Miocene “Monkey”: Pliopithecus canmatensis

What could possibly be a better Christmas present than a new fossil primate?  Nothing, that’s what!

The most recent addition to our family bush is a Pliopithecine from Spain named Pliopithecus canmatensis.  Pliopithecoids are gibbon-like in many ways, including their long limbs, large hands, and maybe the ability to brachiate.  However, the pliopithecoids are much too ancient to be directly related to gibbons, and probably predate the split between monkeys and apes. Resemblance to extant gibbons is almost certainly an example of convergence.  Pliopithecoids have two premolars, which connects them with the catarrhines, but they also have unique dental morphology which places them in their own group.

The scientists describing the fossils- David Alba and his colleagues- have outlined an evolutionary history for the pliopithecoids in which they were the first catarrhines to leave Africa for Eurasia.  In the Early Miocene of Asia, these first pliopithecoids are represented by a group discovered in 1978 called the dionysopithecines.  From this group evolved the true pliopithecines.  By the Middle Miocene, pliopithecines had dispersed quite happily into Europe, and then by the Late Miocene, back into Asia.

P. canmatensis is your typical toothsome primate:  8 individuals are represented from 61 teeth and a few scraps of mandible and maxilla.  From the maxillae, we can tell that the face was short.  The madible is long and skinny, with tooth rows that are almost parallel to each other.

There is only one upper canine tooth represented in the sample, and it is probably from a female.  It has wear facets that show that it occluded with Premolar 3 and the lower canine.   There are a few lower canines, and they probably display sexual dimorphism in size and shape.  The female canines have a blunt surface, while the males have a pointed apex.  The female’s canines are gracile, while those from males are “stout,” but taller than the female’s. Premolar 3 is a typical honing premolar:  It only has one, high cusp with a distinct, steep wear facet which resulted from consistent contact with the upper canine.

In the molars, the distinctive “pliopithecine triangle” is present in the upper 2nd and 3rd molars.  This triangle lies on the cheek side of the tooth, in between the protocone and the hypocone, and even with the beautiful diagram which the authors provide, I have to squint my eyes and cock my head to see what could maybe pass for a triangular shape of some sort.  But that’s what I have to do for all teeth, so I trust the authors that it’s there!

The sites at which these fossils were found-  the Hostalets de Pierola-  are becoming quite the treasure trove of primate fossils!  The area in Catalonia is the home of of the most interesting Miocene hominoids, Pieorolapithecus catalaunicus.  Anoiapithecus brevirostrus and a speices of Dryopithecus also hail from the area around Barcelona.

Alba, D., Moyà-Solà, S., Malgosa, A., Casanovas-Vilar, I., Robles, J., Almécija, S., Galindo, J., Rotgers, C., & Mengual, J. (2009). A new species of Gervais, 1849 (Primates: Pliopithecidae) from the Middle Miocene (MN8) of Abocador de Can Mata (els Hostalets de Pierola, Catalonia, Spain)
American Journal of Physical Anthropology DOI: 10.1002/ajpa.21114

Bones and Genes

John Hawks has a nice post reviewing a recent article (itself a review) by Adam Seipel about the estimates for the chimp-human divergence.  Most of the estimates calculated in the past 5-10 years have put the split at less than 4.5 million years ago!  My brain would like it if that date were older, but Hawks discusses the many reasons why it just can’t be.  Among them is this:

To make the date older, you need to assume there was no demography — an extreme chuman bottleneck. But that would be inconsistent with the evidence of incomplete lineage sorting — those gorilla genes that we share. And it would take some magical rate discontinuities among genetic loci to get them the amount of interlocus variability that they have.

Well, shucks.

(P.S.- If anyone could send me the Seipel paper at amodernprimate AT gmail DOT com, I’d be forever greatful!)

I have the paper now!  Thank you!

The third trochanter and gluteus maximus of Ardipithecus

The femur can be an extremely informative bone when reconstructing the locomotor behaviors of fossil primates. The head and neck are particularly informative. The morphology of the head can tell you how flexible the hip joint is. If you can get a good CT scan, the distribution of cortical bone at the neck can reflect how the primate applied force to the skeleton during locomotion. If you look to the other end of the femur, the condyles can also tell you how a primate loaded the skeleton while moving around by way of the famous bicondylar angle.

Unfortunately, none of those characters were preserved in Ardipithecus. What we do have is a shaft, and as we’ll see, the shaft can give us some information about muscle attachments- especially the ever-important gluteus maximus.

First, a little background: In great apes, the insertion for the gluteus maximus is positioned further down and toward the midline of the femur relative to what we see in humans. A different muscle, the vastus lateralis, attaches to the femur a bit further up.  The two muscle attachment sites are separated from each other by a bump called the lateral spiral pilaster.

