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

Ida Adapoid

Leave it to me to go out camping in the wilderness while the most important discovery in the history of mankind was being reported!  When last I reported on ol’ Darwinius massilae, I thought it was just going to be a tiny little blip on the radar screen of science journalism.  I come home from camping with wild ponies (which I highly recommend) to find that my little corner of science had caused quite a stir- and I missed it!  Laelaps, Pharyngula, The Loom, and many others have written excellent posts on little Ida, which I encourage everyone to go read, and the PLoS article has finally been published as well.

So, who is little Ida, and why all the controversy?  Ida is a primate of modern aspect (!) who lived during the Eocene in what is now Germany.  She scurried about in trees lining a lake which had formed after a volcanic explosion.  Gases from the volcano were still leaking out after the main explosion, and helped to form some of the most beautiful, well-preserved, and famous fossil deposits that we have.  The authors place Ida in the Adapoid superfamily instead of the Omomyid/Tarsioids superfamily.  The Adapoids are further divided into the Adapids and the Notharctids, and Ida is placed in the Notharctid family.  Within Notharctidae, they place Ida in the Cercamonine subfamily because of certain features of her teeth, limbs, and face.  They are able to place Ida in her own genus, Darwinius, because of certain dental characters.  Her first premolar was lost somewhere in her evolutionary history, and her second premolar was heading that way as well.  Many of Ida’s close relatives have a cusp on their molars called the mesostyle, which is a sharp, pointy cusp on the cheek side of the tooth.  Ida lacks the mesostyle in her upper molars, but unlike other close relatives, she has a cheeck-side pointy cusp called the metastyle in both her first and second molars.   I think it’s important to note that, even though Ida is exceptionally complete, we still base most of our species designation on features of the teeth because they are so informative.  Mammalian teeth are all based on the same basic prototype, and possesion or lack of something so trivial as a tiny bump in between two other bumps can tell us so much about familial relationships. This, combined with the fact that many Eocene primates are known only from dentition underscores the importance of teeth in primate paleontology.

Teeth are also extremely important in deciphering Ida’s age at death.  When adult teeth begin to mineralize, they do so while still hidden in the jaw bone, and mineralize from the crown down.  We also know that this process happens in certain teeth before it happens in other teeth.  Ida’s permanent teeth are in many different stages of mineralization- some already have roots, and some are still only crowns.  Most of her baby teeth are still present, except for her first incisors.  So, we know that Ida wasn’t yet an adult.  The rest of the skeleton confirms this, because many of the bones are still not fused together.  The pelvic bones- ilium, ischium, and pubis- are all separate bones, and many of the long bone sutures are still open.

Cercamonines like Ida have been discovered previously with very large penis bones, or bacula.  Ida doesn’t have one, and given the exceptional preservation of the specimen, we can be pretty sure that it wasn’t just lost during the burial process.  That means that she’s a female.

The authors have compared the body proportions of Ida to several different living primates to infer locomotor patterns, and have concluded that she was a generalized arboreal quadruped, with no special adaptations for clinging and leaping, and none for slow clambering, either.

So, why is Ida so controversial?  Modern primates are divided into two big groups: the Strepsirrhines, who have moist noses like a dog, and the Haplorhhines, who have dry noses and a broader face.  Adapids are usually interpreted as the ancestors of the Strepsirrhines, and Omomyids as the ancestors of the Haplorrhines.  The authors of this paper did a phylogenetic analysis and determined that Ida, the Cercamonine, Notharctid, Adapoid shared more traits with modern Haplorrhines than with modern Strepsirrhines.  The only problem is that most of the traits that they analyzed aren’t derived Haplorrhine traits, but basal primate traits.  For instance, Ida lacks a tooth comb.  Some modern Strepsirrhines have tooth combs, and no modern Happlorrhines have them.  Lacking this Strepsirrhine trait isn’t nearly as informative as it would be if Ida had a derived Haplorrhine trait.  Ida also lacks a grooming claw on the first digit, which is a little more tricky.   Because claws are primitive mammalian traits, it would be the most parsimonious if the Strepsirrhines had retained one claw while evolving the rest of them into nails and the Haplorrhines had simply evolved all of the claws into nails.  Ida doesn’t have a grooming claw, which could mean that she lost it in a case of convergent evolution with the Haplorrhines, or that she actually is a Haplorrhine, or that the grooming claw is an atavism, or re-evolved trait in Strepsirrhines.  Given the rest of the traits, I think convergence is the most likely scenario.

It’s a shame that the really remarkable things about Ida are being obscured by the “Missing Link” rhetoric and pretty fringe-y interpretations of morphology.  She has her last meal preserved in her belly, for goodness’ sake!  And all of her little phalanges!  Amazing!

