Dr. Chester received his undergraduate degree from the University of Florida and his doctorate from Yale. He is currently an assistant professor at  Brooklyn College, City University of New York.

People often ask me about my most exciting fossil discovery when they find out that I’m a paleontologist. Their assumption tends to be that I was on a paleontological expedition wiping a combination of sweat and dirt off my brow when I unearthed an amazing specimen that had not seen the light of day for millions of years. Though I have been fortunate to be a part of many exciting fossil discoveries in the field, some of my most important finds have actually been made in more comfortable, climate-controlled, museum settings. The most recent of these finds occurred when searching for bones of the geologically oldest primate in boxes of unsorted vertebrate fossils at the University of California Museum of Paleontology with my former advisor and current collaborator, Dr. Jonathan Bloch.

I first met Jon in 2004 when I was an undergraduate studying biological anthropology at the University of Florida. I still remember our first conversation in his office at the Florida Museum of Natural History. He told me all about his research on the evolution of primates and other mammals, and mentioned numerous research opportunities for volunteers. Jon’s enthusiasm for paleontology was contagious, and I never looked back. Some of the first fossils that I studied were those of plesiadapiforms, a group of very primitive primates that first appear in the fossil record within the first few hundred thousand years following the extinction of the non-avian dinosaurs. It wasn’t long before I couldn’t stop thinking about the origin and earliest evolutionary history of primates. And it turns out that there’s no better place to be than a natural history museum when questions about evolutionary history are on your mind. Along with countless fossil specimens, natural history museums also tend to house an incredible community of people with overlapping interests, including curators, collections managers, preparators, students, volunteers, and visiting scientists. My two years volunteering and interacting with such individuals at the Florida Museum of Natural History were a lot of fun and very important in my preparation for graduate school.

Stephen Chester and Jon Bloch in the lab
Stephen Chester and Jon Bloch in the lab

Speaking of preparation, it was within the prep lab where I gained a great appreciation not only for the fossils themselves, but also for the amount of time and effort that it can take to process such priceless and irreplaceable specimens. Jon showed me how partial skeletons of plesiadapiforms were etched out of fossiliferous freshwater limestone blocks with formic acid. This process requires painstaking attention to detail and takes a considerable amount of time given that the acid may only dissolve one millimeter of limestone every several hours. So why is it worth dedicating months or even years to uncover these skeletons? Well, it turns out that even though approximately 150 species of plesiadapiforms are known from the Paleogene of North America, Europe, and Asia, almost all of these species are known only from dental remains. Fossil mammal teeth can provide us with a lot of information about diet, body mass, and evolutionary relationships, but they are not very informative when it comes to understanding the positional behaviors and substrate preference of these extinct animals. Fortunately, skeletons like those of plesiadapiforms from limestone blocks provide a great deal of information that can help us reconstruct how these mammals moved around and whether these animals lived on the ground or in the trees.

I had the incredible opportunity to study some of these plesiadapiform skeletons in graduate school as a part of my dissertation research. I wanted to reconstruct the skeletal anatomy of the ancestral primate, so I focused on the family Micromomyidae because micromomyids are the most primitive plesiadapiforms for which partial skeletons are known. However, the extraordinary micromomyid skeletons that I was studying were approximately 10 million years more recent and more evolutionary derived than the geologically oldest plesiadapiform, Purgatorius.

Chester and Bloch in the field in Wyoming
Chester and Bloch in the field in Wyoming

Purgatorius is the oldest and most primitive plesiadapiform, and has been viewed as a plausible ancestor for all other primates since it was first discovered 50 years ago. Just like many other plesiadapiforms and mammals from the Paleocene Epoch, it has been known only from isolated teeth and jaw fragments. Dr. William Clemens and members of his field crews have collected many specimens of Purgatorius over the past four decades in Montana. Bill has dedicated much of his career to studying how mammals evolved following the End-Cretaceous mass extinction event and he has built exceptional collections of earliest Paleocene mammals at the University of California Museum of Paleontology. In 2009 in Bristol, England, Jon introduced me to Bill at the annual Society of Vertebrate Paleontology conference. The three of us discussed Purgatorius and other plesiadapiforms and Bill mentioned that he had many cabinets of unsorted bone from the same deposits where many dental specimens of Purgatorius had been discovered. Jon and I told him that given similarities of the dentition between micromomyid plesiadapiforms and Purgatorius, we might be able to use the micromomyid skeletons as Rosetta Stones to search for isolated skeletal elements of Purgatorius. Bill was intrigued and thanks to his generosity, Jon and I visited Berkeley two years later in search for new clues about the paleobiology of the oldest known primate.

The University of California Museum of Paleontology is an incredible museum and it’s very easy to get distracted because there are so many amazing fossils and interesting people to talk to. Jon, who was a member of my Ph.D. committee at the time, and I had to focus because we had an important task in front of us. There were several large cabinets of bone to sort through. Each cabinet contained 10-20 drawers, each drawer contained 20-50 boxes, and each box contained hundreds of bone fragments. This was extremely intimidating at first and it seemed like finding bones of Purgatorius was going to be harder than finding a needle in a haystack. Nevertheless, after a few days, we started finding postcranial elements of mammals, including 65 million year old ankle bones that looked extremely similar to those of micromomyid plesiadapiforms!!!

Stephen Chester holds the tiny ankle bones of Purgatorius, which scientists believe weighed about 3.5 ounces—or as much as a deck of playing cards. Photo courtesy of Stephen Chester
Stephen Chester holds the tiny ankle bones of Purgatorius, which scientists believe weighed about 3.5 ounces—or as much as a deck of playing cards.
Photo courtesy of Stephen Chester

My colleagues and I were able to confidently attribute these ankle bones to Purgatorius based on their size and unique similarities to those of other plesiadapiforms and living primates. These fossils have diagnostic characteristics for mobility that can still be found in the ankle bones of primates and their closest living relatives today. Such features would have allowed Purgatorius enough mobility to grab differently oriented branches with its feet while moving through the trees. In contrast, mammals that are terrestrial typically lack such features for mobility, and have more rigid ankles to effectively propel themselves forward while moving on the ground. We already knew from dental remains that Purgatorius was a fairly small, squirrel-sized mammal that ate a combination of fruit and insects, and these new ankle bones show that Purgatorius had skeletal specializations that would have allowed it to access plant products in the branches of trees. This discovery is really exciting because it is the oldest direct evidence that our earliest primate relatives were arboreal. This study also demonstrates that important paleontological discoveries do not only occur in the field; the next significant find might happen soon at a museum near you!

Further reading:

Stephen’s paper on Purgatorius published in the Proceedings of the National Academy of Sciences

Coverage of the discovery by National Geographic

FLMNH press release on the paper