In humans, the gluteus maximus inserts onto the lateral aspect (outer edge) of the femur.  What we see is a third, very small trochanter that is part of the greater trochanter.  Immediately below that third trochanter is a bumpy depression in the bone  (a hypotrochanteric fossa).  Both of these characters are associated with our enlarged gluteus maximus. Apes never have either of them.

I bet everyone can guess that Ardi shares a more human-like femur morhpology (why else would we be talking about it?), and indeed it does have a third trochanter, as well as the depression below it.  What is surprising is who else has similar morphological characters.  Nacholapithecus has a third trochanter, as does Dryopithecus, and even primitive Proconsul! This is in addition to Orrorin tugenensis, Australopithecus afarensis and A. anamensis.

The femora of Australopithecus afarensis (A- the "Maka" femur) and Ardipithecus ramidus (B). The arrow indicates the third trochanter and the bracket indicates the hypotrochanteric fossa. (From Lovejoy et al. 2009)

Because the back of the African, large-bodied suspensory apes has been so drastically modified since Proconsul, the gluteus maximus also had to be moved around.  The gluteus maximus insertion diverged distomedially away from the vastus lateralus, and the gap was filled in by the lateral spiral pilaster.

…In both lineages!

The gluteus maximus in humans has changed, too.  By the time we get to Australopithecus, the insertion has moved back and to the midline (posteromedially).  The authors suggests that this is because the quadriceps has gotten bigger, while the hamstrings became smaller.

Clearly, we shouldn’t draw any conclusions about bipedality in Ardi from a third trochanter and hypotrochanteric fossa if even Proconsul had similar characters.  Enlargement of the quadriceps hasn’t happened yet, but if we’ve got an animal who was participating in bipedality only some of the time, I’m not sure if we’d expect it yet.  The take-away message here is that we have yet another anatomical detail that supports the idea that humans are more primitive than chimpanzees and gorillas in many aspects.  Using chimps (or bonobos) as an analog for early human behavior- locomotor or otherwise- is becoming less and less defensible.

Lovejoy, C., Suwa, G., Spurlock, L., Asfaw, B., & White, T. (2009). The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking Science, 326 (5949), 71-71 DOI: 10.1126/science.1175831

Skepticism is good, but…

Well-informed skepticism is the best!

Earlier this week, Eric Michael Johnson drew my attention to a post by psychologist Christopher Ryan at his blog Sex At Dawn.  Ryan attacks Lovejoy’s monogamous humans model by citing many different lines of evidence.

I became so distracted by the reported testes:body mass ratio of 1/160 in humans that I couldn’t stop until I had some answers.  I am a female human, but even I thought that 1 kg of testicles would be an awful lot to lug around.  So I got out my books and my calculator and did some math, wrote in, and it was fixed.  Peer review in action!

But then I clicked through to the actual post, and realized that it didn’t get much better from there.  Apart from characterizing Ardi as mere “bits of bone,” Ryan displays many instances of ignorance.

First, a few things that Ryan gets right:

1. Yes, reduced male-male competition can happen in promiscuous mating systems (but I do think that characterizing the Chimpanzee as having reduced competition is dubious).
2. Humans are probably able to produce more sperm per ejaculate than the number that Lovejoy cites.
3. Reproductive anatomy and physiology is particularly labile, so postulating about reproductive physiology 4.4 million years ago is a risky undertaking.

But then…

Lovejoy writes that “Humans have the least complex penis morphology of any primate.” Unfortunately, he never defines what he means by “complex;” nor does he discuss the fact that the human penis is, by most measures, the longest, thickest, most prominently displayed penis among primates. No mention of the unusual flared head or the external scrotum—both strong indications of sperm competition in our species.

Lovejoy clearly states that humans lack “keratinous penile surface mechanoreceptors” (known to you and me as penis spines and ridges) and an os baculum.  So there’s your complexity.

Lovejoy also states in the footnotes:

Flaccid human penis length (13 cm) is unusually great for a hominoid. Length is ~4 cm in Pongo and 3 cm in Gorilla. Its erect size is greater in the multimale Pan (8 cm), but this reflects specialized adaptation to penetrate seminal plugs. Short notes that “(e)ven the pubic hair in the male [human] seems designed to draw attention to the genitalia, rather than to conceal them as in the orangutan and gorilla.”

Lovejoy states these things in support of a rather weird argument against hand-to-hand combat in human ancestors, but why not in the discussion of sperm competition?  Because a huge penis does nothing if you don’t have the testes to back it up!  Clearly a large, pendulous, prominently-displayed penis is something special in humans, but it is not about sperm competition. The external scrotum- by itself- may be indicative of sperm competition, but combined with the fact that human testes are smaller and have fewer seminiferous tubules, that the human sperm midpiece volume is low, and that we have lost the physiology for creating copulatory plugs indicates that there may be something else at work here.

