The unique postcranial of the extinct Old World monkey Paradolichopithecus

The talus (astragalus), distal tibia and the humerus of Paradolichopithecus arvernensis show some unique features, not seen in other monkeys.
The humerus has an increased articulation area on the head compared to Papio, a wide and deep groove for the biceps tendon, a gradually descending capitulum, and an oblique axis for flexion-extension through the elbow joint. During flexion, the ulna deviates from the parasagittal plane, and ends in a position medially to the humerus instead of parallel above it, due to the trochlear shape and axis. This unique feature yields a significant increased mobility.
The distal tibia bears a more massive, square and blunt malleolus that lacks the typically pronounced ball-shaped area, a wider groove (sulcus malleolaris) for the tendon of the M. tibialis posterior, a more square cross-section, clear scars for the fibula, and a double tendon groove on the dorsal surface (either for a bifurcated tendon for the M. flexorum tibialis posterior or a pronounced groove for the long toe flexor), which follows the parasagittal plane. None of these features is unique, and they make Paradolichopithecus resemble Australopithecus, a trained Japanese macaque and to a lesser extent some other macaques. The combination indicates a maintainance of the close-packed situation from dorsiflexion to plantar flexion, an increased importance of the fibula in weight transfer, a stronger plantar flexion, and possibly a slightly abducted foot. The flat tibial malleolus in Paradolichopithecus and Australopithecus, compared to baboons (Papio) and chimps (Pan) respectively, in combination with the corresponding facet on the talus acts as a blocking mechanism, preventing further dorsiflexion rotation during maximal dorsiflexion. This makes this ankle unsuitable for climbing.
The talus has an almost parallel trochlea, a large flap-like, protruding fibular suspensory facet, and a slightly deeper facet for the spring ligament on the talar head. These features are suggestive for a baboon-like ankle joint with the body weight more evenly distributed over the talar trochlea, a greater proportion of the weight transfer through the lateral (fibular) side, and with approximate the same stability in maximal dorsiflexion as in maximal plantar flexion. In these aspects Paradolichopithecus resembles Australopithecus.
Considering the unique features of the ankle and elbow of Paradolichopithecus, it may be expected that its locomotion differed from that of baboons. Main differences are the increased fibular component, the increased stability in plantar flexion, a more evenly distribution of stability during locomotion, and an equal medio-lateral stability in maximal plantiflexion and in maximal dorsiflexion. In our view, such a type of locomotion finds a parallel in Australopithecus and in trained Japanese macaques. The latter appear to develop significant modifications during training, especially in the hind limb, to satisfy the functional requirements for increased habitual bipedalism. Amongst others, the malleolus of the tibia has been remodeled under the influence of the greater stress and became less cusp-shaped, and the talar malleolar facet correspondingly more planar. The varus knee in the trained macaque further requires an increased fibular compound. This may have its parallel in Paradolichopithecus and Australopithecus, in whom we also find an increased fibular component. It should be stressed, however, that the kind of bipedalism of the trained macaque differs essentially from the striding gait bipedalism with erect trunk and straight knees of the genus Homo. The macaque bipedalism is characterised by high energy cost and bent knees. Considering the similar biomechanical features in Paradolichopithecus, Australopithecus and the trained macaque, it is tempting to conclude that also the two former genera had an all-round, energetically expensive bipedal mode with bent knees. This development then was not restricted to the hominoid clade, but appeared also in the papionins, as evidenced by the difference between Australopithecus and Pan on one hand and Paradolichopithecus and Papio on the other hand. The pattern shared indicates similar mechanical stresses, and reflects a shared increased frequency of bipedalism in the daily locomotor behavior, possibly but not necessarily, accompagnied by an increased mobility of the arm.

Read more in SONDAAR P.Y., VAN DER GEER A.A.E., DERMITZAKIS M.D. (2006). The unique postcranial of the Old World monkey Paradolichopithecus: more similar to Australopithecus than to baboons. Hellenic Journal of Geosciences 41, 1: 19-28. Special volume in the memory of Paul Yves Sondaar. Free pdf [868 kb] at http://users.uoa.gr/~geeraae/publications/2006-HJG-Paradolichopithecus.