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A dedication to TM 266 (just because it’s groovy-looking)
Sahelanthropus tchadensis (“Toumai”) - Dated to 7 – 6Ma, found in the Rift Valley of East Africa at the site of Toros-Menalla near Lake Chad.
Remarkably complete skull:
- Thick, continuous supraorbital torus – more projecting that in gracile Australopithecines with no supraorbital sulcus, unlike gorillas
- Upper portion of the face is wide relative to the lower segment
- Wide interorbital “pillar” (unlike Sivapithecus or Pongo)
- Orthognatic face is shorter than in chimps or gorillas, and it’s not as concave as Sivapithecus and organutans. It is more convex than Paranthropus
- Weak subnasal prognathism, less than in gracile Australopithecus; lower face is narrower and less prognathic than in gorillas
- Infraorbital region smaller than in Australopithecus and Paranthropus
- Mastoid and post-glenoid processes are large; condylar process is broad
- Braincase described as “small ape-like” with an endocranial capacity of 320 – 380cc; braincase more elongate than in Australopithecus and Paranthropus
- Long, narrow basicranium
- More centrally located foramen magnum
- More horizontal orientation of the nucal plane
- Large canine fossa
- Relatively small incisors
- Molars with low rounded cusps and enamel thickness of cheek teeth that is intermediate between chimpanzees and Australopithecus.
The difficulty with interpreting TM 266 is that it’s fairly heavily distorted. I think it’s fair to say that its cranial morphology is very unlike what’s usually seen in fossil hominins. Occasionally you’ll come across someone who will argue that S. tchadensis ought to be included as a hominid on the basis of it’s apparent bipedality - position of the foramen magnum, the length of the nucal plane and the foramen magnum-orbital plane angle, when taken on their own, are all suggestive of obligate bipedalism (obligate bipedalism being one of the most significant autapomorphies defining the hominid clade).
Zollikofer et al. 2005 attempted to reconstruct/3D model S. tchadensis without the distortion. On inspection, though, it’s pretty apparent that something is wrong: the angle between the orbital plane and the foramen magnum is about 90 degrees, indicating an upright posture greater than even that of the australopithecines. Highly unusual.
Investigating this anomaly further, Wolpoff et al. (2006) conducted a (very convincing, in my opinion) biomechanical assessment of the reconstructed skull and found that it could not have functioned in a vertical posture because of the length of the nuchal plane and vertical height of inion: the nuchal plane is exceptionally long and even with the greatly reduced nuchal angle of the reconstruction, and its most posterior extent is significantly above the Frankfurt Horizontal. The cranial rear and posterior portion of the cranial base (its size, shape, and orientation of the nuchal plane) of TM 266 reflects nuchal functions similar to those of apes. In essence, all these things are compatible with a chimpanzee mode of locomotion.
Unlike australopithecines, the evidence indicates that Salehanthropus was not bipedal. This on its own contrasts with all known hominids, and even in the absence of postcranial remains (though there is potentially a femur associated with Toumai) this anatomy is sufficient to exclude Sahelanthropus from the human clade as it is currently understood. This compatible with genetic estimates of the chimpanzee/hominid divergence.
So, at least in my somewhat-educated opinion, TM 266 is more closely related to apes than hominids. I imagine that the femur, if it is ever formally described, will probably tell a similar story.
Tags: Skull Sahelanthropus tchadensis Reconstruction
