In Japan, fossils of small mammals, such as those the size of mice or moles, are rarely found in older geological strata. This scarcity is largely due to the fact that fossil searches for small mammals have primarily focused on sediments accumulated in limestone fissures or caves, which are uncommon in strata older than 800,000 years. As a result, discoveries of small mammals from these earlier periods have been limited. Using new methods, I have successfully extracted small mammal fossils from strata dating back 3.5 million and 26 million years. My work contributes new insights into Japan’s fossil mammal record, which has traditionally been dominated by reports of large mammals such as elephants and deer, and aims to shed light on transitions in mammalian fauna across East Asia.