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
I seem to recall that the definition and recognition of the 3rd trochanter is disputed across primates. Some folks call any raised bony area in the area below the greater trochanter a 3rd trochanter, whereas others claim it must be developed from the diaphysis and must show some sort of rugosity. In any case, I definitely remember that it’s highly variable within and between species. These variable traits get somehow “fixed” in the literature as characterizing species A or taxon B, but in fact they are highly variable (e.g., the pterion configuration in plats versus cats also shows more variation than its usually accorded).
People may call anything below the greater trochanter a third trochanter, but I think if we’re going to talk about truly homologous structures, we should limit discussion to the rugose extension of the diaphysis (we can call it the third trochanter!).
You are very correct in pointing out that the third trochanter is highly variable across primate taxa. It’s not even present in all humans. But when it is present, it marks the site of gluteus maximus insertion, which is the important thing here. I think you’re correct that it can’t serve as a useful character to decide taxon designation, but it does provide useful information about functional anatomy.
And I should add that it’s not useful in that it shows how big the gluteus maximus is, but is useful because it shows where it is. Whether or not the animal has the third trochanter is probably the result of genetic variation within the species, but the placement of the gluteus maximus will be relatively conserved within that same species.
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“For reasons that I’ll discuss below, we think that Ardi was a facultative biped who spent most of her time in the trees.” Have I missed something? When do you provide reasons? I only see the assumption that Ardi was a facultative biped.
I think you’ve commented on the wrong post…
No, I meant to comment here because it was your most recent ardipithecus post. In one of your first A. ramidus posts, you state it is your intention to explain why you think Ardi was a facultative biped. In a later entry, you make two arguments for A. ramidus bipedalism, both referencing morphology of bones in the middle foot. I imagine you have more reasons than those.
Among physical anthroplogists, I have yet to hear from anyone (other than White, Lovejoy, et al) who is firmly convinced Ardi was a biped (or hominin). You seem to be. So I wanted to hear why. Do you find all of the arguments offered by the original authors convincing? Are there parts you find less persuasive? Are there certain morphological features you find particularly compelling?
Thanks for the clarification.
My intention in writing these posts is to have some fun describing some cool anatomy from the newest member of my favorite group of animals, the Miocene apes. Yes, I do think that Ardi was probably a biped when terrestrial, and I think the pelvis probably lays out the strongest evidence for that. I haven’t had time to finish a detailed post about the pelvis yet, but it’s in the works. But, I also think that the pelvis alone isn’t a “slam dunk” as it is in later hominids, and we shouldn’t expect it to be. We are talking about some very, very primitive anatomy here, so we can no longer simply compare a bipedal pelvis to a quadrupedal one and figure out which group the pelvis fits into. Taken together, I find the pelvis and the feet to be compelling.
I’ve also found many of the critiques of the author’s arguments to be unconvincing. For example, no, Ardi probably did not have a bicondylar angle, but why would anyone expect one in an animal that spends half (or maybe less than half) of its time loading its femora bipedally, and the other half loading them quadrupedally?
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