Millions of years before the asteroid impact that ended the reign of the dinosaurs, mammals were already beginning to shift from tree-dwelling to ground-based lifestyles.

A groundbreaking study uncovered this evolutionary trend by analyzing tiny limb bone fragments from marsupials and placental mammals in Western North America. These subtle fossil clues reveal that mammals may have been responding to a changing world, especially the spread of flowering plants that transformed habitats on the ground. Surprisingly, this terrestrial transition appears to have played a bigger role in mammalian evolution than direct interactions with dinosaurs.

Early Ground-Dwellers Before Dinosaurs’ Demise.