Re: Platypuses may be older than we think...

["Richard W. Travsky" <rtravsky@uwyo.edu>]

On Wed, 24 Oct 2007, K and T Dykes wrote:
> [...]
> This next quoted sentence puzzles rather than entertains: "Since no 
> other mammal, including echidnas, can surpass the electro-detection 
> abilities of the platypus, researchers had thought this was a relatively 
> new skill that the animal evolved."
>
> As water conducts electricity better than air does, it'd be surprising 
> if echidnas could manage the same levels of this here electro-detection.

A bit dated, but:

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1692308
Sensory receptors in monotremes.
Philos Trans R Soc Lond B Biol Sci. 1998 July 29; 353(1372): 11871198.
U Proske, J E Gregory, and A Iggo
Abstract
...
The structures in the skin responsible for the electric sense have been 
identified as sensory mucous glands with an expanded epidermal portion 
that is innervated by large-diameter nerve fibres. Afferent recordings 
have shown that in both platypuses and echidnas the receptors excited by 
cathodal (negative) pulses and inhibited by anodal (positive) pulses. 
Estimates give a total of 40,000 mucous sensory glands in the upper and 
lower bill of the platypus, whereas there are only about 100 in the tip of 
the echidna snout.
...

400 to 1!

> More interesting is that these terrestrial critters are apparently* 
> rigged up for this art.  Both the long- and the short-beaked echidnas 
> have electro-receptors, with New Guniea's long-beaked ones being the 
> more talented of the pair in this regard.  I can't think of any 
> particular use these abilities might provide for terrestrial echidnas, 
> but perhaps there's something lacking in my imagination.  To me, this 
> suggests it's a relatively old skill inherited from a presumably 
> semi-aquatic common ancestor.  Of course, there may have been some 
> researchers somewhere thinking along the lines the journalist reports.