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Thread: Another Look at the Foramen Magnum in Bipedal Mammals

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    Senior Member Catterick's Avatar
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    Another Look at the Foramen Magnum in Bipedal Mammals

    Unrelated bipedal mammals such as humans, kangaroos, springhares and jerboas have a more forward-positioned foramen magnum than their quadrupedal relatives. So it is a predictor of an upright posture in human ancestors.

    A more anteriorly positioned foramen magnum evolved in concert with bipedalism at least four times within Mammalia: once in macropodid marsupials, once in heteromyid rodents, once in dipodid rodents, and once in hominoid primates. Here, we expand upon previous research on the factors influencing mammalian foramen magnum position (FMP) and angle with four new analyses. First, we quantify FMP using a metric (basioccipital ratio) not previously examined in a broad comparative sample of mammals. Second, we evaluate the potential influence of relative brain size on both FMP and foramen magnum angle (FMA). Third, we assess FMP in an additional rodent clade (Anomaluroidea) containing bipedal springhares (Pedetes spp.) and gliding/quadrupedal anomalures (Anomalurus spp.). Fourth, we determine the relationship between measures of FMP and FMA in extant hominoids and an expanded mammalian sample. Our results indicate that bipedal/orthograde mammals have shorter basioccipitals than their quadrupedal/non-orthograde relatives. Brain size alone has no discernible effect on FMP or FMA. Brain size relative to palate size has a weak influence on FMP in some clades, but effects are not evident in all metrics of FMP and are inconsistent among clades. Among anomaluroids, bipedal Pedetes exhibits a more anterior FMP than gliding/quadrupedal Anomalurus. The relationship between FMA and FMP in hominoids depends on the metric chosen for quantifying FMP, and if modern humans are included in the sample. However, the relationship between FMA and FMP is nonexistent or weak across rodents, marsupials, and, to a lesser extent, strepsirrhine primates. These results provide further evidence that bipedal mammals tend to have more anteriorly positioned foramina magna than their quadrupedal close relatives. Our findings also suggest that the evolution of FMP and FMA in hominins may not be closely coupled.
    http://www.sciencedirect.com/science...47248417300507

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    Senior Member Catterick's Avatar
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    The FM is relevant in the story of the Tuang child. Dart's claims were initially treated with skepticism because chimpanzees also have a human-like FM in childhood. This was not just because of Piltdown which was not as uncritically accepted as you might think - in fact Solly Zuckerman had employed biometrics to try and prove his point. I do wonder exactly how neoteny fits into this and if the human-chimp-gorilla LCA was bipedal. Ususlly bonobos get mentioned though their limb proportions resemble the quadropedal ground monkeys as well as hominins.

    There is no unequivocal knuckle walking in any hominin though some of them (even H. naledi) show signs of arboreal adaptations in the forearm. Whereas Homo and Ardipithecus are/were pronograde on all fours like monkeys and Proconsul. Pan and Gorilla have different adaptations enabling knuckle walking ie. it evolved twice. Orangs are specialised in their own "four handed" direction with arm movements involving lots of reaching out to bridge gaps between branches. (Incidentally so were australopithecine upper bodies). Gibbons are also specialised specifically to arm swinging. Fossil apes are likewise dissimilar to one another in this regard.

    I don't know; its easy to see a direct line of hands and wrists from platyrrhine and catarrhine monkeys through pronograde basal apes like Proconsul through to Ardipithecus and then conservative Homo. All living apes might be specialised but at each node a pronograde quadroped, or maybe a primitive biped at the pan-homo-gorilla node. If the chimp-gorilla-human LCA was quadropedal maybe bonobos lean backwatds but why then do they need knuckle walking if papionins don't? Then the bipedalism of Lucy was too divergent from ours to be our ancestor: what was our shared ancestor like? Certainly upright but in what way - a vertical clinger?

    In tetrapods adaptations for browsing may resemble those for climbing: compare the claws of ornithomimids to those if tree sloths or the "thumbs" of giant pandas to the wrist structures of some arboreal mammals. The combination of hooked fingers and bipedism suggests browsing and also knuckle walking in quadropeds evolved to protect the fingers necessary for feeding (compare the great apes to anteaters). In gibbons and spider monkeys bipedalism achieves the same but most fossil Homo do not have hooked digits.

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