Leptomerycids are one of the earliest families of ruminants; the first members of the group appear in the fossil record in the middle Eocene, around 41 million years ago. Their nearest living relatives are the tragulids, or mouse deer, of equatorial Africa and southeast Asia. Leptomeryx was a small, lightly-built animal that probably lived in wooded environments, feeding on fruits, tender shoots, and occasional insects. Fossils of this animal are quite common in the White River Badlands of South Dakota, where it seems to have been abundant.

The fossil record cannot provide any direct evidence for Leptomeryx possessing the most critical feature of ruminants: a multi-chambered stomach, containing bacteria which assist in the digestion of coarse, cellulose-rich vegetation. However, the fact that their living relatives, the tragulids, possess a small but functional ruminant stomach suggests that this feature evolved at an earlier date and that leptomerycids did ruminate.