Sloths may be slow-moving animals but they're pretty fast when it comes to evolution.  In fact, the speed of how the ancestors of these mammals grew in size is one of the fastest scientists have known among mammals.

About 11,000 years ago, there were a large number species of sloths on Earth. Some of these species were reduced in weigh to about 13 pounds while some grew to sizes as big as modern elephants. For the new study published in the BMC Evolutionary Biology on Sept. 10, John Finarelli, from the School of Biology & Environment Science at the University College Dublin in Ireland, looked at current evolutionary models to reconstruct how sloths diversified.

The researchers found that the models that were based on the living species could not adequately explain the changes in size but models of species on fossil records, revealed that the animals evolved at an extremely fast rate.

"Today's sloths are really the black sheep of the sloth family," said study researcher Anjali Goswami, from the Department of Genetics, Evolution & Environment at the University College London in the UK. "If we ignore the fossil record and limit our studies to living sloths, as previous studies have done, there's a good chance that we'll miss out on the real story and maybe underestimate the extraordinarily complex evolution that produced the species that inhabit our world."

Finarelli and colleagues have found that some of the species grew over 220 pounds every million years, which is among the fastest of body growth rates in the evolution of mammals and this suggests that factors such as environmental conditions and competitions with other animals may have favored sloths with massive size.

A smaller number of sloth species was reduced in size but these did not compete for survival against the bigger animals and whatever killed the giant sloths, possibly hunting, the changes in climate and disease, may have spared the smaller species.

There are only six species of sloths that live today and these are middle-sized mammals that only reach a maximum of 12 lbs. Fossil records, however, show that there are over 50 million species of sloths that thrived between 2.6 million and 11,700 years ago.

"A great deal of debate has centered on the importance of fossil data to accurate analysis of macroevolutionary patterns, and recent studies have demonstrated that excluding extinct taxa can result in misleading reconstructions of diversity dynamics and trait evolution," the researchers wrote.  "This issue can be particularly problematic for clades that were previously diverse, but are presently species-poor, such as tuataras, hyaenas, or sloths."

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