Purgatorius may be from Hell Creek, but it's currently in Limbo:  Why it is so difficult to place early members of a group on the Tree of Life?

Recent discussions on a Facebook Group, Creationism, has centered around a series of yellow memes produced by Creationist and retired pastor Luke Lefebvre.  I've reproduced the latest with his permission, which summarizes Luke's point that since scientists can't exactly agree on whether Purgatorius is a primate, a near-primate or even a true (eutherian) mammal, we can't trust anything those scientists have to say about evolution.

So is this true? And if it is true, why should there be such confusion among paleontologists and paleoanthropologists over the nature of this little critter?  As is so often the case, the Young Earth Creationists like Luke seize upon the usual, healthy give and take of the scientific method as a sign of weakness, and use it to bolster their fear and loathing of science.

Background

First, a little background.  Purgatorius was first described by Leigh Van Valen and Robert E. Sloan in 1965 on the basis of teeth from Purgatory Hill, and placed in two species, Purgatorius unio and Purgatorius ceratops.  P. unio was assigned to the early Paleocene, while P ceratops, because it was a single eroded and weathered tooth, to the underlying Cretaceous part of the Hell Creek Formation.  Both teeth were from a small channel deposit which has a mixture of earliest Paleocene and latest Cretaceous fossils in what is known as a "time-averaged" assemblage. Most of the fossils represent animals that lived at the time the channel was formed, but a few, like the Purgatorius tooth, were eroded out of the older deposit into which the channel was cut.  Most securely dated occurrences of Purgatorius are limited to the second and third phase of the Puercan North American Land Mammal Age (Puercan 2 and Puercan 3), dating between about 64.75 and 64.11 MYA (million years ago), with only a few known from the Puercan 1 phase.

Purgatorius was thought by Van Valen and Sloan to be an early Paramomyid primate, but discussions between 1965 up to today have called Purgatorius a stem primate, or placed it outside the primates; one study has even suggested that it may not be a placental mammal (Eutherian) at all, but should be placed with the Metatheria, which includes the living marsupials as well as a host of extinct groups like the multituberculates.  But why should there be such confusion?  We don't need to get into all the details of dental morphology, although that is where the problem arises.  The answer is really much simpler than that.

The Facebook Meme

Above is Luke Lefebvre's meme entitled Lucy vs. Purgatorius: Who to Believe?  He quotes Don Johanson, then goes on to say "Purgatorius is thought to be an early form of primate (That means that's where you come from) although its exact place in the evolutionary history of primates is much debated.  This is mostly due to the fact that the only fossils we have of this animal are its teeth and jaws. Lefebvre concludes "Ask any evolutionist and wait to get different answers."

Luke is correct in these points:  Purgatorius is thought (by most paleontologists) to be an early primate.  Its exact place on the mammal tree is debated on the basis of different analyses of different data sets. A few paleontologists think it is a proto-primate, or even outside the placental mammals altogether, but the general consensus, using the total evidence available, is that it is a primate.  It is true that the fossil remains of Purgatorius are mostly teeth and jaws, but in 2015 7 astragali and 9 calcanea (both ankle bones) were described by Chester et al.  These were important in being the first postcranial bones attributable to Purgatorius, and in indicating the first Paleocene mammals which were at least partially arboreal.  Note that the tarsal bones were not directly associated in skeletons, but were isolated bones in faunas which had many teeth and jaws of Purgatorius.  How Chester et al. identified each of the particular species of Paleocene mammals is another story, but it is well supported by the evidence.

So why is there any uncertainty?

Think about evolution as the branching pattern often depicted as a phylogentetic tree.  The further back in time you go, the closer you get to the common ancestor, in this case the common ancestor shared by all eutherian mammals.  But remember, evolution isn't a single line from the past to the present. "Decent with modification', Darwin's wonderful phrase, predicts that all the diversity we see in  eutherian mammals traces back to the common ancestor, each modern mammal (as well as all the extinct ones) traces its ancestry back to that same spot on the tree, each by a different route.  Carnivores, proboscideans (elephants), rodents, perissodactyls (horses, rhinos and tapirs), artiodactyls (deer, sheep, cows, giraffes, etc.) all can trace their ancestry back to that same point.  While all those animals are wildly diverse today. as we trace the lines backward toward the common ancestor, they get more and more alike, less different and would be much harder to tell apart if we could see them as living creatures.  Their morphology - teeth, bone structure and all the details that are the basis for understanding the fossil record, also get more and more alike as you move backwards in time and down the tree.  In fact, when you get to the early Paleocene, 61-65 million years ago, all these eutherian mammals, just beginning their evolutionary radiation, are nearly impossible to tell apart.  If we didn't have their descendants identified in some detail, we'd lump them into one group as most closely related to each other.  It is only because we know what they would become later in time, that we can separate them from the other early "primitive" mammals and place them in the proper group known from their descendants.

