Batodonoides

Batodonoides, genus of extinct insectivorous mammals that lived during the Eocene Epoch (56 to 33.9 million years ago) and of which the oldest species, Batodonoides vanhouteni, may have been the smallest mammal that ever lived. The genus includes three other species as well—B. walshi, B. powayensis, and B. rileyi. B. vanhouteni was found in lower Eocene rocks in Wyoming, and fossils of other species are also known from rocks in California.

Batodonoides is classified in the family Geolabididae, which is an extinct subgroup of the Soricomorpha to which living shrews belong. Its molar teeth indicate that it was a placental mammal and that it most likely ate insects. The cusps of the teeth were tall and pointed so that the upper and lower teeth interlocked with many sharp scissorlike edges that first punctured and then sliced its tiny prey. Only the teeth, mandible (lower jaw), and maxilla (upper jaw) have been recovered, but comparisons with its extinct relative Centetodon indicate that Batodonoides was a terrestrial mammal that was probably capable of climbing.

Body mass of extinct animals such as Batodonoides can be estimated based on the statistical relationship between the size of fossilizable structures such as teeth and the body mass of living relatives. Thus, the lower first molars of B. vanhouteni indicate that it weighed 0.93–1.82 grams (0.03–0.06 ounce), which is considerably smaller than the smallest living mammal, the bumblebee bat, Craseonycteris thonglongyai at 1.7–2.0 grams (0.06–0.07 ounce). Batodonoides may have been smaller than this tiny creature, but the statistical uncertainty associated with estimating the size of fossil species overlaps with the known size range of the bumblebee bat.

The tiny Batodonoides was operating near the minimum size limit imposed by mammalian physiology. At this size, animals are hard-pressed to gather food fast enough to maintain a constant body temperature. Compared with larger mammals, Batodonoides had a proportionally larger ratio of surface area to volume, so it lost heat faster.. Batodonoides lived in tropical climates during one of the warmest periods of the last 66 million years, perhaps alleviating its physiological challenges.