Today, sloths take their time, sluggishly making their way from branch to branch in the jungles of South and Central America. Modern sloths are also small. Their ancestors, on the other hand, were giant, and they became that way rather quickly, evolving at an impressive pace.

To get a better understanding of how sloth species diversified over time, scientists from University College London and University College Dublin used mathematical models to fill in the gaps in the family tree and evolutionary timeline -- previously only informed by modern observations and a limited fossil record.

The research helped tell a more comprehensive story about the animals. Today, there are only two sloth species, and both are quite small. But their ancestors were impressively diverse, and some species were as big as elephants. As well, some lineages increased in size at a record-setting pace, adding on more than 220 pounds every million years.

"Today's sloths are really the black sheep of the sloth family," explained Dr. Anjali Goswami, an earth science professor at UCL. "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."

Though sloth species used to number in the dozens, all but two lineages died out around 11,000 years ago. One of those now-extinct species was Megatherium americanum, who could walk upright and weigh up to four tons. Today's sloths max out at 13 pounds.

"There are many other groups, such as hyaenas, elephants and rhinos, that, like sloths, have only a few living species," said Dr. John Finarelli, lead author of the new study and a researcher at the University College Dublin Earth Institute. "But if we look into the distant past, these groups were much more diverse, and in many cases very different to their current forms."

The study was published this week in the latest issue of the journal BMC Evolutionary Biology.