Perhaps the most glaring mistake lies in Ryan’s argument about canine teeth:

For example, much of his thesis hinges on the absence of pronounced canines teeth (fangs) in the fossils found. He writes that we can assume that both males and females lacked these canines (even if the teeth were from a female) because “The SCC [sectorial canine complex] is not male-limited; that is, it is always expressed in both sexes of all anthropoids….” But this is wrong. Male bonobos have long canines, while females don’t (2, 3). Lovejoy also claims an association between reduced canines and pair-bonding, but as this photo of the skull of a monogamous gibbon demonstrates, even this claim is suspect.

When anthropologists refer to the canine complex, they are not merely referring to canine length.  The Sectorial Canine Complex, or the CP3 complex, or the “honing” complex all refer to the way that the top canine tooth fits in with the lower canine and premolar.  It fits in this way in order to sharpen the canine tooth so that it acts as a blade.  In contrast to what Ryan states, both male and female anthropoids (including the bonobo) WILL have this trait if it is present at all.  Yes, the tooth will be longer in males (in many cases, much longer), but the sexual dimorphism here is in size, not in the actual way that these teeth occlude.

Teeth from more than 35 individuals have been found, and not one of them exhibits this Sectorial Canine Complex, so I think it’s safe to say that Ardi and her pals didn’t have it.

And then there’s that beautiful gibbon skull.  Unfortunately, the canine monomorphism in gibbons actually supports Lovejoy’s whole thesis:  In monogamous species, the canine teeth are no longer under strict sexual selection, and natural selection can take over and do its thing.  In gibbons, this has meant that not only do males have large canines, but females have them as well.  Both males and females defend their territory, and so both have enlarged canines.  In humans, the argument is that the energy that goes into maintaining large canine teeth and getting into the fights that result when you have them could be better spent by gathering food for your partner and offspring.  As a general rule for primates, the canines don’t have to be reduced to indicate pair-bonding, but if they are the same in both sexes, something is up.

Lovejoy’s hypothesis is controversial.  It’s a Theory of Everything, and that should raise flags in everyone’s head.  But at the end of the day, it’s really stinkin’ weird that human males abandoned their canine teeth, and it’s really stinkin’ weird that human females abandoned their ovulatory advertisement.  I think monogamous pair-bonding goes further than any other hypothesis in explaining those two weirdnesses of the human.  However, my little red flag pops up when we connect those things to bipedalism.

Zingeser, M. (1969). Cercopithecoid canine tooth honing mechanisms American Journal of Physical Anthropology, 31 (2), 205-213 DOI: 10.1002/ajpa.1330310210
Lovejoy, C. (2009). Reexamining Human Origins in Light of Ardipithecus ramidus Science, 326 (5949), 74-74 DOI: 10.1126/science.1175834
Frisch, J. (1963). Sex-differences in the canines of the gibbon (Hylobates lar) Primates, 4 (2), 1-10 DOI: 10.1007/BF01659148
Leutenegger, W., & Kelly, J. (1977). Relationship of sexual dimorphism in canine size and body size to social, behavioral, and ecological correlates in anthropoid primates Primates, 18 (1), 117-136 DOI: 10.1007/BF02382954

The feet of Ardipithecus ramidus

We’ve already covered Ardi’s hands here, so let’s move on to what is possibly the most interesting aspect of her skeleton: The feet.

As humans, we have pretty special feet. They’re good at dissipating all of the force that comes with walking on only two feet. They’re also good at propelling us forward, since we’ve lost the mechanism that most quadrupedal animals use.

Chimps and gorillas have feet that are essentially hands. Compared to animals like Proconsul, the back of the foot was shortened, and the joints were modified to allow more flexibility at the ankle joint.  They also got rid of a teeny tiny little sesamoid bone embedded in the fibularis longus tendon called the os peroneum, which acts in old world monkeys to adduct the big toe and prevent twisting and buckling of the ankle joint.  Because the great apes have gotten rid of the little bone, their tendon can now grasp with the big toe, while the plantar surface of the foot can conform to whichever substrate they choose: Tree trunk or ground.

Most humans still have the little os peroneum, or at least a cartilaginous mass in the right place.  We clearly don’t use the associated muscle to adduct our big toe since it’s permanently in-line, but fibularis longus stabilizes our longitudinal arch.  We’ve moved fibularis longus to the outside and above the position of where it is in monkeys to aid in this function, and since we don’t need it for toe adduction anymore, the little os peroneum has been allowed to become almost vestigial.

