COSMOS MAGAZINE
Over 300 million years ago, reptile-like synapsids —ancestors of all mammals — first appeared, long predating the dinosaurs.
Credit: Magdalen Mercado, from Brocklehurst RJ, et al., 2025, PLOS Biology, CC-BY 4.0
Some synapsids grew colossal and ruled the planet until a mass extinction 252 million years ago wiped most of them out.
Credit: Magdalen Mercado, from Brocklehurst RJ, et al., 2025, PLOS Biology, CC-BY 4.0
The survivors became small, nocturnal scurriers living in the dinosaurs’ shadow, quietly testing new ways to move.
Credit: Magdalen Mercado, from Brocklehurst RJ, et al., 2025, PLOS Biology, CC-BY 4.0.
By comparing bones from 200 fossil and living species, scientists discovered their shift from a lizard-like sprawl to upright limbs was a late, winding journey rather than a straight line.
That delayed leap in posture set the stage for modern mammals to sprint, climb and ultimately dominate life on land.