Williams, Richard Kay, Christopher Kirk and Callum Ross have published a new paper in the Journal of Human Evolution reassessing the phylogenetic placement of Darwinius masillae, the much-hyped Adapid fossil published last summer. Brian Switek at Laelaps and Eric Michael Johnson at The Primate Diaries have written some excellent posts summarizing the most recent paper, as well as the whole fiasco from the beginning.
Much of the debate about this little fossil surrounds whether it’s a Haplorrhine- which would group it with tarsiers, monkeys, and apes- or a Strepsirrhine- which would group it with lemurs, lorises, and galagos. But how do we decide which is which?
This is the rhinarium on a cat, but it's very similar in Strepsirrhines. Note how the rhinarium attaches to the gums. Image from wikimedia commons.
The first thing we look at is the nose. Strepsirrhines have a wet nose, like a dog. Haplorrhines, on the other hand, have a dry nose, like humans. The wet tissue around the nose is called the rhinarium, and Haplorrhines are defined by the absence of a rhinarium. Pretty straightforward when you have a living animal on your hands- but what if you only have a skeleton or a fossil? The rhinarium attaches to the gum right in between the two upper central incisors, and the bone underneath supports this attachment. In the wet-nosed strespirrhines, there is a gap between the two incisor roots to allow for this bony reinforcement. Since Haplorrhines don’t have a rhinarium, they don’t have this gap between their incisor roots.
Haplorrhines also exhibit a trend toward a reduced reliance on their sense of smell, particularly when searching for a mate. Loss of the rhinarium is part of that, since the rhinarium can help to pick up pheromones from the environment. They’ve also reduced the amount of a special tissue located in the nose that helps them smell things more acutely, the olfactory epithelium. Along with that, they’ve reduced the part of their brain which processes those chemical signals, which is located in paleocortex. If we want to look at manifestations of this in the skeleton, we see that a lot of the bony structures which support that olfactory epithelium have been reduced or lost in Haplorrhines. We can also see that the little nook in the skull where the olfactory bulb of the brain sits is smaller in Haplorrhines.
Haplorrhines have reduced their reliance on smell, but have increased their reliance on vision. The cone photoreceptors are much more densely packed onto their retina than in the mostly-nocturnal Strepsirrhines. They have also developed a little pit in their retina called a fovea centralis. The cones cells here are thinner, which allows them to be more densely packed, and they are very well-connected to the brain by retinal ganglion cells. The areas of the brain which process visual information are relatively much larger in Haplorrhines than in Strepsirrhines, as well.
In the skeleton, this increased reliance on vision manifests itself as a bony cup behind the eye called the postorbital septum. The postorbital septum isolates the eyeball from the chewing muscles, and might be the result of the increased size of the eyes, or perhaps the fact that they’ve moved from the side of the head to the front of it. Strepsirrhines have a bony strut that forms a ring around the eye, but they don’t form a septum like the Haplorrhines do.
Many of the traits that distinguish Haplorrhines from Strepsirrhines represent derived traits in Haplorrhines. However, there are also a few traits which are derived in Strepsirrhines but absent or primitive in Haplorrhines. For example, Strepsirrhines have a tooth comb, but Haplorrhines lack this structure. The tooth comb is formed by the lower two incisors, as well as the lower canines. It sticks directly out and Strepsirrhines use it to groom themselves and each other. Strepsirrhines also have a little claw on their second toe called the toilet claw or the grooming claw, which again, is used for grooming.
And now comes the tricky part. The traits I’ve outlined above (as well as some other more technical traits) are the result of millions of years of evolution. There was an ancestor to all living Haplorrhines who did not possess the full suite of Haplorrhine traits. And it can be tricky to figure out which traits are primitive retentions from an insectivore ancestor or novel traits in the lineage. Take the toilet claw: Is it a primitive retention from a more basal primate, or is it a novel trait?
You also have to keep in mind that the absence of derived traits is not particularly damning for a fossil. Australopithecines do not have very large brains (a derived trait) but that doesn’t exclude them from the human lineage.
So what do we do? We should try to do what the authors of this newest paper have done, and put our new fossil into the context of other discoveries of similar animals from around the same time period. They come down with Frahm et al in describing Darwinius as an adapid. Where they differ is whether or not they would place adapids into the Haplorrhine or Strepsirrhine clade.
Everyone who has looked at Darwinius agrees that she is an adapid, and in particular, a cercamonine adapid. In order to move her over to the Haplorrhine clade, you’d have to take the rest of the cercamonines with her. And probably the rest of the group above that, the Notharctids, and then also the group above that, the Adapids. And most of us are simply not convinced that there is compelling evidence to do this. In fact, the evidence is quite compelling that this should not be done.
Even if this particular adapid shares some convergences with Haplorrhines and not just primitive retentions (and that’s a big if- I’m certainly not convinced), the overall picture of Eocene primate evolution overwhelmingly shows that the adapids are Strepsirrhines.
References:
Williams, B., Kay, R., Christopher Kirk, E., & Ross, C. (2010). Darwinius masillae is a strepsirrhine—a reply to Franzen et al. (2009) Journal of Human Evolution DOI: 10.1016/j.jhevol.2010.01.003
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










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