Slothzilla

***Choloepus*** is a genus of mammals of Central and South America within the monotypic family **Choloepodidae**, consisting of **two-toed sloths** (also less misleadingly known as "two-fingered" sloths).[2][3] The two species of *Choloepus* (which means "lame foot"[4]), Linnaeus's two-toed sloth (*Choloepus didactylus*) and Hoffmann's two-toed sloth (*Choloepus hoffmanni*), were formerly believed on the basis of morphological studies to be the only surviving members of the family Megalonychidae,[5] but have now been shown by molecular results to be closest to extinct ground sloths of the family Mylodontidae.

## Evolution[edit]

A study of retrovirus and mitochondrial DNA suggests that *C. didactylus* and *C. hoffmani* diverged 6 to 7 million years ago.[6] Furthermore, based on cytochrome c oxidase subunit I sequences, a similar divergence date (c. 7 million years ago) between the two populations of *C. hofmanni* separated by the Andes has been reported.[7]

### Relation to the three-toed sloth[edit]

Both types of sloth tend to occupy the same forests; in most areas, a particular species of the somewhat smaller and generally slower-moving three-toed sloth (*Bradypus*) and a single species of the two-toed type will jointly predominate. Although similar in overall appearance, the relationship between the two genera is not close. Recent phylogenetic analyses[8] support analysis of morphological data from the 1970s and 1980s, indicating the two genera are not closely related and adapted to their arboreal lifestyles independently. It was unclear from this work from which, ground-dwelling sloth taxa the three-toed sloths evolved. Based on the morphological comparisons, it was thought the two-toed sloths nested phylogenetically within one of the divisions of Caribbean sloths.[9] Though data has been collected on over 33 different species of sloths by analyzing bone structures, many of the relationships between clades on a phylogenetic tree were unclear.[10]

Much of the morphological evidence to support the hypothesis of diphyly has been based on the structure of the inner ear.[11] Most morphological studies have concluded that convergent evolution is the mechanism that resulted in today’s two genera of tree sloths. This means that the extant genera evolved analogous traits, such as locomotion methods, size, habitat, and many other traits independently from one another as opposed to from their last common ancestor. This makes tree sloths “one of the most striking examples of convergent evolution known among mammals”.[10]

Recently obtained molecular data from collagen[3] and mitochodrial DNA[2] sequences fall in line with the diphyly (convergent evolution) hypothesis, but have overturned some of the other conclusions obtained from morphology. These investigations consistently place two-toed sloths close to mylodontids and three-toed sloths within Megatherioidea, close to *Megalonyx*, megatheriids and nothrotheriids. They make the previously recognized family Megalonychidae polyphyletic, with both two-toed sloths and the Caribbean sloths being moved away from *Megalonyx*. Caribbean sloths are placed in a separate, basal branch of the sloth evolutionary tree.[3][2]