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The sloth’s convergent evolution: Making slow work

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Brian Switek tells us that “The Sloth’s Evolutionary Secret” (Wired, January 3, 2012) is

The distant relationship between the two modern sloth genera – species of two-toed sloth are grouped under Choloepus, and three-toed species under Bradypus – isn’t immediately obvious. Both suspend their bodies beneath tree limbs by their hooked claws, and they slowly clamber along upside-down. If we had no knowledge of the fossil record, we would expect that these animals are the descendants of some relatively recent sloth that moved in the same way.

What actually happened isn’t that simple. In addition to anatomical comparisons between modern and fossil sloths, genetic studies involving DNA extracted from the skin and dung of extinct species have found that two-toed sloths are most closely related to genera such as Megalonyx, and three-toed sloths belong to a different sloth subgroup which includes behemoths akin to Megatherium. Exactly when two- and three-toed sloths last shared a common ancestor isn’t clear, but, on the basis of their relationships, both lineages probably go back to a species which trundled along on the ground. This is consistent with what is known from the fossil record. So far, no one has found an extinct sloth which moved upside-down through the canopy like modern species do.

Perhaps they just kept putting it off.

Comments
I understand some sloths have the rare case of having more veterbrate then other "mammals". Hanging upside however kills the back. sloths were a ground creature and only a few hic cousins went into the trees. Yet they survived the great extinctions because tree life was richer when things went bad. likewise olny camels survived in the mountains of south america though they were everywhere. All sloths are from a single one but tracking them by genetics is a waste of time. There must be innate triggers that bring body changes and not evolutionism. if the body changes quickly the genetics also quickly must change. Yet unrelated to heritage. jUst like with human skin colour.Robert Byers
January 10, 2012
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