In Ardi, the big toe is opposable, and the os peroneum is big.  In contrast to both the human morphology and the great ape morphology, it is comparable in size to what is seen in many old world monkeys, and probably Proconsul as well.  The fibularis longus muscle and tendon adducted the toe and flexed the foot as in OWM, but also added just a little bit of stabilization to the area of the foot that would become the arch in later hominids.

Now, to the lateral foot.  A second metatarsal was preserved, from which we can reconstruct the plantar base.  The second metatarsal articulates with the ankle bones at the tarsometatarsal joing.  In Ardi, there are small little dents in the proximal joint surface of the second metatarsal.  The proximal joint surface second metatarsal is mostly cartilage throughout development, and as such will preserve loading patterns from that time.  The indentations probably show us that these joints were experiencing regular pressure from the joint, which would happen during bipedality.

The third metatarsal has a big “gutter” at the junction of epiphysis and physis.  This gutter reflects loading during dorsiflexion at the ankle, and again, because it is mostly cartilage during development, can preserve important information about loading patterns.  The head shows that it had been consistently rotated during development, which would have occurred during bipedality.

The talus and tibia/fibula of Ardi form an angle (measured from the talus) that is within the range of OWM.  In hominids, this angle is considerably lower because of the angle of the valgus angle of the knee.  The talus in Ardi also preserves the flexor hallucus longus groove, which is shaped like a trapezoid tipped on its side and shows that the FHL approached the talus at an angle.  This is in contrast to the condition of Lucy and other hominids, where the FHL groove is a vertical parallelogram.  This means that, while Lucy was able to keep her knee rather straight during the stance phase of walking, Ardi’s knee was probably rotating.

Ardi's foot

These features, as well as some stuff in the pelvis that we’ll get to later, suggest that when Ardi was on the ground, she placed her foot along the midline of the body, with the big toe pointing inward and the front of the foot pointing outward.  In humans, most of the body weight rotates around the ball of the foot when we “toe-off,” but in Ardi, this pivoting occurred around a diagonal line drawn through the foot, with weight balanced both by the big toe and the fibularis muscle on the lateral edge of the foot.

Much like in the hand, the foot of Ardi suggests that, instead of modifying a foot that looks essentially like a chimpanzee foot, humans, chimpanzees, and gorillas have all taken a generalized Miocene ape foot and done our own special thing to it.

Lovejoy, C., Latimer, B., Suwa, G., Asfaw, B., & White, T. (2009). Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus Science, 326 (5949), 72-72 DOI: 10.1126/science.1175832

Climbing on the branches of the family tree: The hands of Ardipithecus ramidus

As everyone has already heard by now, the long-awaited Ardipithecus ramidus has finally been published, and boy is she a beauty!  So many “anatomical surprises”! There are so many strange and new things about this skeleton that it took an entire issue of Science to describe them.  Clearly then, it will take a few blog posts.  The first post will be about the hands.  Feet will be next, and we’ll move on from there.

But first, a few basics.  The papers report on over 35 individuals represented by various parts of the body.  Lots of teeth.  They were found in Aramis, Ethiopia and date to 4.4 million years ago.  For reasons that I’ll discuss below later, we think that Ardi was a facultative biped who spent most of her time in the trees.  When she was up there, she walked above the branches with her palms and plantar surfaces in contact with the branch.

That Ardi is a palmigrade, above-the-branch walker is surprising. Based on the apes that we have around today (chimps, humans, gorillas, and orangs), most anthropologists have always constructed the common ancestor to chimps and humans as an animal who was suspensory when in the trees (that is, hanging from the branches), and a kunckle-walker when on the ground.  There have been plenty of people who have called this scenario into question (in fact, I talked about one of them here a few weeks ago), but that hasn’t stopped many anthropologists from using the chimp as an analog for what the last common ancestor roughly looked and acted like.  Ardi shows us that the LCA probably looked nothing like either the chimpanzee or the human, but was more likely a generalized Miocene ape which lacked the specializations of either.

(As an aside, I’ve seen a rather questionable quote by Owen Lovejoy mentioned a few times– something like, “Humans didn’t evolve from apes.”  It is a very unfortunate thing for him to have said from the standpoint of science journalism, but taken in the context of the 11 articles that he and the rest of the team have written, as well as numerous other quotations, he clearly means that humans didn’t evolve from anything that looks like any of the extant apes.)

Anyway, on to some anatomy!

Because the extant African apes are knuckle-walkers, they have stiff, inflexible hands and wrists that allow them to support their body weight in sort of a weird position.  Because they also have to climb trees for food and protection, their hands are very long and powerful.  Humans, on the other hand, have pretty mobile hands and wrists which allows us what we call a “power grip.” We are very good graspers, and this has allowed us to become the dextrous tool-wielders that we are.  Because of our close genetic similarity to chimps, and the close morphological similarity between chimps and gorillas, it has been argued that certain features of the Australopithecine wrist- and even the human wrist- were “hold overs” from the period of time when we, too were knuckle-walkers who required a stiff wrist and hand.

