Sixth "toe" for elephants

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

http://www.bbc.co.uk/news/science-environment-16250725

For more than 300 years, the structure has puzzled researchers, but this 
study suggests that it helps to support elephants' colossal weight.

Fossils reveal that this "pre-digit" evolved about 40 million years ago, 
at a point when early elephants became larger and more land-based.

Lead author Professor John Hutchinson, from the UK's structure and motion 
laboratory at the Royal Veterinary College ...

Many people, he said, thought that the structure was a huge lump of 
cartilage, and over the years its purpose or lack of purpose has been 
debated.

"Anyone who has studied elephants' feet has wondered about it. They've 
thought: 'Huh, that's weird,' and then moved on," he added.

But Prof Hutchinson and colleagues used a combination of CT scans, 
histology, dissection and electron microscopy to solve the puzzle.

The researchers said the structure was made of bone, although bone with a 
highly irregular and unusual arrangement.
...


http://www.sciencemag.org/content/334/6063/1699.abstract
Science 23 December 2011:
Vol. 334 no. 6063 pp. 1699-1703
DOI: 10.1126/science.1211437

> From Flat Foot to Fat Foot: Structure, Ontogeny, Function, and Evolution 
of Elephant Sixth Toes

Several groups of tetrapods have expanded sesamoid (small, 
tendon-anchoring) bones into digit-like structures (predigits), such as 
pandas thumbs. Elephants similarly have expanded structures in the fat 
pads of their fore- and hindfeet, but for three centuries these have been 
overlooked as mere cartilaginous curiosities. We show that these are 
indeed massive sesamoids that employ a patchy mode of ossification of a 
massive cartilaginous precursor and that the predigits act functionally 
like digits. Further, we reveal clear osteological correlates of predigit 
joint articulation with the carpals/tarsals that are visible in fossils. 
Our survey shows that basal proboscideans were relatively flat-footed 
(plantigrade), whereas early elephantiforms evolved the more derived 
tip-toed (subunguligrade) morphology, including the predigits and fat pad, 
of extant elephants. Thus, elephants co-opted sesamoid bones into a role 
as false digits and used them for support as they changed their foot 
posture.