Friday, March 16, 2007

What thumb?

Before I forget... why did tetrapods develop divergent digits (thumbs & big toes), and why did they disappear in marine/aquatic returnees?

My guess, divergent digits are important in a terrestrial tetrapod for abrupt reversals, ground perching, left and right turns, and are lost in those which tend toward greater speeds on plains (which tend to encircle rather than sharply turn), but aquatics get no benefit from this since they can't rely on traction, so the thumb disappears, as in early ichthyosaurs. A parallel exists in brachiating arboreal primates, fast forward swinging requires shoulder twisting rather than wrist twisting to turn, so the thumb tends to become vestigial.

That non-brachiating primates retain the divergent functional thumb means they were not derived (recently) from habitual plains walkers. That pandas lost the thumb and then regrew one from the wristbone indicates a plains niche, followed by arboreal habitat niche.

Hmm still thinking on this...

"Speaking of "what thumb?", well see this video of a scuba diver feeding moray eels by hand, so cute until the POP the moray clipped off a thumb. The diver got it replaced with his 2nd toe via surgery. 6 minute video, 1st 3 minutes is feeding, last 3 minutes is the bite and reattachment of the toe to the thumb base. Don't feed the animals...
What thumb?

Don't pet the dolphins: Marine Officials Warn Of Biting Dolphins

Marine researchers are warning about a growing number of dolphin bite cases in Sarasota County, according to a Local 6 News report.Florida experts said wild dolphins are becoming more aggressive because boaters are feeding them."It seems reasonable to understand why you wouldn't feed a bear or something more dangerous-appearing, but these are wild animals," dolphin researcher Jason Allen said. "They are wild animals with lots of sharp teeth." Officials said a dolphin bit a woman from Lakeland earlier this month when she tried to pet it.

A link provided by DM in comments: (link didn't carry over to Blog comments)
Panda's thumb: click here

DD

3 comments:

David Marjanović said...

why did tetrapods develop divergent digits (thumbs & big toes)

Tetrapods?

Take a look around yourself. Very, very, very few tetrapods have divergent – let alone opposable – 1st digits.

David Marjanović said...

Should have mentioned that the pandas haven't lost anything. They have 5 ordinary digits per limb. Their "thumb" is additional. There's a drawing of the skeleton of a panda hand in the banner of http://www.pandasthumb.org.

DDeden said...

I meant "why did any", iow what special function was selected for? I think it's due to fast reverse movements and quick turns on a tractable surface.

Thanks for the Panda ref., I'd thought the original thumb was much more vestigial.

DDeden