Ardi's hand. The inset includes the capitate from a chimpanzee, Ardipithecus, and a human.

However, Ardi’s hand more closely approximates the human hand than the knuckle-walker hand.  It is very flexible.  The midcarpal joint in particular is striking in its flexibility.  Ardi could lay her palms flat on the tree branch and support her entire weight this way.  In this respect, Ardi resembles many generalized Miocene apes, such as Proconsul. Because this wrist is so generalized and resembles the wrist of so many other Miocene apes, it is probably the ancestral condition.

If we look at it that way, then human hands are only moderately derived when compared to those of chimps and gorillas.  Humans don’t enjoy quite the degree of flexibility at the midcarpal joint that Ardi does, and we’ve enlarged our thumbs and shortened our fingers a little.  Chimps and gorillas both had to modify their hands substantially to be able to move around the way that they do.

[Edited to insert link and fix a typo]

Lovejoy, CO., Simpson, S., White, T., Asfaw, B., & Suwa, G. (2009). Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive Science, 326 (5949), 70-70 DOI: 10.1126/science.1175827

What microcephalics can tell us about human evolution

Microcephaly is a disease in which the brain is smaller than normal.  A small brain can result from several different developmental conditions.  Babies can be born with normal-sized crania and brains which then fail to develop with the rest of the head, in which case it’s indicative of a neurodegenerative disorder that may be caused by environmental exposure to some sort of toxin (toxoplasma or alcohol) or some sort of genetic condition which causes deterioration of the brain.  The type of microcephaly that is important to us in this discussion is termed “primary microcephaly,” and is a microcephaly in which infants are born with crania and brains which are already smaller than those of their peers.  It is an autosomal recessive condition with seven identified genetic loci which may or may not be involved in each individual case.  In primary microcephaly the brain is small, but architecturally normal. (That is, they possess the same parts of the brain that humans with developmentally “normal” brains possess, but scaled down and in different ratios.  The cerebral cortex is usually the area of the brain which is most reduced, in terms of both white matter and gyrus complexity.)

Though having microcephaly is strongly correlated with mental retardation, microcephalics exist on a wide spectrum of cognitive abilities.  Retardation is usually only mild.  Social, motor, and speech development may be delayed, but they can progress and function quite well.

A sample of microcephalics taken by Weber and his collleagues in 2005 shows that cranial capacity ranges from 280 to 591 cubic centimeters (cc), with a mean of ~400 cc.  Humans have a pretty wide range, but we usually place it between 1000 and 1800.

Now, I bet you’re thinking that I’m going to talk about Hobbits now, but I’m not!  Right now, I’m going to talk about apes.  Bonobos have a cranial capacity ranging from around 280 to 380 cc.  Chimps range from 280-450.  Orangutans clock in at 280-500cc, and Gorillas at 350-780 cc.

But wait.  None of those guys can talk.  None of them functions normally within a human social group.  None is capable of our fine precision grip.  And yet, many of them have larger brains than microcephalics who can do all of those things.

“Of course!” you may say.  “They are from a different species than us!”  But keep in mind that the accepted knowledge of only a generation ago was that human brains were remarkable because they were big.  Big brains made us smart, especially when scaled for body size.  With our big brains, we conquered tool-making, fire-making, and even monogamous love-making.  The human brain was a chimpanzee brain writ large, as the chimpanzee was a monkey brain writ large, and so on down the line.

The observation that there were humans- humans who could talk and play sports and read and write- with smaller brains than some chimps was a rather profound observation to make.  That observation implies something very important:  Our brains are not special because they are big, they are special because of how they are big.

In terms of simple anatomical repartitioning, human brains are special primarily because of an enlarged and super wrinkly neocortex.  The neocortex became larger while most other parts of the brain either stayed the same or were reduced when scaled for body size.  Our brains are also more spherical, which may reduce the distance that electrical signals need to travel.

While microcephalics generally have a smaller portion of their brain devoted to cerebral cortex than other humans, the proportion is still greater than in the apes.

Paleoanthropologists are often tempted to use brain size as a proxy measure for intelligence, and intelligence as a proxy for “humanness,” but the case of microcephaly should give us a good reason to stop using this measure.  Indeed, most researchers use some sort of “encephalization index” in their work, so that absolute size is no longer the primary reported measure, but a measure of brain size scaled to body size.  However, even this may be inadequate if we consider the vast gulf between a microcephalic human and an ape with a similar body size.

And yet, many researchers still report cranial capacity!  It is, admittedly, only used as a last resort (to my knowledge, at least) when there are no postcranial remains from which an approximate body size can be calculated.  Even if there is a postcranium, calculating body weight for those fossils comes with a rather large margin of error.  So perhaps reporting this number can be forgiven, even if it tells us very little about the individual to whom it belongs.

