Re: Bipedalism in different substrates

From: Bob Keeter (rkeeter_at_earthlink.net)
Date: 06/22/04 

Date: Tue, 22 Jun 2004 02:30:03 GMT

"Pauline M Ross" <pmross@ross-software.co.uk> wrote in message
news:o80dd0dbea8k0pa7m7dd0fvqmrgf7r1cm7@4ax.com...
> On Sun, 20 Jun 2004 23:27:18 GMT, "Bob Keeter" <rkeeter@earthlink.net>
> wrote:
>
> >Let me quote a paragraph. . .
> >"The prolonged persistence of well developed nephrostomes in the
mesonephros
> >of the elephant fetus for at least 2 months could be a result of the very
> >slow rate of embryonic and fetal growth, so that structures that only
appear
> >transiently in other mammals remain for much longer in the elephant.
> >Unfortunately, there is no other mammal with a comparably long gestation
> >length, but, even in the rhinoceros, Diceros bicornis [gestation length
is
> >?16 months (11)], there are no nephrostomes in the fetal mesonephros
(26),
> >nor are they present in the mesonephros of the humpback whale fetus,
> >Megaptera novaeangliae (27) [gestation length is ?11 months (11)]."
> >
> >IOW, this "aquatic" feature of an elephant's renal system is not even
found
> >in whales! 8-) You did of course know that human foetus goes through
> >a brief stage of development with gill slits, right?
>
> You missed the final line of the paragraph you quoted:
>
> "An alternative possibility is that the nephrostomes in the elephant's
> mesonephric kidney are a plesiomorphic character associated with its
> aquatic ancestry."
>
> In other words, the authors are quite properly considering all the
> alternative possible explanations for those nephrostomes.
>
> And they concluded:
>
> "For example, we accept without question the ontogenetic significance
> of the cranial nerves, the branchial arches, and the bones of the
> middle ear, so it seems reasonable to extend the argument to the
> renal, reproductive, and respiratory tracts. Given such an assumption,
> our embryological data strongly suggest that the mesonephric kidney,
> the testis, the trunk, and the lungs of the elephant all originally
> were adapted to its aquatic environment and that some of these unusual
> anatomical adaptations have persisted in present-day terrestrial
> elephants."
>
> In other words, there is evidence from 4 separate anatomical areas of
> a possible aquatic ancestry in elephants, and reasonable grounds for
> accepting that evidence. It's moderately speculative in itself, but,
> combined with the additional evidence the paper described, fairly
> conclusive, wouldn't you say?
>

Conclusive? Not really. I admit it at least makes the possibilty one
to be considered and worthy of debate. Of course if the "aquatic ape
phase" were pushed back into the Permian, we might not have much
of a debate at all! 8-) Seeing as how human embryos (and as far
as I know, all mammalian embryos have vestigal gills at one point
of their development, I would definitely say that there is a swimmer
back there somewhere for us all. . . . . including pachyderms!

Regards
bk