Welcome to the paleoanthropology scene, Darwinius massilae!  May your morphology continue to spark debate amongst scientists and inspire interest in Eocene paleontology amongst the public!

Franzen, J., Gingerich, P., Habersetzer, J., Hurum, J., von Koenigswald, W., & Smith, B. (2009). Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005723

Is a new adapid a “Missing Link”?

I know I said I’d be on a brief hiatus, but a friend sent me this article from the Daily Mail.  Apropos of my previous post/exam question, I thought I’d clear up a few of the misconceptions hidden (or not so hidden) in the article, which just so happens to feature a species of adapid that was recently discovered in the Messel Shales of Frankfurt, Germany.

A good starting place is the term “missing link.”  The term dates back to the medieval concept of the Great Chain of Being, or the Scala Natura.  The Great Chain represents a hierarchy, with each “link” in the chain being higher than the one that preceeded it.  Rocks are down at the bottom, and humans are at the top.  Angels and God are even further up.  “Missing links” are the links in the chain that have gone extinct.  So why don’t modern evolutionary biologists like to talk about “mssing links”?  Apart from the idea of the Great Chain being atiquated, it implies that we are linking some known, lowly form with a known, higher form.  But that isn’t always the case when we find fossils.  We don’t always know the animal that preceeded our new find, and we don’t know which animal succeeded it.  We can know that an animal represents a transitional phase between two different, general kinds of animals, though, so we usually use the term “transitional form” nowadays.  To illustrate my point a little more clearly, I’ll use the famous whale example.  We know that the ancestors of whales were once terrestrial animals, and that they evolved into the aquatic animals that we all know and love.  We know that Ambulocetus is a transitional form between the animals which were fully terrestrial and those which are fully aquatic.  What we don’t know is which particular terrestrial animal evolved into Ambulocetus, and whether or not Ambulocetus eventually evolved into, say, an Orca.

Okay, on to something a bit less general.  As I said before, the article is about a species of adapid which was recently discovered in Germany.   The generally accepted theory is that the adapids are the precursors to modern-day strepsirrhines (lemurs, bushbabies, lorises) because of certain shared anatomical traits.  They have long, projecting snouts like lemurs, smallish eyes like lemurs, and shared a number of features of the wrist and ankle with lemurs and their close cousins.  Happlorrhiness such as ourselves, the other apes, monkeys and tarsiers evolved from a different primate that was around at the same time.  This primate was probably an omomyid, and we can say that because omomyids share a certain number of features with modern happlorrhines: they have short, broad faces, huge eyes, and some even have a fused tibia and fibula like modern tarsiers.

The actual journal article hasn’t been published yet (or, I can’t find it if it has been!), but it seems that they are suggesting that their new fossil, dubbed Darwinius masillae, may be a stem Happlorrhine, even though it’s an adapid!  They say this because the fossil lacks a tooth comb, which is a highly specialized set of lower incisors used for grooming, and a also lacks a toilet claw, which is a retained claw on the first digit that is also used for grooming.

Hmm.  Absence of Strepsirrhine traits doesn’t a Happlorrhine make.  I will have to wait for the journal article before I can say anything more on that subject.

It sounds like they’ve got an exciting fossil, but not quite for the reasons stated in this Daily Mail article.  It’s not that this fossil is possibly a human ancestor- it’s much, much broader than that.  This fossil might be the common ancestor to monkeys, apes, humans, and tarsiers- or it might not be.  A rather annoying graphic shows our new little fossil evolving directly into an ape, skipping all the really interesting and diverse animals in between.  Animals like Aegyptopithecus, Eosimias, Proconsul, and Oreopithecus.

Annoying graphic from the Daily Mail

I understand the emphasis on the human connection, but in the effort to make sure that that angle of the story was represented, the rest of the story became convoluted and confusing, and in many places inaccurate (Humans did not evolve from tarsiidae!  We just have a more recent common ancestor with tarsiers than we do with lemurs.).  I have no doubt that David Attenborough will present the story much more elegantly and accurately.  And I REALLY want to read the article by Phillip Gingerich and Jorn Hurum!

Finals Week

Next week is finals week here, so it may be a little quieter than usual around here for a few days.  In the mean time, try your hand at one of the questions I give my undergrads for their Human Evolution lab practical:

The fossil in front of you [er, below] belongs to a species of Adapis. Give one anatomical feature that this fossil shares with the extant lemur next to it.

Adapis spp.