The next logical step would be to skip reporting the simple size of the brain cavity, and look at the actual brain itself.  Fossilized brains, or endocasts, may be useful for gleaning information about anatomical structure of the brain, but these are very, very few, very far between, and usually very difficult to interpret (see the debate over the lunate sulcus of the Taung child, for example).  For these reasons, comparative neurology may be the best way to find out about the neurology of fossil hominids.  Figuring out the patterns and structures present in different taxa, or even within one taxon as in the case of the microcephalic, can lead us to information that we simply can’t get from rocks and bones.

WOODS, C., BOND, J., & ENARD, W. (2005). Autosomal Recessive Primary Microcephaly (MCPH): A Review of Clinical, Molecular, and Evolutionary Findings The American Journal of Human Genetics, 76 (5), 717-728 DOI: 10.1086/429930
Sherwood, C., Subiaul, F., & Zawidzki, T. (2008). A natural history of the human mind: tracing evolutionary changes in brain and cognition Journal of Anatomy, 212 (4), 426-454 DOI: 10.1111/j.1469-7580.2008.00868.x
HOLLOWAY, R. (2004). Posterior lunate sulcus in Australopithecus africanus: was Dart right?*1 Comptes Rendus Palevol, 3 (4), 287-293 DOI: 10.1016/j.crpv.2003.09.030
Falk, D. (1983). The Taung endocast: A reply to Holloway American Journal of Physical Anthropology, 60 (4), 479-489 DOI: 10.1002/ajpa.1330600410
Weber, J. (2005). Comment on “The Brain of LB1, Homo floresiensis” Science, 310 (5746), 236-236 DOI: 10.1126/science.1114789

Suminia getmanov: A false primate

Suppose I were to describe to you a little animal who lived in the trees, had lovely, long hindlimbs and forelimbs, and had grasping hands, feet, and even a grasping tail.  Your first instinct would probably tell you that I was describing a primate- and if you were particularly clever, you might even conclude that I was describing a platyrrhine.  But you’d be wrong!  The animal I just described is actually a 260 million-year-old Synapsid named Suminia getmanov. That’s 200 million years older than the first primates!  This little guy is even older than the first animals that we conclusively call “mammals,” though it probably fits quite well into the “mammal-like reptile” vs. “reptile-like mammal” debate.

Suminia getmanov is not new to the paleontological scene, but what is new is a description of 12-15 complete individualswhich were encased in a single, large block of rock in what is now the Kirov region of Russia.  The pristine condition of the skeletons suggests that this group was buried rapidly, before any scavengers or wind or water could get to them and disturb the skeletons.  This block of rock and the fossils inside allows us to look at a small sample of a population of these guys, and preserves features of their skeletal anatomy which were previously unknown.

S. getmanov, as it turns out, has a long neck, with particularly wide cervical vertebrae.  The scapula is “tall and slender,” or what we may think of as a more rectangular shape than our big triangles.  The limbs are long when compared with the trunk.

Hands and fingers are particularly exciting to the paleontologist who studies small mammals.  They are so small and fragile that they are usually the first things to go when a gust of wind or trickle of water happens to find our decaying little animal.  The block of fossils described here has lots of hand bones, which is exciting both because of their rarity, and also because they are such an informative part of the skeleton.  The hands described here are very special hands.  They make up about 40% of the entire forelimb!  They were also flexible, as shown by their rounded articular facets, and extremely curved- a hallmark of the arboreal animal.  So these hands were long, but how were they long?  The bones themselves are longer, but the number of bones in each ray, or finger, is also higher.  In the third and fourth fingers and the fifth toe, there are extra bones when compared with S. getmanov’s close relatives.  Fewer finger bones is the derived condition (on the way to full mammalhood!), so S. getmanov is perhaps displaying an atavism, and re-evolved these bones.

And then, there is the widely divergent thumb.  The angle that the thumb and big toe (hallux and pollux in anatomy-speak) form with the rest of the hand is 30-40 degrees.  The joint surfaces indicate that the thumb and big toe could be flexed underneath, in opposition to the palm, and also that it could be adducted and abducted freely as well.  Flexing, abducting, and adducting are all very important motions for an animal that makes a living holding on to branches.  I also mentioned that S. getmanov possibly had a prehensile tail, and the evidence for this comes in the form of robust caudal vertebrae, with large transverse and spinous processes which could possibly have served as muscle attachments for a strong and possibly prehensile tail.

