New species of Batodonoides (Lipotyphla, Geolabididae) from the early Eocene of Wyoming: Smallest known mammal?

A new early Eocene insectivore, Batodonoides vanhouteni, sp. nov., has molar teeth indicating a body size smaller than that of any mammal known to date. B. vanhouteni is the oldest known species assigned to the genus, which was known previously from the middle Eocene of California. The type, all associated maxilla and mandible, preserves P2-M2 and c1-p2, dp4, p4-m3, with alveoli for i1-3. Some teeth (P4, c1, and p4) are still erupting, indicating that the specimen is a juvenile at about the age of dispersal. B. vanhouteni had two sets of functional teeth, unlike extant mammals of very small size. it retains the primitive pattern of cheek-tooth replacement, erupting P3 before P4, unlike the P4-before-P3 sequence of almost all Lipotyphla. The distribution of body masses for Clarkforkian and Wasatchian insectivores indicates that late Paleocene and early Eocene lipotyphlans occupied a lower range of body masses (ca. 1.3-53 g) compared to extant Lipotyphla (ca. 2.5-1,100 g); the upper range of insectivore body masses was occupied by now-extinct "Proteutheria."