Lemur catta

Navigating the Bony Labyrinth

When many of us think of paleontology, we think of static reconstructions suspended by wire from cavernous ceilings in our favorite museums.  Some of us think of little trilobites entombed in stone and sitting on our father’s desk as a paperweight.  We might think of stately, dignified creatures striking one final pose for us to admire for an eternity, or perhaps of a dinosaur curling its head back in agony one last time.  But the final in situ position of an animal can only tell us so much about its lifestyle- usually, how it died.  Paleontologists are interested in that, of course, but we are even more interested in how an animal lived.

Luckily, the skeleton is a good place to look for clues.  Fossil primates can pose some especially interesting questions to a paleoprimatologist.  Because they live in trees, many different kinds of locomotion are possible.  We can look at limb proportions to see if the little guys were clinging to vertical supports and then leaping from them, or perhaps walking on top of thick, horizontal branches, or maybe even swinging below these brances.   We can look at the shape of the scapula to see whether the animal kept its arms underneath itself or used them to reach out to the side or above itself.  We can look at the fingers to see if they were grasping branches or balancing above them.  In species known only from cranial bones, we can also look at the ear bones to see how these guys positioned themselves while in the trees.

The Bony Labyrinth of the Inner Ear

The ear has three main parts.  The outer ear is the part we can see.  It directs sound energy into the middle ear, which is an air-filled cavity in which three tiny bones called the incus, malleus, and stapes vibrate against each other and the tympanic membrane to transmit sound.  The inner ear includes the organ of hearing called the cochlea, and an organ of balance which includes the vestibule and a bony labyrinth of semicircular tubes.  These tubes, called the semicircular canals, are the darlings of paleoprimatology because they can tell us so much about balance and locomotion.

In life, the vestibular system (including the canals) is filled with fluid called endolymph and lined with hair-like cilia which sense where the fluid is and when.  The semicircular canals are positioned orthogonally to each other so that, when used together, they can sense both vertical and horizontal movements.  We think that this is especially important in stabilizing gaze.  If you draw a dot on a piece of paper and then shake your head back and forth and up and down, you can still focus on the dot because the cillia in your semicircular canals are sensing the movement of the endolymph and telling your brain what your head is doing.  If you’re a small primate moving through the trees and trying to jump from one branch to another, your brain needs to know where your head is so it can tell your body how to land safely.

Mammals which move more quickly and with more agility tend to have larger semicicular canals with more cilia, which allows them to detect changes in direction, attitude, or speed more quickly.  In living primates, we know that leapers like sifakas and tarsiers have much larger semicircular canals in terms of both radius and length than arboreal quadrupeds or slow-climbing animals like the loris.  Mary Silcox and her colleagues used data from living primates to examine the locomotor patterns of extinct primates, many of whom are known only from cranial remains.  She examined five families of the Plesiadapiforms (pre-primate animals resembling modern tree shrews) and several Euprimates as well, including omomyids (the tarsier-like guys with small snouts and large eyes) and adapoids (the lemur-like animals with long snouts).

From post-cranial bones, we have reconstructed the locomotor repertoires of a number of these species. Plesiadapiforms had a diverse locomotor repertoire.  Some were living on or clinging to the underside of broad horizontal supports like modern tree shrews; some were “bounding and scampering” about like modern marmosets; others were slowly and deliberately moving on broad supports or the ground.  Among the Euprimates, the adapoids displayed a clear divide between the North American group, called the Notharctids, and the European group, called the adapids.  The North American notharctids were active arborealists, and most of them practiced leaping to at least some degree.  The European adapids were less active.  They most likely stayed on top of the branches.  The omomyids almost certainly practiced leaping in all species, but some were much more specialized than the others.

From these post-cranial reconstructions, we expect the Plesiadapiforms and the European adapids to have rather small semicircular canals, and notharctids and omomyids to have larger canals.  Using CT scans, Silcox and her colleagues measured the radius of the semicircular canals, scaled them for estimated body mass, and compared them with those of 91 extant species.  Generally, the predictions were borne out. Plesiadapiforms had smaller canals that were consistent with slower, less agile movements than the omomyids, who had larger canals, and the adapoids fell in between.  Silcox and her colleagues looked at Rooneyia, a fossil omomyid who is only known from cranial elements, and determined it to be an “occasional leaper,” similar to other omomyids with known post-crania. We can thus make the prediciton that, when a more complete Rooneyia skeleton is found, it will have a body consistent with a leaper.

Paleontologists love to take the static images of fossilized museum specimens and reconstruct them as dynamic creatures, quickly bounding from branch to branch or slowly scampering across a big tree branch.  A good paleontologist will be able to do this not only in her imagination, but by knowing the primate body inside and out (pun intended, maybe) and being able to wield some statistical weapons to make her case.

Literature cited:

Silcox M, Bloch J, Boyer D, Godinot M, Ryan T, Spoor F, Walker A (2009).  Semicircular canal system in early primates. Journal of Human Evolution.  56:315-327.