The authors describe the spine as flexible, and cite evidence that in some of the individuals, the vertebrae were not fully fused together (the neural arches were not fused to the centrum).  The authors say that this lack of fusion has nothing to do with body size, and hence, nothing to do with ontogenetic age as we might suspect.  This particular piece of evidence is odd to me.  Arboreal animals generally do not want a flexible spine.  Apes, for example, have very short backs to minimize the effects of all of the bending moments that they experience.  But apes swing in the branches and there’s no evidence that S. getmanov was swinging.  Probably more likely to be clinging or scansorial.  But I also think it’s a pretty risky thing to use body size as a proxy for ontogenetic age.  Perhaps we have a sexually dimorphic species here.  Plus, I’m not really sure what kind of flexibility an unfused vertebra would allow one to enjoy.  It sounds dangerous to me!

The authors clearly don’t have the PR team that little Ida had, which is probably a good thing.  This small population is as much a “missing link” as Ida was, and has just about as much of a connection to the human family tree.  Plus, it’s older, and there are more bones.  Perhaps there’s no fur, but they did find some coprolites!

Plus, it’s an engaging example of convergent evolution, or homoplasy.  Paleontologists have to be on the look out for homoplasies because they are everywhere!  If a synapsid from the Permian can have opposable thumbs and claw-like digits, well then we better believe that lots of other true mammals could have as well, and could possibly muck up our interpretation of who goes where and who is related to whom.  For that reason, it’s particulary important for us to construct phylogenies around non-adaptive trais as much as we possibly can.  If we choose something that is probably an adaptation, it has probably evolved in more than one lineage.

A good example of a non-adaptive trait which is a reliable phylogenetic marker is the primate auditory bulla.  The bulla is a bony little cup that surrounds the ear.  In primates, the cup is an extension of the petrosal bone.  Other mammals have this little cup as well, but theirs are all either extensions of other bones, or a distinct, separate bone.  There’s really no adaptive reason why an animal should extend one region of bone as opposed to another if they’re going to do the same thing, so we get a nice, non-adaptive trait from which to identify primates.

The trick, of course, is in finding a trait which everyone can agree is non-adaptive.

Frobisch, J., & Reisz, R. (2009). The Late Permian herbivore Suminia and the early evolution of arboreality in terrestrial vertebrate ecosystems Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.0911

Ganlea megacanina: Saki of the Eocene

Pithecia pithecia

Meet the White-faced Saki, Pithecia pithecia.  P. pithecia lives in South America, where it scampers about the low canopy eating the seeds of fruit with tough outer shells.  To get through those tough outer shells, it has robust, stout canines that are able to pierce the skins and dig out the soft fruit and seeds inside.

Now let’s jump back in time to the Eocene, and across space to Southeastern Asia.  Here, we’ll find a group of primates that we would probably recognize as monkeys. These are the Amphipithecidae.  One of those early monkeys would be Ganlea megacanina, a new fossil primate described by Chris Beard and colleagues in the latest issue of the Proceedings of the Royal Society B.   G. megacanina was found in the Pondaung Formation in Burma, and is known only from teeth and a little bit of jaw.

I can’t tell you exactly what G. megacanina looked like because we don’t have most of that information.  This is typical for primate paleontology (which was what made Darwinius such an exciting discovery).   But, we can enjoy a few brief glimpses of what it may have eaten and how it may have behaved.

G. megacanina has, as its name would imply, a massive canine tooth.  The authors looked at the canine:molar ratio and determined that the individual was a male, but also found that the tooth is bigger than would be expected simply by being a male in a sexually dimorphic species.  They also observed that the apex, or tip, of the tooth was worn almost flat.  This type of wear is most likely dietary, as the wear pattern that results from simply closing your moth and having your teeth rub against each other is usually more oblique than the wear patterns present here.  Based on the anatomy of this canine tooth, it is a pretty good inference to make that these primates were eating a diet very similar to modern sakis: soft fruit and seeds covered by a tough outer husk.

Buccal view of Ganlea megacanina teeth. From Beard et al 2009.

We can also tell a bit about this guy’s relationships to other primates.  Together with two other primates from the Eocene of Burma, he is part of the Amphipithecinae subfamily of Amphipithecidae.  We group them together based on the anatomy of their premolars, which are shortened mesiodistally (which means from the front of the mouth to the back) so much so that they are actually wider bucco-lingually (on the cheek-tongue “axis”).  This feature, along with several features of the cusps of the teeth, ally the entire group more closely with anthropoid primates- specifically either modern Platyrrhines or the extinct Propliopithecids- than with adapiforms.  This analysis puts this group of primates close to being some of the first monkeys.  It looks as though they are a bit more evolved in the anthropoid direction than Beard’s famous Eosimias, or Dawn Monkey, though, so they are not quite at the root of our clade.

