Chiniquodon fossils suggest mammal ancestors birthed live young earlier than thought

Researchers have discovered evidence suggesting that mammal ancestors may have transitioned from egg-laying to live birth at least 90 million years earlier than previously estimated.

Analysis of growth rings in the fossilized bones of Chiniquodon theotonicus, a species of cynodont that lived approximately 236 million years ago, indicates that these animals were relatively large at birth. This characteristic is typical of modern mammals but differs from the hatchlings of reptiles or birds.

According to findings published in Frontiers in Mammal Science, this discovery challenges the previous timeline which suggested viviparity emerged around 160 million years ago with therian mammals. The cynodonts, which appeared roughly 260 million years ago, possessed advanced evolutionary traits such as differentiated teeth and a secondary palate, allowing for simultaneous chewing and breathing to support higher metabolic rates.