Many find sloths cute, while some find them just plain bizarre. In The Upside-Down Book of Sloths, Elizabeth Shreeve uncovers their less-well-known evolutionary history and how they became the beloved--and unique--creatures of today. She pairs and compares the six extant modern species, like the pygmy sloth, the brown-throated sloth, and the ai, with their prehistoric counterparts, such as Thalassocnus, the tough seafaring sloth; Paramylodon, which...