Re: An alternative to ST and AAT

pete (VINCENT@reg.Triumf.CA)
1 Nov 1996 22:15:37 GMT

Rohinton Collins ( sez:
`pete <VINCENT@reg.Triumf.CA> wrote in article
`> The idea of an arboreal quadruped which spends much of its time standing
`> on two feet is somewhat ambiguous. What exactly do you have in mind here?
`> Do you consider the gibbon to be an arboreal quadruped? It uses all four
`> limbs for locomotion, but uses the front pair quite differently from the
`> back.

`Fair enough Pete, I may have not explained myself well enough. What I meant
`by an arboreal quadruped is an ape which uses all four limbs for
`locomotion, but being arboreal, has much flexibility of locomotion and
`posture (the orang-utan would probably be a good model). The chimpanzee has
` a rather restricted mode of locomotion, since it is a fully adapted
`terrestrial quadruped (N.B. small valgus angle). So, what I am saying is
`that my LCA would have been more predisposed to the transformation into a
`terrestrial biped than would be a proto-chimp LCA. And no, the gibbon is a
`brachiator. I would say that a gibbon-like LCA (in its mode of locomotion)
`would be unlikely because brachiation is specialised. What it comes down to
`is that a more generalised LCA (in all ways, as well as locomotion) would
`be more predisposed to take advantage of a new (or a forced) mode of
`locomotion than a more specialised ape would be.

Ok, but the orangutan has a pretty unique method of locomotion, because
of its size. One must suspect from the size of extant a-pith fossils
that the lca was a smaller creature. When one considers the gibbon
style of locomotion, which is divided between brachiation with the
fore limbs and running along branches with the hind limbs, as well as
having an erect-posture pelvis resulting from its brachiating niche,
it doesn't seem unreasonable to postulate an LCA which used a similar
repertoire of motive strategies, but had not evolved so far along that
line, so that for instance its forelimbs were not so elongated, might
be well positioned to transfer to a bipedal terrestrial niche when
(not yet specified) pressures forced it in that direction. ...I wonder
if the gibbon family was fully developed to its current fore/hind limb
ratio 6 to 8 Mya...

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