Ancestors of sloths were large animals and evolved faster than other mammals, a new study has found.

Sloths living in the dense rainforests of Central and South America may be one of the slowest animals on land, but they were also at one point one of the largest. A new study by researchers at the University College London and University College has found that some sloth ancestors were as big as modern elephants.

The researchers used existing models to reconstruct sloths' evolutionary history. They found that sloth lineages increased their size by as much as 100 kilos every million years, which is some of the fastest rates of body size evolution among mammals.

"Today's sloths are really the black sheep of the sloth family. 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," Dr. Anjali Goswami from UCL Earth Sciences, an author on the paper, said in a news release.

Two species of sloths inhabit rainforests of Central and South America: two toed and three toed. Most sloths are around the size of a small dog and have a short and flat head. Their hair is grey in color, but appears greenish due to the growth of algae on their coats.

But according to the researchers, one extinct species of sloths, Megatherium americanum, weighed about four tons. Another species, Eremotherium eomigrans, weighed around five tons, boasting claws that were a foot long. These large-bodied sloth species, as well as others except for the two remaining today, died some 11,000 years ago.

In the study, the researchers used data on both modern-day sloths and their ancestors. The team said that the models can be used to study the evolution of other animals as well.

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

The study is published in the journal BMC Evolutionary Biology.