Darwinius as a “Missing Link”
I’ve finally gotten around to reading the PLoS ONE paper describing Darwnius masillae, the newly descrived adapoid from Messel, Germany, represented by a remarkably (95%) complete skeleton of an immature (approximately 1 y/o) female. The media blitz that accompanied the announcement of the fossil on May 20th put heavy emphasis on the notion that this fossil was a “missing link,” often put as being between “us and lemurs.” I’m sure the documentary soon to be aired and the book by Colin Tudge (which I have bought a copy of but haven’t yet read) will as well.
See any resemblance?
“Missing link” is a typical expression in any news report of a discovery of a (supposed) human ancestor, of course. And I will point out that the authors of the description don’t put their case as dramatically as the media does. Their basic thrust is to suggest that the constellation of features in D. masillae warrant a realignment of the Adapoidae (or at least the Cercomoniinae) with the group that includes living tarsiers and humans, that Haplorhini, instead of the group that contains modern lemurs, lorises, and galagos, the Strepsirhini. That’s not exactly a ‘missing link.’ More like a ‘missing cousin.’ However, they do suggest that D. masillae should force paleontologists to recognize that “the adapoid primates it represents deserve more careful comparison with higher primates than they have received in the past” and make allusion to the possibility that they “could represent a stem group from which later anthropoid primates evolved.”
As I mentioned before, Phillip Gingerich was on the team that described D. masillae, and in addition to being a fine mammalian paleontologist, he also believes on the basis of prior fossil evidence that anthropoids descended from certain adapoid primates of this subfamily Cercomoniinae. This fossil absolutley does not enter into a vacuum, either on the part of the theoretical orientations of its discoverers or the state of the debate on Eocene primates. Gingerich has been a partisan for a number of years in a rather intractable war. D. masillae is a cercomoniine adapoid that the describers claim to have found features on it that point toward anthropoids. This would be a clincher for Gingerich; an astonishingly complete fossil cercomoniine that is transitional to modern anthropoids. The ‘missing link’ with our prosimian past.
This combination, media frenzy, prior convictions, lent me to skepticism. I don’t feel unalterably assuaged.
First, let’s look at the features already cited by Gingerich and others in cercomoniine adapoids to support the theory that they are ancestral to anthropoids. The genus Mahgarita was a previous favorite for it’s “deep snout, fused mandibular symphesis, and enlarged canal for the promontary artery” (Fleagle, 1999). These are indeed all typical anthropoid traits: a high face (deep snout, in this context, as the length of the face still hasn’t been substantially reduced), a single bone in the lower jaw as opposed to the primitive two, and blood supply to the brain provided by the promontory as opposed to the stapedial artery (although this condition also exists in prosimian lorises). To make the point I intend to make here, I don’t particularly need to dispute the likelihood of any of these features forming synapomorphies (unique derived traits) with anthropoids. But, needless to say, the paleontologists who reject the adapoid theory for anthropoid origins are able to explain them away as convergences sufficiently so that no consensus has formed. As it stands, we have a stale mate.
For D. masillae to deserve the media hype (beyond being a beautiful primate fossil) and the confident moniker “missing link” it would need to provide some new evidence, by virtue of its completeness, of synapomorphies with anthropoids. So does it? The technical description is littered with non-committal little allusions to similarities with anthropoids, but I will limit myself to the features the authors’ put forth as definitive synapomorphies. I provide the relevant segment of Table 3 from the describing paper, detailing the binary (yes, no) presence of a list of traits present in Old World monkeys, apes, and humans and mostly present in New World Monkeys (i.e., anthropoid traits) in some living prosimians (note that tarsiers are commonly grouped with anthropoids as haplorhines), an assessment as to their status as primitive or derived, and their status in D. masillae. Say all that five times fast.
Anyway, the table:
Now, again, I don’t particularly hold myself competent to judge the quality of the synapomorphies identified by the describing team to link D. masillae to anthropoids. The criteria I’m holding “Ida” up to in determine whether-or-not the public and scientists should be hailing her as a “missing link” is, “Does she tell us anything new?” There is no indication of post-orbital closer or a narrowing of the space of bone between the eyes, features that anthropoids share with other potential prosimian ancestor groups and that have traditionally tripped up the adapoid theory. In honesty, save one, all the other features listed have been described in other cercomoniines and used to link them with anthropoids. albeit perhaps not all in the same individual.
The deep mandibular ramus, fused symphesis, and vertically implanted incisors are all reflections of a relatively sturdy, deep lower jaw and have been pointed out many times before (they form perhaps the core of the usual Gingerich case). All are present, for example, in Mahgarita. The short rostrum is somewhat less usual because it is a cranial feature, and there are few cercomoniine crania, but it is linked with the usual suggestion of a shorter, higher face in both adapoids and anthropoids. The post-cranial features are likely to get the most attention (flipping through the Tudge book I can see a number of illustrations detailing the hands and feet of ‘Ida’), and deserve it. However, the absence of a grooming claw has already been demonstrated in a cercomoniine, Europolemur koenigswaldi, though it seems to be present in Europolemur kelleri (of the same genus, no less). The fibular morphology has also already been commented on as seperating the cercomoniines from lemurs as well as the other family of European adapoids, the Adapidae, and alligning them with haplorhines. The authors don’t make a bad case over all, I suppose, but it doesn’t seem to me to be a particularly new one.
So, while the fossil is certainly extraordinary in what it has to tell us about Eocene primates and how they lived (all the parts of the paper I didn’t talk about), I don’t see as the discovers’ made a sufficiently original case using her unusually complete anatomy to, put frankly, change much in the way we look at anthropoid origins or the Haplorhini. Maybe the authors’ have a good point, and the Adapoidae should be included with tarsiers and anthropoids. But, the case they make in this paper doesn’t seem to use any evidence that wasn’t already available from other fossils. Before I sound too harsh, let me conclude by saying that the general proposition that this fossil is remarkably complete ergo has unusual potential for informing our understanding of the systematics of Eocene primates is quite possibly sound. But this provisional description (though thorough) doesn’t quite make that case. The mere fact that the fossil they’re working off of is really complete doesn’t make their argument from the same characters as were already known any more especially sound. This only becomes an indictment of the scientists if they fail to follow up with more comprehensive (espcially cladistic) analyses. For now, all we can say is, surprise, surprise, the media jumped the gun.
References:
Beard, KC (2004) The hunt for the dawn monkey: unearthing the origins of monkeys, apes, and humans. Berkeley: University of California Press.
(1999) Primate adaptation and evolution, second edition. San Diego: Academic Press.
Franzen JL, Gingerich PD, Habersetzer J, Hurum J, von Koenigswald W, Smith, BH (2009) “Complete primate skeleton from the middle Eocene of Messel in Germany: morphology and paleobiology.” PLoS ONE 4(5): e5723. doi:10.1371/journal.pone.0005723
Tudge, Colin (2009) The link: uncovering our earliest ancestor. New York: Little, Brown, and Co.
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