So, most of the confusion - all of the confusion! - comes from disagreements between researchers as to the legitimate place of Purgatorius on the tree.  It isn't resolved now, and likely won't be until better evidence is available, such as a complete skull and skeleton of Purgatorius and some of the other Paleocene mammals.

In order to illustrate better what I'm talking about, I've taken a slide from a PowerPoint presentation by the Joint Experimental Molecular Unit and the Royal Museum for Central Africa entitled "Introductory seminar on the use of molecular tools in natural history collections" dated November 6-7, 2007.  On a part of this slide I have added more information, as follows:

Each red dot in a red circle indicates a possible positions that Purgatorius could be placed in, with the present hypothesis most favored by the evidence being a stem or near-stem primate.  Another possible position is at the base of the unresolved archontan trichotomy - which just means that the three groups, primates, tree shrews (Scandentia) and the colugo (Dermoptera) are closely related, but the evidence doesn't clearly tell us which two of the three are more closely related to each other than either one is to the third.  Yet another possibility is that Purgatorius represents a common ancestor of all the Euarchontoglires - a group composed of the colugo, tree shrews, primates, rabbits and rodents.  A final possibility which no one has suggested and which has no evidence is that Purgatorius could stand at or near the common ancestry of the rodents and rabbits.

The diagram also has some dashed green lines between the living taxa (blue dot to blue dot) which show that the distance between those dots indicates roughly the morphological "distance" or divergence between the evolving lineage leading up to those dots.  Remembering that each of those blue lines represents a long series of ancestor / descendant species, genera and families, we can also draw those dashed lines between the blue lineages at any time point along the line, demonstrating that as we move back in time - towards any of the common ancestors, the morphological difference between them gets to be less and less as shown by the green dashed lines between the blue Primate line and the blue Dermoptera line.  By the time we reach the early Paleocene - the time of Purgatorius - an early tree shrew is going to be morphologically quite close to an early relative of the colugo, and both will be very similar to an early primate.

The final figure provides yet a different illustration of the problem.  In this diagram, the three living orders, Primates, The Colugo and the Tree Shrews, are shown across the top.  The blue line under each one represents the evolutionary history of that group, called a "clade".  The red oval includes a cloud of fossil species - some are on the direct line back to the earliest member of each clade; others are either just a little off that mainline ("first cousins") or aberrant members, dead ends with no surviving descendants.  One such fossil species is represented for each clade in the diagram by a red dot.  

Now, as is often the case for the most primitive members of the major clades, the earliest members look very much like the earliest members of other related clades.  In fact, we often can't tell to which surviving clade they belong because they lack the specialized characters which help us define the clade.  So all these primitive species get lumped together in a group called a "grade"  It's much like a small dense cloud of species which we can't differentiate and connect to their eventual descendants until sufficient material is collected, a careful analysis is done, and unique shared characters are identified which will connect a given "red-cloud" species to a particular clade.

Summary

So Luke Lefebvre is correct - you can get different hypothses when you read different papers.  But you have to read them carefully, mindful of when each was written, and look at what new evidence each brings to bear on the problem.  That's what science is about.  Science would be no fun at all if there weren't new discoveries to be made, new connections to be drawn and new hypotheses to be proposed - and then tested.

References

Chester, Stephen G. B., Jonathan I. Bloch, Doug M. Boyer, William A. Clemens, 2015, "Oldest known euarchontan tarsals and affinities of Paleocene Purgatorius and Primates" Proceedings of the Academy of Science, U.S.A. Volume 112 (5): 1487-1492

Van Valen, Leigh and Robert Sloan, 1965, "The earliest primates". Science. 150 (3697): 743–745. <>

I thank Luke Lefebvre for allowing me to use his "Yellow Meme" for the anchor point of this blog entry.