Xenarthra (Edentata) is an extremely diverse mammalian order whose modern r
epresentatives are the armadillos, anteaters, and sloths. The phylogeny of
these groups is poorly resolved. This is particularly true for the sloths (
phyllophagans), originally a large and diverse group now reduced to two gen
era in two different families. Both morphological analyses and molecular an
alyses of rDNA genes of living and extinct sloths have been used with limit
ed success to elucidate their phylogeny. In an attempt to clarify relations
hips among the sloths, DNA was extracted and mitochondrial cytochrome b gen
e sequences were determined from representatives of two extinct groups of s
loths (Mylodontidae and Megatheriidae), their two living relatives (two-toe
d sloths [Megalonychidae], three-toed sloths [Bradypodidae]), anteaters and
armadillos. A consistent feature of the latter two species was the nuclear
copies of cytochrome b gene sequences. Several methods of phylogenetic rec
onstruction were applied to the sequences determined, and the results were
compared with 12S rDNA sequences obtained in previous studies. The cytochro
me b gene exhibited a phylogenetic resolving power similar to that of the 1
2S rDNA sequences. When both data sets were combined, they tended to suppor
t the grouping of two-toed sloths with mylodontids and three-toed sloths wi
th megatheriids, The results strengthen the view that the two families of l
iving sloths adapted independently to an arboreal lifestyle. (C) 2000 Acade
mic Press.