Knuckle-walking in Sahelanthropus?
Locomotor inferences from the ulnae of fossil hominins and other hominoids
MR Meyer, JP Jung, JK Spear, IF Araiza, J Galway-Witham & SA Williams 2023 JHE 179,103355
doi org/10.1016/j.jhevol.2023.103355

The ulna supports & transmits forces during movement: its morphology can signal aspects of functional adaptation.
Do, like extant apes, some hominins habitually recruit the forelimb in locomotion?
We separate the ulna-shaft & ulna proximal complex, for independent shape analyses (elliptical Fourier methods) to identify functional signals.
We examine the relative influence of locomotion, taxonomy & body-mass on ulna contours in 22 Hs, 33 extant apes (5 spp), 2 Miocene apes (Hispanopith., Danuvius) & 17 fossil hominin spms:
Sahelanthr., Ardipith., Australopith., Paranthropus, early Homo.

Ulna proximal complex contours correlate with body-mass, but not locomotor patterns,
ulna shafts significantly correlate with locomotion.
African apes' ulna shafts are more robust & curved than Asian apes', unlike other terrestrial mammals (incl. other primates), curving ventrally rather than dorsally,
this distinctive curvature is absent in orangs & hylobatids:
it is likely a function of powerful flexors engaged in wrist & hand stabilization during KWing, not an adaptation to climbing or suspension.
OH-36 (purported Par.boisei) & TM-266 (assigned to Sahelanthr.tchadensis) differ from other hominins,
they fall within the KWing morpho-space, and thus appear to show forelimb morphology cons.x terrestrial locomotion.
Discriminant function analysis classifies OH-36 & TM-266 with Pan & Gorilla with high posterior probability.
Along with its associated femur, the TM-266 ulna shaft & and its deep, keeled trochlear notch comprise a suite of traits signaling Afr.ape-like QPism.
Implications for the phylogenetic position & hominin status of S.tchadensis remain equivocal,
but this study supports the growing body of evidence indicating:
S.tchadensis was not an obligate biped, but instead represents a late Miocene hominid with KWing adaptations.

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:-)

This paper once more confirms our view that apiths were evolving into the Afr.ape direction:
they were fossil relatives of Pan (S.Africa) or Gorilla (E.Africa), not Homo, evolving from aquarboreal to knuckle-walking (P//G).
https://www.gondwanatalks.com/l/the-waterside-hypothesis-wading-led-to-upright-walking-in-early-humans/

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