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Sixth "toe" for elephants


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.

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.