Beard, K., Marivaux, L., Chaimanee, Y., Jaeger, J., Marandat, B., Tafforeau, P., Soe, A., Tun, S., & Kyaw, A. (2009). A new primate from the Eocene Pondaung Formation of Myanmar and the monophyly of Burmese amphipithecids Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.0836

The big hole in your head (Picture-heavy post!)

When an animal is walking around doing whatever that particular animal does for a living, its eyes are usually looking forward, toward the horizon.  The animal needs to see whatever it is that the animal needs to see- predators if you’re a prey animal; prey if you’re a predator- and the best way to do this is to position your head so that you can see in front of you without having to flex or extend too many muscles.  When the animal is positioned in the normal old looking-in-front-of-you position (norma lateralus in anatomy-speak), certain points in the head line up to form a plane which is horizontal to the ground.  These points are the bottom-most point of the eye orbit, called the orbitale, and the top-most point of the external bony auditory meatus, called the porion. So, you draw a line through the left and right porion and orbitale, slice through the skull in a horizontal plane, and you have a plane which is horizontal to the ground.  This plane is called the Frankfort Horizontal Plane, and the first step in most cranial morphometric studies is to put the skull in Frankfort Horizontal. (Some people like to use the horizontal semicircular canals to put the skull into a horizontal plane, but sometimes that requires expensive CT technology, or bones that didn’t survive millennia in the ground or centuries of being slammed around in big specimen drawers in museums.  The Frankfort Horizontal is the cheap and easy way, and in primates, it’s usually pretty accurate.)

There are many reasons why you would want to orient a skull so that it’s in the same horizontal position that it took to during life.  What we’re going to talk about here is the position of the foramen magnum- the big hole at the base of your skull where your spine meets your head.  In most quadrupedal animals, the foramen magnum is at the back of the skull, which makes a certain kind of intuitive sense.  Your eyes have to be at the front of your face so you can see, and if your spinal column is a horizontal line, it makes sense to have it coming out of the back of your head because where else would it be?  Here, look at this Mexican Wolf skull.  The first picture is the bottom of the skull, and you can’t see the foramen magnum.  That’s because it’s at the back of the head, as shown in the second picture.

Mexican Wolf Skull, bottom view

Mexican Wolf Skull, back view

Human Skull

In humans, the foramen magnum is tucked under the head so that the head is right on top of our vertical spinal column.  This movement of the foramen magnum forward and under is actually, in developmental terms, a lack of movement.  All baby apes, including humans, start out with foramina magna in about the same position.  As the rest of the apes develop into adults, their foramina magna migrate toward the back of the head.  But ours stay in the same, baby-like position.

Giraffe

So, is the foramen magnum at the bottom of our heads because we need to balance our head on top of our erect spinal column?  Or might it have to do with something else?  Let’s think about some other animals.  Giraffes are quadrupedal, right?  But they also have really long, erect spines.  So where is their foramen magnum?  Pretty much at the back of the head, as it turns out.  Same with kangaroos, llamas, birds, and some other guys.  So, it might not have as much to do with posture as we thought.

What else might it be?  Maybe it’s the weight of the face.  When you have a heavy face, you need a lot of muscle at the back of your head and neck to balance it and hold it up. The spinal column is basically a fulcrum over which the front and back sides of your head are balanced.  If you move the fulcrum more toward the center of the weight, the weight will be more evenly distributed and you won’t need to use as much muscle to keep your head up.  Sounds pretty good.  The only problem is the gibbon.  Gibbons have a heavier head relative to their body weight than chimps or even humans, yet their foramina magna are more posteriorly positioned than either one.  And, since they’re suspension brachiators, their posture is very upright as well.  And yet, that foramen is still pointing out the back of the head instead of the bottom.

That leaves us with brain size and encephalization.  As the cerebral cortex (the “thinky” parts of the brain) gets bigger and more complicated, it tends to not only get longer and taller, but it changes shape as well.  Alligator brains are basically arranged in a straight line with the forebrain at the front and the hindbrain at the back.  As brains have expanded, the brain curves around so that it forms a reflex angle (greater than 180 degrees).  Now, instead of the hindbrain being behind the forebrain, it’s under it.  Sounds a lot like a certain hole I know…

The jury’s still out on what exactly causes this shift in the position of the foramen magnum, but it’s an important question for paleoanthropologists to ask.  Some of our most famous fossils have been described as bipedal based on the position of the foramen magnum alone- fossils like the Taung Child and Sahelanthropus.  In the case of the Taung Child, we now have lots of corroborating evidence that members of the Child’s species were bipedal, but that’s not the case with Sahelanthropus.  We need to make sure that we know what exactly an anteriorly positioned foramen magnum means before we use it as a defining character of members of our family!

In parting, I leave you with these images of a cat and a macaque.  Both are quadrupedal animals, but the differences in foramen magnum position and brain size (by way of skull size) are striking.

Cat

Macaque